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| LTC2496 1 2496fa typical application features applications description 16-bit 8-/16-channel ? adc with easy drive input current cancellation the ltc ? 2496 is a 16-channel (8-differential) 16-bit no latency ? ? adc with easy drive? technology. the pat- ented sampling scheme eliminates dynamic input current errors and the shortcomings of on-chip buffering through automatic cancellation of differential input current. this allows large external source impedances, and rail-to-rail input signals to be directly digitized while maintaining exceptional dc accuracy. the LTC2496 includes an integrated oscillator. this device can be con? gured to measure an external signal (from combinations of 16 analog input channels operating in single ended or differential modes). it automatically rejects line frequencies of 50hz and 60hz, simultaneously. the LTC2496 allows a wide common mode input range (0v to v cc ), independent of the reference voltage. any combination of single-ended or differential inputs can be selected and the ? rst conversion after a new channel is selected is valid. access to the multiplexer output enables optional external ampli? ers to be shared between all analog inputs and auto calibration continuously removes their associated offset and drift. data acquisition system up to 8 differential or 16 single-ended inputs easy drive technology enables rail-to-rail inputs with zero differential input current directly digitizes high impedance sensors with full accuracy 600nv rms noise (0.02 lsb transition noise) gnd to v cc input/reference common mode range simultaneous 50hz/60hz rejection 2ppm inl, no missing codes 1ppm offset and 15ppm full-scale error no latency: digital filter settles in a single cycle, even after a new channel is selected single supply 2.7v to 5.5v operation (0.8mw) internal oscillator qfn 5mm 7mm package direct sensor digitizer direct temperature measurement instrumentation industrial process control , lt, ltc and ltm are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners. +fs error vs r source 0.1 f 10 f f 0 ref + v cc muxout/ adcin muxout/ adcin 2.7v to 5.5v com ref 16-bit ? adc with easy-drive 16-channel mux in + in 2496 ta01a ch0 ch1 ch7 ch8 ch15 osc sdi sck sdo cs 4-wire spi interface r source ( ) 1 +fs error (ppm) ?0 0 20 1k 100k 2498 ta01b ?0 ?0 ?0 10 100 10k 40 60 80 v cc = 5v v ref = 5v v in + = 3.75v v in = 1.25v f o = gnd t a = 25 c c in = 1 f
LTC2496 2 2496fa package/order information electrical characteristics absolute maximum ratings supply voltage (v cc ) ................................... ?0.3v to 6v analog input voltage (ch0 to ch15, com) ................................................. ?0.3v to (v cc + 0.3v) reference input voltage ................................................. ?0.3v to (v cc + 0.3v) adcinn, adcinp, muxoutp, muxoutn ................................................. ?0.3v to (v cc + 0.3v) digital input voltage ...................... ?0.3v to (v cc + 0.3v) digital output voltage ................... ?0.3v to (v cc + 0.3v) operating temperature range LTC2496c ................................................ 0c to 70c LTC2496i ............................................. ?40c to 85c storage temperature range ................... ?65c to 150c (notes 1, 2) 13 14 15 16 top view 39 uhf package 38-lead (5mm 7mm) plastic qfn 17 18 19 38 37 36 35 34 33 32 24 25 26 27 28 29 30 31 8 7 6 5 4 3 2 1gnd nc gnd gnd gnd gnd com ch0 ch1 ch2 ch3 ch4 gnd ref e ref + v cc muxoutn adcinn adcinp muxoutp ch15 ch14 ch13 ch12 sck sdo cs f o sdi gnd gnd ch5 ch6 ch7 ch8 ch9 ch10 ch11 23 22 21 20 9 10 11 12 t jmax = 125c, e ja = 34c/w exposed pad (pin 39) is gnd, must be soldered to pcb order part number qfn part marking* LTC2496cuhf LTC2496iuhf 2496 2496 order options tape and reel: add #tr lead free: add #pbf lead free tape and reel: add #trpbf lead free part marking: http://www.linear.com/leadfree/ consult ltc marketing for parts speci? ed with wider operating temperature ranges. *the temperature grade is identi? ed by a label on the shipping container. the o denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. (notes 3, 4) parameter conditions min typ max units resolution (no missing codes) 0.1v v ref v cc , ?fs v in +fs (note 5) 16 bits integral nonlinearity 5v v cc 5.5v, v ref = 5v, v in(cm) = 2.5v (note 6) 2.7v v cc 5.5v, v ref = 2.5v, v in(cm) = 1.25v (note 6) o 2 1 20 ppm of v ref ppm of v ref offset error 2.5v v ref v cc , gnd in + = in ? v cc (note 14) o 0.5 5 v offset error drift 2.5v v ref v cc , gnd in + = in ? v cc 10 nv/oc positive full-scale error 2.5v v ref v cc , in + = 0.75v ref , in ? = 0.25v ref o 32 ppm of v ref positive full-scale error drift 2.5v v ref v cc , in + = 0.75v ref , in ? = 0.25v ref 0.1 ppm of v ref /c negative full-scale error 2.5v v ref v cc , in + = 0.25v ref , in ? = 0.75v ref o 32 ppm of v ref negative full-scale error drift 2.5v v ref v cc , in + = 0.25v ref , in ? = 0.75v ref 0.1 ppm of v ref /c total unadjusted error 5v v cc 5.5v, v ref = 2.5v, v in(cm) = 1.25v 5v v cc 5.5v, v ref = 5v, v in(cm) = 2.5v 2.7v v cc 5.5v, v ref = 2.5v, v in(cm) = 1.25v 15 15 15 ppm of v ref ppm of v ref ppm of v ref output noise 5.5v v cc 2.7v, 2.5v v ref v cc , gnd in + = in ? v cc (note 13) 0.6 v rms LTC2496 3 2496fa the denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. (note 3) converter characteristics the denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. (note 3) parameter conditions min typ max units input common mode rejection dc 2.5v v ref v cc , gnd in + = in C v cc (note 5) 140 db input common mode rejection 60hz 2% 2.5v v ref v cc , gnd in + = in C v cc (note 5) 140 db input common mode rejection 50hz 2% 2.5v v ref v cc , gnd in + = in C v cc (note 5) 140 db input normal mode rejection 50hz 2% 2.5v v ref v cc , gnd in + = in C v cc (notes 5, 7) 110 120 db input normal mode rejection 60hz 2% 2.5v v ref v cc , gnd in + = in C v cc (notes 5, 8) 110 120 db input normal mode rejection 50hz/60hz 2% 2.5v v ref v cc , gnd in + = in C v cc (notes 5, 9) 87 db reference common mode rejection dc 2.5v v ref v cc , gnd in + = in C v cc (note 5) 120 140 db power supply rejection dc v ref = 2.5v, in + = in C = gnd 120 db power supply rejection, 50hz 2% v ref = 2.5v, in + = in C = gnd (notes 7, 9) 120 db power supply rejection, 60hz 2% v ref = 2.5v, in + = in C = gnd (notes 8, 9) 120 db analog input and reference symbol parameter conditions min typ max units in + absolute/common mode in + voltage (in + corresponds to the selected positive input channel) gnd C 0.3v v cc + 0.3v v in C absolute/common mode in C voltage (in C corresponds to the selected positive input channel) gnd C 0.3v v cc + 0.3v v v in input differential voltage range (in + C in C ) Cfs +fs v fs full scale of the differential input (in + C in C ) 0.5 v ref v lsb least signi? cant bit of the output code fs/2 16 ref + absolute/common mode ref + voltage 0.1 v cc v ref C absolute/common mode ref C voltage gnd ref + C 0.1v v v ref reference voltage range (ref + C ref C ) 0.1 v cc v cs(in + )in + sampling capacitance 11 pf cs(in C )in C sampling capacitance 11 pf cs(v ref )v ref sampling capacitance 11 pf i dc_leak (in + )in + dc leakage current sleep mode, in + = gnd C10 1 10 na i dc_leak (in C )in C dc leakage current sleep mode, in C = gnd C10 1 10 na i dc_leak (ref + )ref + dc leakage current sleep mode, ref + = v cc C100 1 100 na i dc_leak (ref C )ref C dc leakage current sleep mode, ref C = gnd C100 1 100 na t open mux break-before-make 50 ns qirr mux off isolation v in = 2v p-p dc to 1.8mhz 120 db LTC2496 4 2496fa the denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. (note 3) digital inputs and digital outputs symbol parameter conditions min typ max units v ih high level input voltage ( ? c ? s, f o , sdi) 2.7v v cc 5.5v v cc C 0.5 v v il low level input voltage ( ? c ? s, f o , sdi) 2.7v v cc 5.5v 0.5 v v ih high level input voltage (sck) 2.7v v cc 5.5v (notes 10, 15) v cc C 0.5 v v il low level input voltage (sck) 2.7v v cc 5.5v (notes 10, 15) 0.5 v i in digital input current ( ? c ? s, f o , sdi) 0v v in v cc C10 10 a i in digital input current (sck) 0v v in v cc (notes 10, 15) C10 10 a c in digital input capacitance ( ? c ? s, f o , sdi) 10 pf c in digital input capacitance (sck) (notes 10, 17) 10 pf v oh high level output voltage (sdo) i o = C800a v cc C 0.5 v v ol low level output voltage (sdo) i o = 1.6ma 0.4 v v oh high level output voltage (sck) i o = C800a (notes 10, 17) v cc C 0.5 v v ol low level output voltage (sck) i o = 1.6ma (notes 10, 17) 0.4 v i oz hi-z output leakage (sdo) C10 10 a symbol parameter conditions min typ max units v cc supply voltage 2.7 5.5 v i cc supply current conversion current (note 12) sleep mode (note 12) 160 1 275 2 a a the denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. (note 3) power requirements the denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. (note 3) digital inputs and digital outputs symbol parameter conditions min typ max units f eosc external oscillator frequency range (note 16) 10 4000 khz t heo external oscillator high period 0.125 100 s t leo external oscillator low period 0.125 100 s t conv conversion time simultaneous 50/60hz external oscillator 144.1 146.9 41036/f eosc (in khz) 149.9 ms ms f isck internal sck frequency internal oscillator (note 10) external oscillator (notes 10, 11) 38.4 f eosc /8 khz khz d isck internal sck duty cycle (note 10) 45 55 % f esck external sck frequency range (note 10) 4000 khz t lesck external sck low period (note 10) 125 ns t hesck external sck high period (note 10) 125 ns t dout_isck internal sck 24-bit data output time internal oscillator external oscillator 0.61 0.625 192/f eosc (in khz) 0.64 ms ms t dout_esck external sck 24-bit data output time (note 10) 24/f esck (in khz) ms LTC2496 5 2496fa note 1: stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. exposure to any absolute maximum rating condition for extended periods may affect device reliability and lifetime. note 2: all voltage values are with respect to gnd. note 3: v cc = 2.7v to 5.5v unless otherwise speci? ed. v refcm = v ref /2, fs = 0.5v ref v in = in + C in C , v in(cm) = (in + C in C )/2, where in + and in C are the selected input channels note 4: use internal conversion clock or external conversion clock source with f eosc = 307.2khz unless other wise speci? ed. note 5: guaranteed by design, not subject to test. note 6: integral nonlinearity is de? ned as the deviation of a code from a straight line passing through the actual endpoints of the transfer curve. the deviation is measured from the center of the quantization band. note 7: f eosc = 256khz 2% (external oscillator). note 8: f eosc = 307.2khz 2% (external oscillator). the denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. (note 3) digital inputs and digital outputs symbol parameter conditions min typ max units t 1 ? c ? s to sdo low 0 200 ns t 2 ? c ? s to sdo high z 0 200 ns t 3 ? c ? s to sck internal sck mode 0 200 ns t 4 ? c ? s to sck external sck mode 50 ns t kqmax sck to sdo valid 200 ns t kqmin sdo hold after sck (note 5) 15 ns t 5 sck set-up before ? c ? s 50 ns t 6 sck hold after ? c ? s 50 ns t 7 sdi setup before sck (note 5) 100 ns t 8 sdi hold after sck (note 5) 100 ns note 9: simultaneous 50hz/60hz (internal oscillator) or f eosc = 280khz 2% (external oscillator). note 10: the sck can be con? gured in external sck mode or internal sck mode. in external sck mode, the sck pin is used as a digital input and the driving clock is f esck . in the internal sck mode, the sck pin is used as a digital output and the output clock signal during the data output is f isck . note 11: the external oscillator is connected to the f o pin. the external oscillator frequency, f eosc , is expressed in khz. note 12: the converter uses its internal oscillator. note 13: the output noise includes the contribution of the internal calibration operations. note 14: guaranteed by design and test correlation. note 15: the converter is in external sck mode of operation such that the sck pin is used as a digital input. the frequency of the clock signal driving sck during the data output is f esck and is expressed in hz. note 16: refer to applications information section for performance vs data rate graphs. note 17: the converter is in internal sck mode of operation such that the sck pin is used as a digital output. LTC2496 6 2496fa input voltage (v) ?2 tue (ppm of v ref ) ? 4 12 ? 0 8 0.75 0.25 0.25 0.75 2496 g23 1.25 ?.25 v cc = 5v v ref = 5v v in(cm) = 1.25v f o = gnd 85 c 25 c ?5 c input voltage (v) ?2 tue (ppm of v ref ) ? 4 12 ? 0 8 ?.5 ?.5 0.5 1.5 2496 g22 2.5 ? ?.5 ? 0 1 2 v cc = 5v v ref = 5v v in(cm) = 2.5v f o = gnd 85 c 25 c ?5 c input voltage (v) ?2 tue (ppm of v ref ) ? 4 12 ? 0 8 0.75 0.25 0.25 0.75 2496 g24 1.25 ?.25 v cc = 2.7v v ref = 2.5v v in(cm) = 1.25v f o = gnd 85 c 25 c ?5 c input voltage (v) ? inl (ppm of v ref ) ? 1 3 ? 0 2 0.75 0.25 0.25 0.75 2496 g21 1.25 ?.25 v cc = 2.7v v ref = 2.5v v in(cm) = 1.25v f o = gnd ?5 c, 25 c, 90 c input voltage (v) ? inl (ppm of v ref ) ? 1 3 ? 0 2 0.75 0.25 0.25 0.75 2496 g20 1.25 ?.25 v cc = 5v v ref = 2.5v v in(cm) = 1.25v f o = gnd ?5 c, 25 c, 90 c input voltage (v) ? inl (ppm of v ref ) ? 1 3 ? 0 2 ?.5 ?.5 0.5 1.5 2496 g19 2.5 ? ?.5 ? 0 1 2 v cc = 5v v ref = 5v v in(cm) = 2.5v f o = gnd 85 c ?5 c 25 c typical performance characteristics integral nonlinearity (v cc = 5v, v ref = 5v) integral nonlinearity (v cc = 5v, v ref = 2.5v) integral nonlinearity (v cc = 2.7v, v ref = 2.5v) total unadjusted error (v cc = 5v, v ref = 5v) total unadjusted error (v cc = 5v, v ref = 2.5v) total unadjusted error (v cc = 2.7v, v ref = 2.5v) LTC2496 7 2496fa v in(cm) (v) ? offset error (ppm of v ref ) 0.1 0.2 0.3 24 2496 g25 0 0.1 01 356 0.2 0.3 v cc = 5v v ref = 5v v in = 0v t a = 25 c temperature ( c) ?5 0.3 offset error (ppm of v ref ) 0.2 0 0.1 0.2 ?5 15 30 90 2496 g26 0.1 ?0 0 45 60 75 0.3 v cc = 5v v ref = 5v v in = 0v v in(cm) = gnd f o = gnd v cc (v) 2.7 offset error (ppm of v ref ) 0.1 0.2 0.3 3.9 4.7 2496 g27 0 0.1 3.1 3.5 4.3 5.1 5.5 0.2 0.3 ref + = 2.5v ref = gnd v in = 0v v in(cm) = gnd t a = 25 c offset error vs v in(cm) offset error vs temperature offset error vs v cc v ref (v) 0 0.3 offset error (ppm of v ref ) 0.2 0.1 0 0.1 0.2 0.3 1234 2496 g28 5 v cc = 5v ref = gnd v in = 0v v in(cm) = gnd t a = 25 c typical performance characteristics offset error vs v ref on-chip oscillator frequency vs temperature on-chip oscillator frequency vs v cc temperature ( c) ?5 ?0 300 frequency (khz) 304 310 ?5 30 45 2496 g29 302 308 306 15 0 60 75 90 v cc = 4.1v v ref = 2.5v v in = 0v v in(cm) = gnd f o = gnd v cc (v) 2.5 300 frequency (khz) 302 304 306 308 310 3.0 3.5 4.0 4.5 2496 g30 5.0 5.5 v ref = 2.5v v in = 0v v in(cm) = gnd f o = gnd t a = 25 c LTC2496 8 2496fa typical performance characteristics temperature ( c) ?5 100 conversion current ( a) 120 160 180 200 ?5 15 30 90 2496 g34 140 ?0 0 45 60 75 v cc = 5v v cc = 2.7v f o = gnd cs = gnd sck = nc sdo = nc temperature ( c) ?5 0 sleep mode current ( a) 0.2 0.6 0.8 1.0 2.0 1.4 ?5 15 30 90 2496 g35 0.4 1.6 1.8 1.2 ?0 0 45 60 75 v cc = 5v v cc = 2.7v f o = gnd cs = v cc sck = nc sdo = nc output data rate (readings/sec) 0 supply current ( a) 500 450 400 350 300 250 200 150 100 80 2496 g36 20 40 60 100 70 10 30 50 90 v cc = 5v v cc = 3v v ref = v cc in + = gnd in = gnd sck = nc sdo = nc cs = gnd f o = ext osc t a = 25 c frequency at v cc (hz) 0 0 ?0 ?0 ?0 ?0 ?00 ?20 ?40 1k 100k 2496 g31 10 100 10k 1m rejection (db) v cc = 4.1v dc v ref = 2.5v in + = gnd in = gnd f o = gnd t a = 25 c frequency at v cc (hz) 0 ?40 rejection (db) ?20 ?0 ?0 ?0 0 20 100 140 2496 g32 ?00 ?0 80 180 220 200 40 60 120 160 v cc = 4.1v dc 1.4v v ref = 2.5v in + = gnd in = gnd f o = gnd t a = 25 c frequency at v cc (hz) 30600 ?0 ?0 0 30750 2496 g33 ?0 ?00 30650 30700 30800 ?20 ?40 ?0 rejection (db) v cc = 4.1v dc 0.7v v ref = 2.5v in + = gnd in = gnd f o = gnd t a = 25 c psrr vs frequency at v cc psrr vs frequency at v cc psrr vs frequency at v cc conversion current vs temperature sleep mode current vs temperature conversion current vs data output rate LTC2496 9 2496fa pin functions gnd (pins 1, 3, 4, 5, 6, 31, 32, 33): ground. multiple ground pins internally connected for optimum ground cur- rent ? ow and v cc decoupling. connect each one of these pins to a common ground plane through a low impedance connection. all 8 pins must be connected to ground for proper operation. nc (pin 2): no connection, this pin can be left ? oating or tied to gnd. com (pin 7): the common negative input (in C ) for all single-ended multiplexer con? gurations. the voltage on ch0 to ch15 and com pins can have any value between gnd C 0.3v to v cc + 0.3v. within these limits, the two selected inputs (in + and in C ) provide a bipolar input range (v in = in + C in C ) from C0.5 ? v ref to 0.5 ? v ref . outside this input range, the converter produces unique over-range and under-range output codes. ch0 to ch15 (pins 8 to 23): analog inputs. may be pro- grammed for single-ended or differential mode. muxoutp (pin 24): positive multiplexer output. used to drive an external buffer/ampli? er or can be shorted directly to adcinp. adcinp (pin 25): positive adc input. tie to the output of a buffer/ampli? er driven by muxoutp or short directly to muxoutp. adcinn (pin 26): negative adc input. tie to the output of a buffer/ampli? er driven by muxoutn or short directly to muxoutn. muxoutn (pin 27): negative multiplexer output. used to drive an external buffer/ampli? er or can be shorted directly to adcinn. v cc (pin 28): positive supply voltage. bypass to gnd with a 10f tantalum capacitor in parallel with a 0.1f ceramic capacitor as close to the part as possible. ref + (pin 29), ref C (pin 30): differential reference input. the voltage on these pins can have any value between gnd and v cc as long as the reference positive input, ref + , remains more positive than the negative reference input, ref C , by at least 0.1v. the differential voltage (ref = ref + C ref C ) sets the full-scale range for all input channels. sdi (pin 34): serial data input. this pin is used to select the input channel. the serial data input is applied under control of the serial clock (sck) during the data output operation. the ? rst conversion following a new input is valid. f o (pin 35): frequency control pin. digital input that controls the internal conversion clock rate. when f o is connected to v cc or gnd, the converter uses its internal oscillator running at 307.2khz. the conversion clock may also be overridden by driving the f o pin with an external clock in order to change the output rate and the digital ? lter rejection null. ? c ? s (pin 36): active low chip select. a low on this pin enables the digital input/output and wakes up the adc. following each conversion, the adc automatically enters the sleep mode and remains in this low power state as long as ? c ? s is high. a low-to-high transition on ? c ? s during the data output aborts the data transfer and starts a new conversion. sdo (pin 37): three-state digital output. during the data output period, this pin is used as the serial data output. when the chip select pin is high, the sdo pin is in a high impedance state. during the conversion and sleep periods, this pin is used as the conversion status output. when the conversion is in progress this pin is high; once the conversion is complete sdo goes low. the conversion status is monitored by pulling ? c ? s low. sck (pin 38): bidirectional, digital i/o, clock pin. in internal serial clock operation mode, sck is generated internally and is seen as an output on the sck pin. in external serial clock operation mode, the digital i/o clock is externally applied to the sck pin. the serial clock operation mode is determined by the logic level applied to the sck pin at power up and during the most recent falling edge of ? c ? s. exposed pad (pin 39): ground. this pin is ground and must be soldered to the pcb ground plane. for prototyping purposes, this pin may remain ? oating. LTC2496 10 2496fa test circuits functional block diagram autocalibration and control differential 3rd order ? modulator decimating fir address internal oscillator serial interface gnd v cc ch0 ch1 ch15 com mux sdo sck ref + ref cs sdi f o (int/ext) 2496 bd + muxoutp muxoutn adcinp adcinn figure 1. functional block diagram 1.69k sdo 2496 tc01 hi-z to v oh v ol to v oh v oh to hi-z c load = 20pf 1.69k sdo 2496 tc02 hi-z to v ol v oh to v ol v ol to hi-z c load = 20pf v cc LTC2496 11 2496fa timing diagram using internal sck (sck high with ? c ? s ) timing diagram using external sck (sck low with ? c ? s ) timing diagrams cs sdo sck sdi t 1 t 3 t 7 t 8 sleep t kqmax conversion data in/out t kqmin t 2 2496 td01 cs sdo sck sdi t 1 t 5 t 6 t 4 t 7 t 8 sleep t kqmax conversion data in/out t kqmin t 2 2496 td02 LTC2496 12 2496fa applications information converter operation converter operation cycle the LTC2496 is a multi-channel, low power, delta-sigma analog-to-digital converter with an easy to use 4-wire interface and automatic differential input current can- cellation. its operation is made up of three states (see figure 2). the converter operating cycle begins with the conversion, followed by the sleep state and ends with the data input/output cycle. the 4-wire interface consists of serial data output (sdo), serial clock (sck), chip select ( ? c ? s) and serial data input (sdi).the interface, timing, operation cycle, and data output format is compatible with linears entire family of ? converters. initially, at power up, the LTC2496 performs a conversion. once the conversion is complete, the device enters the sleep state. while in this sleep state, if ? c ? s is high, power consumption is reduced by two orders of magnitude. the part remains in the sleep state as long as ? c ? s is high. the conversion result is held inde? nitely in a static shift register while the part is in the sleep state. once ? c ? s is pulled low, the device powers up, exits the sleep mode, and enters the data input/output state. if ? c ? s is brought high before the ? rst rising edge of sck, the device returns to the sleep state and the power is reduced. if ? c ? s is brought high after the ? rst rising edge of sck, the data output cycle is aborted and a new conversion cycle begins. the data output corresponds to the conversion just completed. this result is shifted out on the serial data output pin (sdo) under the control of the serial clock pin (sck). data is updated on the falling edge of sck allowing the user to reliably latch data on the rising edge of sck (see figure 3). the channel selection data for the next conversion is also loaded into the device at this time. data is loaded from the serial data input pin (sdi) on each rising edge of sck. the data input/output cycle is concluded once 24 bits are read out of the adc or when ? c ? s is brought high. the device automatically initiates a new conversion and the cycle repeats. through timing control of the ? c ? s and sck pins, the LTC2496 offers several ? exible modes of operation (internal or external sck and free-running conversion modes). these various modes do not require programming and do not disturb the cyclic operation described above. these modes of operation are described in detail in the serial interface timing modes section. ease of use the LTC2496 data output has no latency, ? lter settling delay or redundant data associated with the conversion cycle. there is a one-to-one correspondence between the conversion and the output data. therefore, multiplexing multiple analog inputs is straightforward. each conversion, immediately following a newly selected input, is valid and accurate to the full speci? cations of the device. the LTC2496 automatically performs offset and full scale calibration every conversion cycle independent of the input channel selected. this calibration is transparent to the user and has no effect with the operation cycle de- scribed above. the advantage of continuous calibration is extreme stability of offset and full-scale readings with respect to time, supply voltage variation, input channel, and temperature drift. easy drive input current cancellation the LTC2496 combines a high precision delta-sigma adc with an automatic, differential, input current cancellation figure 2. LTC2496 state transition diagram convert sleep channel select data output power up in + = ch0, in = ch1 2496 f02 cs = low and sck LTC2496 13 2496fa applications information front end. a proprietary front end passive sampling network transparently removes the differential input current. this enables external rc networks and high impedance sen- sors to directly interface to the LTC2496 without external ampli? ers. the remaining common mode input current is eliminated by either balancing the differential input impedances or setting the common mode input equal to the common mode reference (see automatic differential input current cancellation section). this unique archi- tecture does not require on-chip buffers thereby enabling signals to swing beyond ground or up to v cc . moreover, the cancellation does not interfere with the transparent offset and full-scale auto-calibration and the absolute accuracy (full-scale + offset + linearity + drift) is maintained even with external rc networks. power-up sequence the LTC2496 automatically enters an internal reset state when the power supply voltage v cc drops below ap- proximately 2v. this feature guarantees the integrity of the conversion result, input channel selection, and serial clock mode. when v cc rises above this threshold, the converter creates an internal power-on-reset (por) signal with a duration of approximately 4ms. the por signal clears all internal registers. the conversion immediately following a por cycle is performed on the input channel in + = ch0, in C = ch1. the ? rst conversion following a por cycle is accurate within the speci? cation of the device if the power supply voltage is restored to (2.7v to 5.5v) before the end of the por interval. a new input channel, can be programmed into the device during this ? rst data input/output cycle. reference voltage range this converter accepts a truly differential external reference voltage. the absolute/common mode voltage range for ref + and ref C pins covers the entire operating range of the device (gnd to v cc ). for correct converter operation, v ref must be positive (ref + > ref C ) the LTC2496 differential reference input range is 0.1v to v cc . for the simplest operation, ref + can be shorted to v cc and ref C can be shorted to gnd. the converter output noise is determined by the thermal noise of the front end circuits. since the transition noise is well below 1lsb (0.02lsb), a decrease in reference voltage will proportionally improve the converters effective resolution and improve the inl. input voltage range the analog input is truly differential with an absolute, com- mon mode range for ch0 to ch15 and com input pins extending from gnd C 0.3v to v cc + 0.3v. outside these limits, the esd projection devices begin to turn on and the errors due to input leakage current increase rapidly. within these limits, the LTC2496 converts the bipolar differential input signal v in = in + + in C (where in + and in C are the selected input channels), from Cfs = C0.5 ? v ref to +fs = 0.5 ? v ref where v ref = ref + C ref C . outside this range, the converter indicates the over range or the under range condition using distinct output codes. signals applied to the input (ch0 to ch15, com) may extend 300mv below ground and above v cc . in order to limit any fault current, resistors of up to 5k may be added in series with the input. the effect of series resistance on the converter accuracy can be evaluated from the curves presented in the input current/reference current sections. in addition, series resistors will introduce a temperature dependent error due to input leakage current. a 1na input leakage current will develop a 1ppm offset error on a 5k resistor if v ref = 5v. this error has a very strong tem- perature dependency. muxout/adcin the output of the multiplexer (muxout) and the input to the adc (adcin) can be used to perform input signal conditioning on any of the selected input channels or sim- ply shorted together for direct digitization. if an external ampli? er is used, the LTC2496 automatically calibrates both the offset and drift of this circuit and the easy drive sampling scheme enables a wide variety of ampli? ers to be used. LTC2496 14 2496fa applications information in order to achieve optimum performance, if an external ampli? er is not used, short these pins directly together (adcinp to muxoutp and adcinn to muxoutn) and minimize their capacitance to ground. serial interface pins the LTC2496 transmits the conversion result, reads the input channel selection, and receives a start of conversion command through a synchronous 3- or 4-wire interface. during the conversion and sleep states, this interface can be used to access the converter status. during the data output state, it is used to read the conversion result and program the input channel for the next conversion cycle. serial clock input/output (sck) the serial clock pin (sck) is used to synchronize the data input/output transfer. each bit is shifted out of the sdo pin on the falling edge of sck and data is shifted into the sdi pin on the rising edge of sck. the serial clock pin (sck) can be con? gured as either a master (sck is an output generated internally) or a slave (sck is an input and applied externally). master mode (internal sck) is selected by simply ? oating the sck pin. slave mode (external sck) is selected by driving sck low during power up and each falling edge of ? c ? s. speci? c details of these sck modes are described in the serial interface timing modes section. serial data output (sdo) the serial data output pin (sdo) provides the result of the last conversion as a serial bit stream (msb ? rst) during the data output state. in addition, the sdo pin is used as an end of conversion indicator during the conversion and sleep states. when ? c ? s is high, the sdo driver is switched to a high impedance state in order to share the data output line with other devices. if ? c ? s is brought low during the conversion phase, the ? e ? o ? c bit (sdo pin) will be driven high. once the conversion is complete, if ? c ? s is brought low ? e ? o ? c will be driven low indicating the conversion is complete and the result is ready to be shifted out of the device. chip select ( ? c ? s) the active low ? c ? s pin is used to test the conversion status, enable i/o data transfer, initiate a new conversion, control the duration of the sleep state, and set the sck mode. at the conclusion of a conversion cycle, while ? c ? s is high, the device remains in a low power sleep state where the supply current is reduced several orders of magnitude. in order to exit the sleep state and enter the data output state, ? c ? s must be pulled low. data is now shifted out the sdo pin under control of the sck pin as described previously. a new conversion cycle is initiated either at the conclusion of the data output cycle (all 24 data bits read) or by pulling ? c ? s high any time between the ? rst and 24th rising edges of the serial clock (sck). in this case, the data output is aborted and a new conversion begins. serial data input (sdi) the serial data input (sdi) is used to select the input channel. data is shifted into the device during the data output/input state on the rising edge of sck while ? c ? s is low. output data format the LTC2496 serial output stream is 24 bits long. the ? rst bit indicates the conversion status, the second bit is always zero, and the third bit conveys sign information. the next 17 bits are the conversion result, msb ? rst. the remaining 4 bits are always low. bit 23 (? rst output bit) is the end of conversion ( ? e ? o ? c) indicator. this bit is available on the sdo pin during the conversion and sleep states whenever ? c ? s is low. this bit is high during the conversion cycle, goes low once the conversion is complete, and is high-z when ? c ? s is high. bit 22 (second output bit) is a dummy bit (dmy) and is always low. bit 21 (third output bit) is the conversion result sign indica- tor (sig). if the selected input (v in = in + C in C ) is greater than 0v, this bit is high. if v in < 0, this bit is low. LTC2496 15 2496fa applications information bit 20 (fourth output bit) is the most signi? cant bit (msb) of the result. this bit in conjunction with bit 21 also provides under range and over range indication. if both bit 21 and bit 20 are high, the differential input voltage is above +fs. if both bit 21 and bit 20 are low, the differential input voltage is below Cfs. the function of these bits is summarized in table 1. table 1. LTC2496 status bits input range bit 23 ? e ? o ? c bit 22 dmy bit 21 sig bit 20 msb v in 0.5 ? v ref 0011 0v v in < 0.5 ? v ref 0010 C0.5 ? v ref v in < 0v 0001 v in < C0.5 ? v ref 0000 bits 20 to 4 are the 16-bit plus sign conversion result msb ? rst. bit 4 is the least signi? cant bit (lsb 16 ). bits 3 to 0 are always low. data is shifted out of the sdo pin under control of the serial clock (sck), see figure 3. whenever ? c ? s is high, sdo remains high impedance and sck is ignored. in order to shift the conversion result out of the device, ? c ? s must ? rst be driven low. ? e ? o ? c is seen at the sdo pin of the device once ? c ? s is pulled low. ? e ? o ? c changes in real time from high to low at the completion of a conversion. this signal may be used as an interrupt for an external microcontroller. bit 23 ( ? e ? o ? c) can be captured on the ? rst rising edge of sck. bit 22 is shifted out of the device on the ? rst falling edge of sck. the ? nal data bit (bit 0) is shifted out on the on the falling edge of the 23rd sck and may be latched on the rising edge of the 24th sck pulse. on the falling edge of the 24th sck pulse, sdo goes high indicating the initiation of a new conversion cycle. this bit serves as ? e ? o ? c (bit 23) for the next conversion cycle. table 2 summarizes the output data format. as long as the voltage on the in + and in C pins remains between C0.3v and v cc + 0.3v (absolute maximum op- erating range) a conversion result is generated for any differential input voltage v in from Cfs = C0.5 ? v ref to +fs = 0.5 ? v ref . for differential input voltages greater than +fs, the conversion result is clamped to the value corresponding to +fs + 1lsb. for differential input volt- ages below Cfs, the conversion result is clamped to the value Cfs C 1lsb. eoc cs sck (external) sdi sdo 2496 f03 conversion sleep data input/output conversion hi-z hi-z msb lsb bit 23 bit 22 bit 21 bit 20 bit 19 sig ? 1 0 en sgl a2 a1 a0 odd bit 18 bit 17 bit 16 bit 15 bit 4 bit 3 bit 2 bit 1 bit 0 123456789 192021222324 don't care don't care figure 3. channel selection and data output timing LTC2496 16 2496fa applications information input data format the LTC2496 serial input word is 8 bits long. the input data (sgl, odd, a2, a1, a0) is used to select the input channel. after power up, the device initiates an internal reset cycle which sets the input channel to ch0 C ch1 (in + = ch0, in C = ch1), the ? rst conversion automatically begins at power up using the default input channel. once the conversion is complete a new word can be written into the device in order to select the input channel for the next conversion cycle. the ? rst 3 bits shifted into the device consist of two preen- able bits and one enable bit. as demonstrated in figure 3, the ? rst three bits shifted into the device enable the device input channel selection. valid settings for these three bits are 000, 100, and 101. other combinations should be avoided. if the ? rst three bits are 000 or 100, the following data is ignored (dont care) and the previously selected input channel remains valid for the next conversion if the ? rst 3 bits shifted into the device are 101, then the next 5 bits select the input channel for the next conversion cycle, see table 3. the ? rst input bit following the 101 sequence (sgl) de- termines if the input selection is differential (sgl = 0) or single-ended (sgl = 1). for sgl = 0, two adjacent chan- nels can be selected to form a differential input. for sgl = 1, one of 16-channels is selected as the positive input. the negative input is com for all single ended operations. the remaining 4 bits (odd, a2, a1, a0) determine which channel(s) is/are selected and the polarity (for a differential input). this data sequence is backward compatible with the ltc2448 and ltc2418 families of delta sigma adcs. serial interface timing modes the LTC2496s 4-wire interface is spi and microwire compatible. this interface offers several ? exible modes of operation. these include internal/external serial clock, 3- or 4-wire i/o, single cycle or continuous conversion. the following sections describe each of these timing modes in detail. in all cases, the converter can use the internal oscillator (f o = low or f o = high) or an external oscillator connected to the f o pin. for each mode, the operating cycle, data input format, data output format, and performance remain the same. refer to table 4 for a summary. table 2. LTC2496 output data format differential input voltage v in * bit 23 eoc bit 22 dmy bit 21 sig bit 20 msb bit 19 bit 18 bit 17 bit 4 bits 3 to 0 v in * fs** 0 0 1 1 0 0 0 0 0000 fs** C 1lsb 0 0 1 0 1 1 1 1 0000 0.5 ? fs** 0 0 1 0 1 0 0 0 0000 0.5 ? fs** C 1lsb 0 0 1 0 0 1 1 1 0000 0 0 0 1 0 0 0 0 0 0000 C1lsb 0 0 0 1 1 1 1 1 0000 C0.5 ? fs** 0 0 0 1 1 0 0 0 0000 C0.5 ? fs** C 1lsb 0 0 0 1 0 1 1 1 0000 Cfs** 0 0 0 1 0 0 0 0 0000 v in * < Cfs** 0 0 0 0 1 1 1 1 0000 *the differential input voltage v in = in + C in C . **the full-scale voltage fs = 0.5 ? v ref . LTC2496 17 2496fa table 3 channel selection mux address channel selection sgl odd/ signa2a1a00123456789101112131415com *00000in + in C 00001 in + in C 00010 in + in C 00011 in + in C 00100 in + in C 00101 in + in C 00110 in + in C 00111 in + in C 01000in C in + 01001 in C in + 01010 in C in + 01011 in C in + 01100 in C in + 01101 in C in + 01110 in C in + 01111 in C in + 10000in + in C 10001 in + in C 10010 in + in C 10011 in + in C 10100 in + in C 10101 in + in C 10110 in + in C 10111 in + in C 11000 in + in C 11001 in + in C 11010 in + in C 11011 in + in C 11100 in + in C 11101 in + in C 11110 in + in C 11111 in + in C *default at power up applications information LTC2496 18 2496fa table 4. serial interface timing modes configuration sck source conversion cycle control data output control connection and waveforms external sck, single cycle conversion external ? c ? s and sck ? c ? s and sck figures 4, 5 external sck, 3-wire i/o external sck sck figure 6 internal sck, single cycle conversion internal ? c ? s ? c ? s figures 7, 8 internal sck, 3-wire i/o, continuous conversion internal continuous internal figure 9 applications information external serial clock, single cycle operation this timing mode uses an external serial clock to shift out the conversion result and ? c ? s to monitor and control the state of the conversion cycle, see figure 4. the external serial clock mode is selected during the power- up sequence and on each falling edge of ? c ? s. in order to enter and remain in the external sck mode of operation, sck must be driven low both at power up and on each ? c ? s falling edge. if sck is high on the falling edge of ? c ? s, the device will switch to the internal sck mode. the serial data output pin (sdo) is hi-z as long as ? c ? s is high. at any time during the conversion cycle, ? c ? s may be pulled low in order to monitor the state of the converter. while ? c ? s is low, ? e ? o ? c is output to the sdo pin. ? e ? o ? c = 1 while a conversion is in progress and ? e ? o ? c = 0 if the conversion is complete and the device is in the sleep state. independent of ? c ? s, the device automatically enters the sleep state once the conversion is complete; however, in order to reduce the power, ? c ? s must be high. 10 f 0.1 f 2.7v to 5.5v hi-z 2496 f04 cs sck (external) sdi sdo conversion sleep data input/output conversion v cc f o ref + ref ch0 ch7 ch8 ch15 com sck sdi sdo cs gnd 28 35 29 30 8 15 16 23 7 38 37 1,3,4,5,6,31,32,33,39 36 34 reference voltage 0.1v to v cc analog inputs = external oscillator = internal oscillator LTC2496 4-wire spi interface eoc 123456789 192021222324 1 0 en sgl a2 a1 a0 odd don't care don't care msb sig ? lsb bit 23 bit 22 bit 21 bit 20 bit 19 bit 18 bit 17 bit 16 bit 15 bit 4 bit 3 bit 2 bit 1 bit 0 figure 4. external serial clock, single cycle operation LTC2496 19 2496fa applications information when the device is in the sleep state, its conversion re- sult is held in an internal static shift register. the device remains in the sleep state until the ? rst rising edge of sck is seen while ? c ? s is low. the input data is then shifted in via the sdi pin on each rising edge of sck (including the ? rst rising edge). the channel selection will be used for the following conversion cycle. if the input channel is changed during this i/o cycle, the new settings take effect on the conversion cycle following the data input/output cycle. the output data is shifted out the sdo pin on each falling edge of sck. this enables external circuitry to latch the output on the rising edge of sck. ? e ? o ? c can be latched on the ? rst rising edge of sck and the last bit of the conversion result can be latched on the 24th rising edge of sck. on the 24th falling edge of sck, the device begins a new conversion and sdo goes high ( ? e ? o ? c = 1) indicating a conversion is in progress. at the conclusion of the data cycle, ? c ? s may remain low and ? e ? o ? c monitored as an end-of-conversion interrupt. typically, ? c ? s remains low during the data output/input state. however, the data output state may be aborted by pulling ? c ? s high any time between the 1st falling edge and the 24th falling edge of sck, see figure 5. on the rising edge of ? c ? s, the device aborts the data output state and immediately initiates a new conversion. in order to program a new input channel, 8 sck clock pulses are required. if the data output sequence is aborted prior to the 8th falling edge of sck, the new input data is ignored and the previously selected input channel remains valid. if the rising edge of ? c ? s occurs after the 8th falling edge of sck, the new input channel is loaded and valid for the next conversion cycle. figure 5. external serial clock, reduced output data length and valid channel selection 10 f 0.1 f 2.7v to 5.5v hi-z 2496 f05 cs sck (external) sdi sdo conversion sleep data input/output sleep conversion v cc f o ref + ref ch0 ch7 ch8 ch15 com sck sdi sdo cs gnd 28 35 29 30 8 15 16 23 7 38 37 1,3,4,5,6,31,32,33,39 36 34 reference voltage 0.1v to v cc analog inputs = external oscillator = internal oscillator LTC2496 4-wire spi interface eoc bit 20 bit 19 bit 18 bit 17 bit 16 bit 15 bit 21 bit 22 bit 23 12345678 1 0 en sgl a2 a1 a0 odd don't care don't care msb sig ? LTC2496 20 2496fa applications information 10 f 0.1 f 2.7v to 5.5v v cc f o ref + ref ch0 ch7 ch8 ch15 com sck sdi sdo cs gnd 28 35 29 30 8 15 16 23 7 38 37 1,3,4,5,6,31,32,33,39 36 34 reference voltage 0.1v to v cc analog inputs = external oscillator = internal oscillator LTC2496 3-wire spi interface eoc cs sck (external) sdi sdo 2496 f06 conversion sleep data input/output conversion 1 0 en sgl a2 a1 a0 odd bit 0 don't care don't care msb sig ? 123456789 192021222324 lsb bit 23 bit 22 bit 21 bit 20 bit 19 bit 18 bit 17 bit 16 bit 15 bit 4 bit 3 bit 2 bit 1 bit 0 figure 6. external serial clock, 3-wire operation ( ? c ? s = 0) external serial clock, 3-wire i/o this timing mode uses a 3-wire serial i/o interface. the conversion result is shifted out of the device by an externally generated serial clock (sck) signal, see figure 6. ? c ? s is permanently tied to ground, simplifying the user interface or isolation barrier. the external serial clock mode is selected at the end of the power-on reset (por) cycle. the por cycle is typically concluded 4ms after v cc exceeds 2v. the level applied to sck at this time determines if sck is internally generated or externally applied. in order to enter the external sck mode, sck must be driven low prior to the end of the por cycle. since ? c ? s is tied low, the end-of-conversion ( ? e ? o ? c) can be continuously monitored at the sdo pin during the convert and sleep states. ? e ? o ? c may be used as an interrupt to an external controller. ? e ? o ? c = 1 while the conversion is in progress and ? e ? o ? c = 0 once the conversion is complete. on the falling edge of ? e ? o ? c, the conversion result is load- ing into an internal static shift register. the output data can now be shifted out the sdo pin under control of the externally applied sck signal. data is updated on the fall- ing edge of sck. the input data is shifted into the device through the sdi pin on the rising edge of sck. on the 24th falling edge of sck, sdo goes high, indicating a new conversion has begun. this data now serves as ? e ? o ? c for the next conversion. LTC2496 21 2496fa applications information figure 7. internal serial clock, single cycle operation internal serial clock, single cycle operation this timing mode uses the internal serial clock to shift out the conversion result and ? c ? s to monitor and control the state of the conversion cycle, see figure 7. in order to select the internal serial clock timing mode, the serial clock pin (sck) must be ? oating or pulled high before the conclusion of the por cycle and prior to each falling edge of ? c ? s. an internal weak pull-up resistor is active on the sck pin during the falling edge of ? c ? s; therefore, the internal sck mode is automatically selected if sck is not externally driven. the serial data output pin (sdo) is hi-z as long as ? c ? s is high. at any time during the conversion cycle, ? c ? s may be pulled low in order to monitor the state of the converter. once ? c ? s is pulled low, sck goes low and ? e ? o ? c is output to the sdo pin. ? e ? o ? c =1 while the conversion is in progress and ? e ? o ? c = 0 if the device is in the sleep state when testing ? e ? o ? c, if the conversion is complete ( ? e ? o ? c = 0), the device will exit sleep state. in order to return to the sleep state and reduce the power consumption, ? c ? s must be pulled high before the device pulls sck high. when the device is using its own internal oscillator (f o is tied low), the ? rst rising edge of sck occurs 12s (t eoctest = 12s) after the falling edge of ? c ? s. if f o is driven by an external oscillator of frequency f eosc , then t eoctest = 3.6/f eosc . if ? c ? s remains low longer than t eoctest , the ? rst rising edge of sck will occur and the conversion result is shifted out the sdo pin on the falling edge of sck. the serial input word (sdi) is shifted into the device on the rising edge of sck. after the 24th rising edge of sck a new conversion au- tomatically begins. sdo goes high ( ? e ? o ? c = 1) and sck remains high for the duration of the conversion cycle. once the conversion is complete, the cycle repeats. 10 f 0.1 f 2.7v to 5.5v hi-z 2496 f07 cs sck (internal) sdi sdo conversion sleep data input/output conversion v cc f o ref + ref ch0 ch7 ch8 ch15 com sck sdi sdo cs gnd 28 35 29 30 8 15 16 23 7 38 37 1,3,4,5,6,31,32,33,39 36 34 reference voltage 0.1v to v cc analog inputs = external oscillator = internal oscillator LTC2496 4-wire spi interface eoc 1 0 en sgl a2 a1 a0 odd don't care don't care msb sig ? optional 10k v cc LTC2496 24 2496fa applications information the digital input signal range is 0.5v to v cc C 0.5v. during transitions, the cmos input circuits draw dynamic cur- rent. for optimal performance, application of signals to the serial data interface should be reserved for the sleep and data output periods. during the conversion period, overshoot and undershoot of fast digital signals applied to both the serial digital in- terface and the external oscillator pin (f o ) may degrade the converter performance. undershoot and overshoot occur due to impedance mismatch of the circuit board trace at the converter pin when the transition time of an external control signal is less than twice the propagation delay from the driver to the input pin. for reference, on a regular fr-4 board, the propagation delay is approximately 183ps/inch. in order to prevent overshoot, a driver with a 1ns transition time must be connected to the converter through a trace shorter than 2.5 inches. this becomes dif? cult when shared control lines are used and multiple re? ections occur. parallel termination near the input pin of the LTC2496 will eliminate this problem, but will increase the driver power dissipation. a series resistor from 27 to 54 (depend- ing on the trace impedance and connection) placed near the driver will also eliminate over/undershoot without additional driver power dissipation. for many applications, the serial interface pins (sck, sdi, ? c ? s, f o ) remain static during the conversion cycle and no degradation occurs. on the other hand, if an external oscillator is used (f o driven externally) it is active during the conversion cycle. moreover, the digital ? lter rejection is minimal at the clock rate applied to f o . care must be taken to ensure external inputs and reference lines do not cross this signal or run near it. these issues are avoided when using the internal oscillator. driving the input and reference the input and reference pins of the LTC2496 are connected directly to a switched capacitor network. depending on the relationship between the differential input voltage and the differential reference voltage, these capacitors are switched between these four pins. each time a capacitor is switched between two of these pins, a small amount of charge is transferred. a simpli? ed equivalent circuit is shown in figure 10. figure 10. LTC2496 equivalent analog input circuit iin iin vv r avg avg in cm ref cm eq + () = () = ? ? () () . 05 i iref vv v r avg ref ref cm in cm + () + () 15 05 . . () () e eq in ref eq ref ref cm v vr where v ref ref v : ( 2 =? +? ) ) , = ? ? ? ? ? ? ? ? =? +? +? + ref ref v in in where in an in 2 d d in are the selected input channels v in in cm ? + = () ?n ? ? ? ? ? ? ? ? ? = 2 r 2.98m internal oscillator r eq e eq 12 eosc 0.833 10 /f external oscillator =? () in + in 10k internal switch network 10k c eq 12pf 10k i in ref + i ref + i in + i ref 2496 f10 switching frequency f sw = 123khz internal oscillator f sw = 0.4 ?f eosc external oscillator ref 10k 100 input multiplexer external connection 100 muxoutp adcinp external connection muxoutn adcinn LTC2496 25 2496fa applications information when using the LTC2496s internal oscillator, the input capacitor array is switched at 123khz. the effect of the charge transfer depends on the circuitry driving the in- put/reference pins. if the total external rc time constant is less then 580ns the errors introduced by the sampling process are negligible since complete settling occurs. typically, the reference inputs are driven from a low imped- ance source. in this case complete settling occurs even with large external bypass capacitors. the inputs (ch0 to ch15, com), on the other hand, are typically driven from larger source resistances. source resistances up to 10k may interface directly to the LTC2496 and settle completely; however, the addition of external capacitors at the input terminals in order to ? lter unwanted noise (anti-aliasing) results in incomplete settling. the LTC2496 offers two methods of removing these errors. the ? rst is an automatic differential input current cancella- tion (easy drive) and the second is the insertion of buffer between the muxout and adcin pins, thus isolating the input switching from the source resistance. automatic differential input current cancellation in applications where the sensor output impedance is low (up to 10k with no external bypass capacitor or up to 500 with 0.001f bypass), complete settling of the input occurs. in this case, no errors are introduced and direct digitization is possible. for many applications, the sensor output impedance combined with external input bypass capacitors produces rc time constants much greater than the 580ns required for 1ppm accuracy. for example, a 10k bridge driving a 0.1f capacitor has a time constant an order of magnitude greater than the required maximum. the LTC2496 uses a proprietary switching algorithm that forces the average differential input current to zero independent of external settling errors. this allows direct digitization of high impedance sensors without the need of buffers. the switching algorithm forces the average input current on the positive input (i in + ) to be equal to the average input current in the negative input (i in C ). over the complete conversion cycle, the average differential input current (i in + C i in C ) is zero. while the differential input current is zero, the common mode input current (i in + + i in C )/2 is proportional to the difference between the common mode input voltage (v in(cm) ) and the common mode reference voltage (v ref(cm) ). in applications where the input common mode voltage is equal to the reference common mode voltage, as in the case of a balanced bridge, both the differential and com- mon mode input current are zero. the accuracy of the converter is not compromised by settling errors. in applications where the input common mode voltage is constant but different from the reference common mode voltage, the differential input current remains zero while the common mode input current is proportional to the difference between v in(cm) and v ref(cm) . for a reference common mode voltage of 2.5v and an input common mode of 1.5v, the common mode input current is approximately 0.74a. this common mode input current does not degrade the accuracy if the source impedances tied to in + and in C are matched. mismatches in source impedance lead to a ? xed offset error but do not effect the linearity or full scale reading. a 1% mismatch in a 1k source resistance leads to a 74v shift in offset voltage. in applications where the common mode input voltage varies as a function of the input signal level (single ended type sensors), the common mode input current varies proportionally with input voltage. for the case of balanced input impedances, the common mode input current effects are rejected by the large cmrr of the LTC2496, leading to little degradation in accuracy. mismatches in source impedances lead to gain errors proportional to the dif- ference between the common mode input and common mode reference. 1% mismatches in 1k source resistances lead to gain errors on the order of 15ppm. based on the stability of the internal sampling capacitors and the ac- curacy of the internal oscillator, a one-time calibration will remove this error. in addition to the input sampling current, the input esd protection diodes have a temperature dependent leakage current. this current, nominally 1na (10na max) results in a small offset shift. a 1k source resistance will create a 1v typical and a 10v maximum offset voltage. LTC2496 26 2496fa applications information automatic offset calibration of external buffers/ ampli? ers in addition to the easy drive input current cancellation, the LTC2496 enables an external ampli? er to be inserted between the multiplexer output and the adc input, see figure 11. this is useful in applications where balanced source impedances are not possible. one pair of external buffers/ampli? ers can be shared between all 17 analog inputs. the LTC2496 preforms an internal offset calibration every conversion cycle in order to remove the offset and drift of the adc. this calibration is performed through a combination of front end switching and digital process- ing. since the external ampli? er is placed between the multiplexer and the adc, it is inside this correction loop. this results in automatic offset correction and offset drift removal of the external ampli? er. the ltc6078 is an excellent ampli? er for this function. it operates with supply voltages as low as 2.7v and its noise level is 18nv/hz. the easy drive input technology of the LTC2496 enables an rc network to be added directly to the output of the ltc6078. the capacitor reduces the magnitude of the current spikes seen at the input to the adc and the resistor isolates the capacitor load from the op-amp output enabling stable operation. reference current similar to the analog inputs, the LTC2496 samples the differential reference pins (ref + and ref C ) transferring small amounts of charge to and from these pins, thus producing a dynamic reference current. if incomplete set- tling occurs (as a function the reference source resistance and reference bypass capacitance) linearity and gain errors are introduced. for relatively small values of external reference capacitance (c ref < 1nf), the voltage on the sampling capacitor settles for reference impedances of many k (if c ref = 100pf up to 10k will not degrade the performance), see figures 12 and 13. figure 11. external buffers provide high impedance inputs and ampli? er offsets are automatically cancelled. 0.1 f 0.1 f + + 1/2 ltc6078 1/2 ltc6078 1 2 3 5 6 7 ? adc with easy drive inputs input mux muxoutp muxoutn 17 2496 f11 LTC2496 analog inputs sdi sck sdo cs 1k 1k figure 12. +fs error vs r source at v ref (small c ref ) figure 13. Cfs error vs r source at v ref (small c ref ) r source ( ) 0 +fs error (ppm) 50 70 90 10k 2496 f12 30 10 40 60 80 20 0 ?0 10 100 1k 100k v cc = 5v v ref = 5v v in + = 3.75v v in = 1.25v f o = gnd t a = 25 c c ref = 0.01 f c ref = 0.001 f c ref = 100pf c ref = 0pf r source ( ) 0 ?s error (ppm) ?0 ?0 10 10k 2496 f13 ?0 ?0 ?0 ?0 0 ?0 ?0 ?0 10 100 1k 100k v cc = 5v v ref = 5v v in + = 1.25v v in = 3.75v f o = gnd t a = 25 c c ref = 0.01 f c ref = 0.001 f c ref = 100pf c ref = 0pf LTC2496 27 2496fa applications information in cases where large bypass capacitors are required on the reference inputs (c ref > 0.01f), full-scale and linear- ity errors are proportional to the value of the reference resistance. every ohm of reference resistance produces a full-scale error of approximately 0.5ppm, see figures 14 and 15. if the input common mode voltage is equal to the reference common mode voltage, a linearity error of approximately 0.67ppm per 100 of reference resistance results, see figure 16. in applications where the input and reference common mode voltages are different, the errors increase. a 1v difference in between common mode input and common mode reference results in a 6.7ppm inl error for every 100 of reference resistance. in addition to the reference sampling charge, the reference r source ( ) 0 +fs error (ppm) 300 400 500 800 2496 f14 200 100 0 200 400 600 1000 v cc = 5v v ref = 5v v in + = 3.75v v in = 1.25v f o = gnd t a = 25 c c ref = 1 f, 10 f c ref = 0.1 f c ref = 0.01 f r source ( ) 0 ?s error (ppm) ?00 ?00 0 800 2496 f15 ?00 ?00 ?00 200 400 600 1000 v cc = 5v v ref = 5v v in + = 1.25v v in = 3.75v f o = gnd t a = 25 c c ref = 1 f, 10 f c ref = 0.1 f c ref = 0.01 f figure 14. +fs error vs r source at v ref (large c ref ) figure 15. Cfs error vs r source at v ref (large c ref ) v in /v ref (v) 0.5 inl (ppm of v ref ) 2 6 10 0.3 2496 f16 ? ? 0 4 8 ? ? ?0 0.3 0.1 0.1 0.5 v cc = 5v v ref = 5v v in(cm) = 2.5v t a = 25 c c ref = 10 f r = 1k r = 100 r = 500 figure 16. inl vs differential input voltage and reference source resistance for c ref > 1f esd projection diodes have a temperature dependent leak- age current. this leakage current, nominally 1na (10na max) results in a small gain error. a 100 reference resistance will create a 0.5v full scale error. normal mode rejection and anti-aliasing one of the advantages delta-sigma adcs offer over conventional adcs is on-chip digital ? ltering. combined with a large oversample ratio, the LTC2496 signi? cantly simpli? es anti-aliasing ? lter requirements. additionally, the input current cancellation feature allows external low pass ? ltering without degrading the dc performance of the device. the sinc 4 digital ? lter provides excellent normal mode rejection at all frequencies except dc and integer multiples of the modulator sampling frequency (f s ). the modulator sampling frequency is f s = 15,360hz while operating with its internal oscillator and f s = f eosc /20 when operating with an external oscillator of frequency f eosc . when using the internal oscillator, the LTC2496 is designed to reject line frequencies. as shown in figure 17, rejection nulls occur at multiples of frequency f n , where f n is 55hz for simultaneous 50hz/60hz rejection). multiples of the modulator sampling rate (f s = f n ? 256) only reject noise to 15db (see figure 18), if noise sources are present at these frequencies anti-aliasing will reduce their effects. LTC2496 28 2496fa applications information the user can expect to achieve this level of performance using the internal oscillator, as shown in figure 19. measured values of normal mode rejection are shown superimposed over the theoretical rejection. traditional high order delta-sigma modulators suffer from potential instabilities at large input signal levels. the proprietary architecture used for the LTC2496 third order modulator resolves this problem and guarantees stability with input signals 150% of full-scale. in many industrial applications, it is not uncommon to have mi- crovolt level signals superimposed over unwanted error sources with several volts of peak-to-peak noise. figure 20 shows measurement results for the rejection of a 7.5v peak-to-peak noise source (150% of full scale) applied to the LTC2496. from these curves, it is shown that the rejection performance is maintained even in extremely noisy environments. input frequency (hz) 0 20 40 60 80 100 120 140 160 180 200 220 normal mode rejection (db) 2496 f19 0 ?0 ?0 ?0 ?0 100 120 v cc = 5v v ref = 5v v in(cm) = 2.5v v in(p-p) = 5v t a = 25 c measured data calculated data input signal frequency (hz) input normal mode rejection (db) 2496 f17 0 ?0 ?0 ?0 ?0 ?0 ?0 ?0 ?0 ?0 100 110 120 f n 0 2f n 3f n 4f n 5f n 6f n 7f n 8f n f n = f eosc/5120 figure 17. input normal mode rejection at dc input signal frequency (hz) 250f n 252f n 254f n 256f n 258f n 260f n 262f n input normal mode rejection (db) 2496 f18 0 ?0 ?0 ?0 ?0 ?0 ?0 ?0 ?0 ?0 100 110 120 figure 18. input normal mode rejection at f s = 256 ? f n figure 19. input normal mode rejection vs input frequency with input perturbation of 100% (50hz/60hz notch) output data rate when using its internal oscillator, the LTC2496 produces up to 6.9 samples per second (sps) with a notch frequency of 55hz. the actual output data rate depends upon the length of the sleep and data output cycles which are controlled by the user and can be made insigni? cantly short. when operating with an external conversion clock (f o connected to an external oscillator), the LTC2496 output data rate can be increased. the duration of the conversion cycle is 41036/f eosc . if f eosc = 307.2khz, the converter notch frequency is 60hz. an increase in f eosc over the nominal 307.2khz will trans- late into a proportional increase in the maximum output data rate (up to a maximum of 100sps). the increase in output rate leads to degradation in offset, full-scale error, and effective resolution as well as a shift in frequency rejection. input frequency (hz) 0 15 30 45 60 75 90 105 120 135 150 165 180 195 210 225 240 normal mode rejection (db) 2496 f20 0 ?0 ?0 ?0 ?0 100 120 v cc = 5v v ref = 5v v in(cm) = 2.5v t a = 25 c v in(p-p) = 5v v in(p-p) = 7.5v (150% of full scale) figure 20. input normal mode rejection vs input frequency with input perturbation of 150% (60hz notch) LTC2496 29 2496fa output data rate (readings/sec) 0 ?0 offset error (ppm of v ref ) ? 5 10 20 10 50 70 2496 f26 0 15 40 90 100 20 30 60 80 v cc = 5v, v ref = 2.5v v cc = v ref = 5v v in(cm) = v ref(cm) v in = 0v f o = ext clock t a = 25 c output data rate (readings/sec) 0 10 resolution (bits) 12 16 18 22 10 50 70 2496 f25 14 20 40 90 100 20 30 60 80 t a = 85 c t a = 25 c v in(cm) = v ref(cm) v cc = v ref = 5v f o = ext clock res = log 2 (v ref /inl max ) output data rate (readings/sec) 0 10 resolution (bits) 12 16 18 20 24 10 50 70 2496 f24 14 22 40 90 100 20 30 60 80 v in(cm) = v ref(cm) v cc = v ref = 5v v in = 0v f o = ext clock res = log 2 (v ref /noise rms ) t a = 85 c t a = 25 c t a = 25 c, 85 c output data rate (readings/sec) 0 ?500 ?s error (ppm of v ref ) ?000 ?000 ?500 ?000 0 10 50 70 2496 f23 ?500 ?00 40 90 100 20 30 60 80 v in(cm) = v ref(cm) v cc = v ref = 5v f o = ext clock t a = 85 c t a = 25 c output data rate (readings/sec) 0 0 +fs error (ppm of v ref ) 500 1500 2000 2500 3500 10 50 70 2496 f22 1000 3000 40 90 100 20 30 60 80 v in(cm) = v ref(cm) v cc = v ref = 5v f o = ext clock t a = 85 c t a = 25 c applications information a change in f eosc results in a proportional change in the internal notch position. this leads to reduced differential mode rejection of line frequencies. the common mode rejection of line frequencies remains unchanged, thus fully differential input signals with a high degree of symmetry on both the in + and in C pins will continue to reject line frequency noise. an increase in f eosc also increases the effective dynamic input and reference current. external rc networks will continue to have zero differential input current, but the time required for complete settling (580ns for f eosc = 307.2khz) is reduced, proportionally. once the external oscillator frequency is increased above 1mhz (a more than 3x increase in output rate) the effective- ness of internal auto calibration circuits begins to degrade. this results in larger offset errors, full-scale errors, and decreased resolution, see figures 21 to 28. figure 22. +fs error vs output data rate and temperature figure 23.Cfs error vs output data rate and temperature figure 24. resolution (noise rms 1lsb) vs output data rate and temperature figure 25. resolution (inl max 1lsb) vs output data rate and temperature figure 26. offset error vs output data rate and reference voltage output data rate (readings/sec) ?0 offset error (ppm of v ref ) 10 30 50 0 20 40 20 40 60 80 2496 f21 100 10 030507090 v in(cm) = v ref(cm) v cc = v ref = 5v v in = 0v f o = ext clock t a = 85 c t a = 25 c figure 21. offset error vs output data rate and temperature LTC2496 30 2496fa applications information figure 27. resolution (noise rms 1lsb) vs output data rate and reference voltage figure 28. resolution (inl max 1lsb) vs output data rate and reference voltage output data rate (readings/sec) 0 10 resolution (bits) 12 16 18 20 24 10 50 70 2496 f27 14 22 40 90 100 20 30 60 80 v in(cm) = v ref(cm) v in = 0v f o = ext clock t a = 25 c res = log 2 (v ref /noise rms ) v cc = 5v, v ref = 2.5v v cc = v ref = 5v v cc = 5v, v ref = 5v, 2.5v output data rate (readings/sec) 0 10 resolution (bits) 12 16 18 22 10 50 70 2496 f28 14 20 40 90 100 20 30 60 80 v cc = 5v, v ref = 2.5v v cc = v ref = 5v v in(cm) = v ref(cm) v in = 0v ref = gnd f o = ext clock t a = 25 c res = log 2 (v ref /inl max ) LTC2496 31 2496fa information furnished by linear technology corporation is believed to be accurate and reliable. however, no responsibility is assumed for its use. linear technology corporation makes no representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. package description 5.00 0.10 (2 sides) note: 1. drawing conforms to jedec package outline m0-220 variation whkd 2. drawing not to scale 3. all dimensions are in millimeters pin 1 top mark (see note 6) 0.40 0.10 37 1 2 38 bottom view?xposed pad 5.15 0.10 (2 sides) 7.00 0.10 (2 sides) 0.75 0.05 r = 0.115 typ 0.25 0.05 (uh) qfn 0205 0.50 bsc 0.200 ref 0.200 ref 0.00 ?0.05 recommended solder pad layout 3.15 0.10 (2 sides) 0.40 0.10 0.00 ?0.05 0.75 0.05 0.70 0.05 0.50 bsc 5.15 0.05 (2 sides) 3.15 0.05 (2 sides) 4.10 0.05 (2 sides) 5.50 0.05 (2 sides) 6.10 0.05 (2 sides) 7.50 0.05 (2 sides) 0.25 0.05 package outline 4. dimensions of exposed pad on bottom of package do not include mold flash. mold flash, if present, shall not exceed 0.20mm on any side 5. exposed pad shall be solder plated 6. shaded area is only a reference for pin 1 location on the top and bottom of package pin 1 notch r = 0.30 typ or 0.35 45 chamfer uhf package 38-lead plastic qfn (5mm 7mm) (reference ltc dwg # 05-08-1701) LTC2496 32 2496fa linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2006 lt 0407 rev a ? printed in usa typical application related parts part number description comments lt1236a-5 precision bandgap reference, 5v 0.05% max initial accuracy, 5ppm/c drift lt1460 micropower series reference 0.075% max initial accuracy, 10ppm/c max drift lt1790 micropower sot-23 low dropout reference family 0.05% max initial accuracy, 10ppm/c max drift ltc2400 24-bit, no latency ? adc in so-8 0.3ppm noise, 4ppm inl, 10ppm total unadjusted error, 200a ltc2410 24-bit, no latency ? adc with differential inputs 0.8v rms noise, 2ppm inl ltc2411/ltc2411-1 24-bit, no latency ? adcs with differential inputs in msop 1.45v rms noise, 4ppm inl, simultaneous 50hz/60hz rejection (ltc2411-1) ltc2413 24-bit, no latency ? adc with differential inputs simultaneous 50hz/60hz rejection, 800nv rms noise ltc2415/ltc2415-1 24-bit, no latency ? adcs with 15hz output rate pin compatible with the ltc2410 ltc2414/ltc2418 8-/16-channel 24-bit, no latency ? adcs 0.2ppm noise, 2ppm inl, 3ppm total unadjusted errors 200a ltc2440 high speed, low noise 24-bit ? adc 3.5khz output rate, 200nv noise, 24.6 enobs ltc2480 16-bit ? adc with easy drive inputs, 600nv noise, programmable gain, and temperature sensor pin compatible with ltc2482/ltc2484 ltc2481 16-bit ? adc with easy drive inputs, 600nv noise, i 2 c interface, programmable gain, and temperature sensor pin compatible with ltc2483/ltc2485 ltc2482 16-bit ? adc with easy drive inputs pin compatible with ltc2480/ltc2484 ltc2483 16-bit ? adc with easy drive inputs, and i 2 c interface pin compatible with ltc2481/ltc2485 ltc2484 24-bit ? adc with easy drive inputs pin compatible with ltc2480/ltc2482 ltc2485 24-bit ? adc with easy drive inputs, i 2 c interface, and temperature sensor pin compatible with ltc2481/ltc2483 ltc2498 24-bit 8-/16-channel ? adc with easy drive input current cancellation pin compatible with LTC2496/ltc2449 0.1 f 0.1 f + + 1/2 lt6078 1/2 lt6078 1 2 3 5 6 7 ? adc with easy drive inputs input mux muxoutp muxoutn 17 2496 ta02 LTC2496 analog inputs sdi sck sdo cs 1k 1k external buffers provide high impedance inputs and ampli? er offsets are automatically cancelled. |
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