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1 LTC2355-12/ltc2355-14 2355f applicatio s u features descriptio u block diagra w serial 12-bit/14-bit, 3.5msps sampling adcs with shutdown 3.5msps conversion rate 74.2db sinad at 14-bits, 71.1db sinad at 12-bits low power dissipation: 18mw 3.3v single supply operation 2.5v internal bandgap reference can be overdriven 3-wire spi-compatible serial interface sleep (13 w) shutdown mode nap (4mw) shutdown mode 80db common mode rejection 0v to 2.5v unipolar input range tiny 10-lead msop package communications data acquisition systems uninterrupted power supplies multiphase motor control multiplexed data acquisition rfid thd, 2nd, 3rd and sfdr vs input frequency the ltc ? 2355-12/ltc2355-14 are 12-bit/14-bit, 3.5msps serial adcs with differential inputs. the devices draw only 5.5ma from a single 3.3v supply and come in a tiny 10-lead msop package. a sleep shutdown feature further reduces power consumption to 13 w. the combination of speed, low power and tiny package makes the LTC2355-12/ ltc2355-14 suitable for high speed, portable applications. the 80db common mode rejection allows users to elimi- nate ground loops and common mode noise by measuring signals differentially from the source. the devices convert 0v to 2.5v unipolar inputs differentially. the absolute voltage swing for a in + and a in C extends from ground to the supply voltage. the serial interface sends out the conversion results during the 16 clock cycles following a conv rising edge for compatibility with standard serial interfaces. if two addi- tional clock cycles for acquisition time are allowed after the data stream in between conversions, the full sampling rate of 3.5msps can be achieved with a 63mhz clock. C + 1 2 7 3 4 s & h gnd exposed pad ltc2355-14 v ref 10 f a in C a in + 14-bit adc 3.3v 10 f 14 14-bit latch 8 10 9 three- state serial output port 2.5v reference timing logic v dd sdo conv sck 2355 ta01 5 6 11 , lt, ltc and ltm are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners. frequency (mhz) 0.1 ?0 thd, 2nd, 3rd (db) ?4 ?8 ?2 ?6 1 10 100 2355 g02 ?6 ?2 ?8 ?04 ?10 ?0 thd 2nd 3rd
2 LTC2355-12/ltc2355-14 2355f t jmax = 125 c, ja = 150 c/ w exposed pad is gnd (pin 11) must be soldered to pcb 1 2 3 4 5 a in + a in C v ref gnd gnd 10 9 8 7 6 conv sck sdo v dd gnd top view 11 mse package 10-lead plastic msop (notes 1, 2) supply voltage (v dd ) ................................................. 4v analog and v ref input voltages (note 3) ....................................C0.3v to (v dd + 0.3v) digital input voltages ................. C 0.3v to (v dd + 0.3v) digital output voltage .................. C 0.3v to (v dd + 0.3v) power dissipation .............................................. 100mw operation temperature range ltc2355c-12/ltc2355c-14 ................... 0 c to 70 c ltc2355i-12/ltc2355i-14 ................ C 40 c to 85 c storage temperature range ................. C 65 c to 150 c lead temperature (soldering, 10 sec).................. 300 c order part number mse part marking ltcvx ltcvx ltcvy ltcvy ltc2355cmse-12 ltc2355imse-12 ltc2355cmse-14 ltc2355imse-14 absolute axi u rati gs w ww u package/order i for atio uu w the denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25 c. with internal reference. v dd = 3.3v. LTC2355-12 ltc2355-14 parameter conditions min typ max min typ max units resolution (no missing codes) 12 14 bits integral linearity error (notes 4, 5, 18) C2 0.25 2 C4 0.5 4 lsb offset error (notes 4, 18) C10 1 10 C20 220 lsb gain error (note 4, 18) C30 5 30 C80 10 80 lsb gain tempco internal reference (note 4) 15 15 ppm/ c external reference 1 1 ppm/ c the denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25 c. with internal reference. v dd = 3.3v. symbol parameter conditions min typ max units v in analog differential input range (notes 3, 8, 9) 3.1v v dd 3.6v 0 to 2.5 v v cm analog common mode + differential 0 to v dd v input range (note 10) i in analog input leakage current 1 a c in analog input capacitance (note 19) 13 pf t acq sample-and-hold acquisition time (note 6) 39 ns t ap sample-and-hold aperture delay time 1 ns t jitter sample-and-hold aperture delay time jitter 0.3 ps cmrr analog input common mode rejection ratio f in = 1mhz, v in = 0v to 3v C60 db f in = 100mhz, v in = 0v to 3v C15 db co verter characteristics u a alog i put u u consult factory for parts specified with wider operating temperature ranges. 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/
3 LTC2355-12/ltc2355-14 2355f the denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25 c with external reference = 2.55v. v dd = 3.3v LTC2355-12 ltc2355-14 symbol parameter conditions min typ max min typ max units sinad signal-to-noise plus 100khz input signal 71.1 74.2 db distortion ratio 1.4mhz input signal 69 71.1 71 73.8 db thd total harmonic 100khz first 5 harmonics C86 C86 db distortion 1.4mhz first 5 harmonics C82 C76 C82 C78 db sfdr spurious free 100khz input signal 86 86 db dynamic range 1.4mhz input signal 82 82 db imd intermodulation 1.25v to 2.5v 1.25mhz into a in + , 0v to 1.25v, C82 C82 db distortion 1.2mhz into a in C code-to-code v ref = 2.5v (note 18) 0.25 1 lsb rms transition noise full power bandwidth v in = 2.5v p-p , sdo = 11585lsb p-p (note 15) 50 50 mhz full linear bandwidth s/(n + d) 68db 5 5 mhz the denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25 c. v dd = 3.3v parameter conditions min typ max units v ref output voltage i out = 0 2.5 v v ref output tempco 15 ppm/ c v ref line regulation v dd = 3.1v to 3.6v, v ref = 2.5v 600 v/v v ref output resistance load current = 0.5ma 0.2 ? v ref settling time c ref = 10 f2ms external v ref input range 2.55 v dd v the denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25 c. v dd = 3.3v symbol parameter conditions min typ max units v ih high level input voltage v dd = 3.6v 2.4 v v il low level input voltage v dd = 3.1v 0.6 v i in digital input current v in = 0v to v dd 10 a c in digital input capacitance 5pf v oh high level output voltage v dd = 3.3v, i out = C 200 a 2.5 2.9 v v ol low level output voltage v dd = 3.1v, i out = 160 a 0.05 v v dd = 3.1v, i out = 1.6ma 0.10 0.4 v i oz hi-z output leakage d out v out = 0v to v dd 10 a c oz hi-z output capacitance d out 1pf i source output short-circuit source current v out = 0v, v dd = 3.3v 20 ma i sink output short-circuit sink current v out = v dd = 3.3v 15 ma dy a ic accuracy u w i ter al refere ce characteristics uu u digital i puts a d digital outputs u u
4 LTC2355-12/ltc2355-14 2355f symbol parameter conditions min typ max units f sample(max) maximum sampling rate per channel 3.5 mhz (conversion rate) t throughput minimum sampling period (conversion + acquisiton period) 286 ns t sck clock period (note 16) 15.872 10000 ns t conv conversion time (note 6) 16 18 sclk cycles t 1 minimum high or low sclk pulse width (note 6) 2 ns t 2 conv to sck setup time (notes 6, 10) 3 ns t 3 nearest sck edge before conv (note 6) 0 ns t 4 minimum high or low conv pulse width (note 6) 4 ns t 5 sck to sample mode (note 6) 4 ns t 6 conv to hold mode (notes 6, 11) 1.2 ns t 7 16th sck to conv interval (affects acquisition period) (notes 6, 7, 13) 45 ns t 8 delay from sck to valid bits 0 through 13 (notes 6, 12) 8 ns t 9 sck to hi-z at sdo (notes 6, 12) 6 ns t 10 previous sdo bit remains valid after sck (notes 6, 12) 2 ns t 12 v ref settling time after sleep-to-wake transition (note 14) 2 ms ti i g characteristics w u the denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25 c. v dd = 3.3v power require e ts w u the denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25 c. (note 17) symbol parameter conditions min typ max units v dd supply voltage 3.1 3.3 3.6 v i dd supply current active mode 5.5 8 ma nap mode 1.1 1.5 ma sleep mode (LTC2355-12) 4 15 a sleep mode (ltc2355-14) 4 12 a p d power dissipation 18 mw 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: when these pins are taken below gnd or above v dd , they will be clamped by internal diodes. this product can handle input currents greater than 100ma below gnd or greater than v dd without latchup. note 4: offset and full-gain specifications are measured for a single-ended a in + input with a in C grounded and using the internal 2.5v reference. note 5: integral linearity is tested with an external 2.55v reference and is defined as the deviation of a code from the straight line passing through the actual endpoints of a transfer curve. the deviation is measured from the center of quantization band. note 6: guaranteed by design, not subject to test. note 7: recommended operating conditions. note 8: the analog input range is defined for the voltage difference between a in + and a in C . note 9: the absolute voltage at a in + and a in C must be within this range. note 10: if less than 3ns is allowed, the output data will appear one clock cycle later. it is best for conv to rise half a clock before sck, when running the clock at rated speed. note 11: not the same as aperture delay. aperture delay is smaller (1ns) because the 2.2ns delay through the sample-and-hold is subtracted from the conv to hold mode delay. note 12: the rising edge of sck is guaranteed to catch the data coming out into a storage latch. note 13: the time period for acquiring the input signal is started by the 16th rising clock and it is ended by the rising edge of convert. note 14: the internal reference settles in 2ms after it wakes up from sleep mode with one or more cycles at sck and a 10 f capacitive load. note 15: the full power bandwidth is the frequency where the output code swing drops to 3db with a 2.5v p-p input sine wave. note 16: maximum clock period guarantees analog performance during conversion. output data can be read with an arbitrarily long clock. note 17: v dd = 3.3v, f sample = 3.5msps. note 18: the ltc2355-14 is measured and specified with 14-bit resolution (1lsb = 152 v) and the LTC2355-12 is measured and specified with 12-bit resolution (1lsb = 610 v). note 19: the sampling capacitor at each input accounts for 4.1pf of the input capacitance.
5 LTC2355-12/ltc2355-14 2355f sinad vs input frequency sfdr vs input frequency t a = 25 c, v dd = 3.3v (ltc2355-14) thd, 2nd and 3rd vs input frequency typical perfor a ce characteristics uw 1.4mhz sine wave 8192 point fft plot differential linearity vs output code 100khz sine wave 8192 point fft plot integral linearity vs output code snr vs input frequency frequency (mhz) 0.1 ?0 thd, 2nd, 3rd (db) ?4 ?8 ?2 ?6 1 10 100 2355 g02 ?6 ?2 ?8 ?04 ?10 ?0 thd 2nd 3rd frequency (mhz) 0.1 74 sfdr (db) 80 86 92 1 10 100 2355 g03 68 62 56 50 frequency (mhz) 0.1 62 sinad (db) 65 68 71 74 1 10 100 2355 g01 59 56 53 50 77 frequency (mhz) 0.1 62 snr (db) 65 68 71 74 1 10 100 2355 g04 59 56 53 50 77 frequency (mhz) 0.00 magnitude (db) C90 C30 C20 C10 0 0.50 1.00 1.25 2355 g05 C110 C50 C70 C100 C40 C120 C60 C80 0.25 0.75 1.50 1.75 frequency (mhz) 0.00 magnitude (db) C90 C30 C20 C10 0 0.50 1.00 1.25 2355 g06 C110 C50 C70 C100 C40 C120 C60 C80 0.25 0.75 1.50 1.75 2355 g07 output code 0 ?.0 differential linearity (lsb) ?.8 ?.4 ?.2 0 1.0 0.4 4096 8192 ?.6 0.6 0.8 0.2 12288 16384 output code 0 integral linearity (lsb) 0 1 2 16384 2355 g08 ? ? ? 4096 8192 12288 ? 4 3
6 LTC2355-12/ltc2355-14 2355f t a = 25 c, v dd = 3.3v (ltc2355-14) typical perfor a ce characteristics uw differential and integral linearity vs conversion rate sinad vs conversion rate, input frequency = 1.4mhz internal reference voltage vs load current internal reference voltage vs v dd v dd supply current vs conversion rate 2.5v p-p power bandwidth cmrr vs frequency psrr vs frequency conversion rate (msps) 2.0 linearity (lsb) 3 4 2 1 0 ? ? ? ? 3.6 2355 g09 2.4 2.8 3.2 4.0 3.4 2.2 2.6 3.0 3.8 max inl max dnl min inl min dnl conversion rate (msps) 2 sinad (db) 73 74 75 3.6 2355 g10 72 71 70 2.2 2.4 2.6 2.8 3 3.2 3.4 3.8 4 frequency (hz) 1m 10m 100m 1g C18 amplitude (db) C12 C6 0 2355 g11 C24 C30 C36 6 12 frequency (hz) 100 cmrr (db) 0 C20 C40 C60 C80 C100 C120 1k 10k 100k 1m 2355 g12 10m 100m frequency (hz) 110 C50 psrr (db) C45 C40 C35 C30 100 1k 10k 100k 1m 2355 g13 C55 C60 C65 C70 C25 load current (ma) 0.4 0.8 1.2 1.6 2355 g14 2.0 0.2 0 0.6 1.0 1.4 1.8 2.4890 v ref (v) 2.4894 2.4898 2.4902 2.4892 2.4896 2.4900 v dd (v) 2.4890 v ref (v) 2.4894 2.4898 2.4902 2.4892 2.4896 2.4900 2.8 3.0 3.2 3.4 2355 g15 2.6 3.6 conversion rate (mps) 0 0 v dd supply current (ma) 1 0.5 2 1.5 3 2.5 4 3.5 6 5.5 0.5 1 1.5 2 2355 g16 2.5 3.5 34 5 4.5 t a = 25 c, v dd = 3.3v (LTC2355-12 and ltc2355-14)
7 LTC2355-12/ltc2355-14 2355f a in + (pin 1): noninverting analog input. a in + operates fully differentially with respect to a in C with a 0v to 2.5v differential swing and a 0v to v dd common mode swing. a in (pin 2): inverting analog input. a in C operates fully differentially with respect to a in + with a C 2.5v to 0v differential swing and a 0v to v dd common mode swing. v ref (pin 3): 2.5v internal reference. bypass to gnd and to a solid analog ground plane with a 10 f ceramic capacitor (or 10 f tantalum in parallel with 0.1 f ce- ramic). can be overdriven by an external reference be- tween 2.55v and v dd . gnd (pins 4, 5, 6, 11): ground and exposed pad. these ground pins and the exposed pad must be tied directly to the solid ground plane under the part. keep in mind that analog signal currents and digital output signal currents flow through these pins. v dd (pin 7): 3.3v positive supply. this single power pin supplies 3.3v to the entire device. bypass to gnd and to a solid analog ground plane with a 10 f ceramic capacitor (or 10 f tantalum in parallel with 0.1 f ceramic). keep in mind that internal analog currents and digital output signal currents flow through this pin. care should be taken to place the 0.1 f bypass capacitor as close to pins 6 and 7 as possible. sdo (pin 8): three-state serial data output. each set of output data words represents the difference between a in + and a in C analog inputs at the start of the previous conversion. sck (pin 9): external clock input. advances the conver- sion process and sequences the output data on the rising edge. responds to ttl ( 3.3v) and 3.3v cmos levels. one or more sck pulses wakes the adc from sleep mode. conv (pin 10): convert start. holds the analog input signal and starts the conversion on the rising edge. responds to ttl ( 3.3v) and 3.3v cmos levels. two conv pulses with sck in fixed high or fixed low state start nap mode. four or more conv pulses with sck in fixed high or fixed low state start sleep mode. uu u pi fu ctio s block diagra w 2355 bd C + 1 2 7 3 4 s & h gnd exposed pad ltc2355-14 v ref 10 f a in C a in + 14-bit adc 3.3v 10 f 14 14-bit latch 8 10 9 three- state serial output port 2.5v reference timing logic v dd sdo conv sck 5 6 11
8 LTC2355-12/ltc2355-14 2355f nap mode and sleep mode waveforms sck to sdo delay LTC2355-12 timing diagram ti i g diagra u ww ltc2355-14 timing diagram sck conv internal s/h status sdo t 7 t 3 t 1 1 17 16 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 t 2 t 6 t 8 t 10 t 4 t 5 t 8 t 9 t acq sample hold hold hi-z hi-z t conv 14-bit data word sdo represents the analog input from the previous conversion *bits marked "x" after d0 should be ignored. t throughput 2355 td01 d11 d10 d8 d7 d6 d5 d4 d3 d2 d1 d0 x x d9 sample 1 sck conv internal s/h status sdo t 7 t 3 t 1 1 17 16 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 t 2 t 6 t 8 t 10 t 4 t 5 t 8 t 9 t acq sample hold hold hi-z hi-z t conv 14-bit data word sdo represents the analog input from the previous conversion t throughput 2355 td01b d13 d12 d10 d9 d8 d7 d6 d5 d4 d3 d2 d1 d0 d11 sample 1 slk conv nap sleep v ref t 1 t 12 t 1 note: nap and sleep are internal signals 2355 td02 t 8 t 10 sck sdo 2355 td03 v ih v oh v ol t 9 sck sdo v ih 90% 10%
9 LTC2355-12/ltc2355-14 2355f driving the analog input t he differential analog inputs of the LTC2355-12/ltc2355-14 may be driven differentially or as a single-ended input (i.e., the a in C input is grounded). both differential analog inputs, a in + and a in C , are sampled at the same instant. any unwanted signal that is common to both inputs of each input pair will be reduced by the common mode rejection of the sample-and- hold circuit. the inputs draw only one small current spike while charging the sample-and-hold capacitors at the end of conversion. during conversion, the analog inputs draw only a small leakage current. if the source impedance of the driving circuit is low, then the LTC2355-12/ltc2355-14 inputs can be driven directly. as source impedance increases, so will acquisition time. for minimum acquisition time with high source impedance, a buffer amplifier must be used. the main requirement is that the amplifier driving the analog input(s) must settle after the small current spike before the next conversion starts (settling time must be 39ns for full through- put rate). also keep in mind while choosing an input amplifier the amount of noise and harmonic distortion added by the amplifier. choosing an input amplifier choosing an input amplifier is easy if a few requirements are taken into consideration. first, to limit the magnitude of the voltage spike seen by the amplifier from charging the sam- pling capacitor, choose an amplifier that has a low output impedance (<100 ? ) at the closed-loop bandwidth frequency. for example, if an amplifier is used in a gain of 1 and has a unity-gain bandwidth of 50mhz, then the output impedance at 50mhz must be less than 100 ? . the second requirement is that the closed-loop bandwidth must be greater than 40mhz to ensure adequate small-signal settling for full throughput rate. if slower op amps are used, more time for settling can be provided by increasing the time between conversions. the best choice for an op amp to drive the LTC2355-12/ltc2355-14 will depend on the application. generally, applications fall into two categories: ac applica- tions where dynamic specifications are most critical and time domain applications where dc accuracy and settling time are most critical. the following list is a summary of the op amps that are suitable for driving the LTC2355-12/ltc2355-14. (more detailed information is available in the linear technol- ogy databooks and on the linearview tm cd-rom.) ltc1566-1: low noise 2.3mhz continuous time low-pass filter. lt 1630: dual 30mhz rail-to-rail voltage fb amplifier. 2.7v to 15v supplies. very high a vol , 500 v offset and 520ns settling to 0.5lsb for a 4v swing. thd and noise are C93db to 40khz and below 1lsb to 320khz (a v = 1, 2v p-p into 1k ? , v s = 5v), making the part excellent for ac applications (to 1/3 nyquist) where rail-to-rail performance is desired. quad version is available as lt1631. lt1632: dual 45mhz rail-to-rail voltage fb amplifier. 2.7v to 15v supplies. very high a vol , 1.5mv offset and 400ns settling to 0.5lsb for a 4v swing. it is suitable for applica- tions with a single 5v supply. thd and noise are C93db to 40khz and below 1lsb to 800khz (a v = 1, 2v p-p into 1k ? , v s = 5v), making the part excellent for ac applications where rail-to-rail performance is desired. quad version is available as lt1633. lt1813: dual 100mhz 750v/ s 3ma voltage feedback amplifier. 5v to 5v supplies. distortion is C86db to 100khz and C77db to 1mhz with 5v supplies (2v p-p into 500 ? ). excellent part for fast ac applications with 5v supplies. lt1801: 80mhz gbwp, C75dbc at 500khz, 2ma/amplifier, 8.5nv/ hz. lt1806/lt1807: 325mhz gbwp, C80dbc distortion at 5mhz, unity-gain stable, r-r in and out, 10ma/amplifier, 3.5nv/ hz. lt1810: 180mhz gbwp, C90dbc distortion at 5mhz, unity-gain stable, r-r in and out, 15ma/amplifier, 16nv/ hz. lt1818/lt1819: 400mhz, 2500v/ s,9ma, single/dual volt- age mode operational amplifier. lt6200: 165mhz gbwp, C85dbc distortion at 1mhz, unity- gain stable, r-r in and out, 15ma/amplifier, 0.95nv/ hz. lt6203: 100mhz gbwp, C80dbc distortion at 1mhz, unity-gain stable, r-r in and out, 3ma/amplifier, 1.9nv/ hz. lt6600-10: amplifier/filter differential in/out with 10mhz cutoff. applicatio s i for atio wu u u linearview is a trademark of linear technology corporation.
10 LTC2355-12/ltc2355-14 2355f input filtering and source impedance the noise and the distortion of the input amplifier and other circuitry must be considered since they will add to the ltc2355 -12/ltc2355-14 noise and distortion. the small-signal bandwidth of the sample-and-hold circuit is 50mhz. any noise or distortion products that are present at the analog inputs will be summed over this entire bandwidth. noisy input circuitry should be filtered prior to the analog inputs to minimize noise. a simple 1-pole rc filter is sufficient for many applications. for example, figure 1 shows a 47pf capacitor from a in + to ground and a 51 ? source resistor to limit the input bandwidth to 47mhz. the 47pf capacitor also acts as a charge reservoir for the input sample-and-hold and isolates the adc input from sampling-glitch sensitive circuitry. high quality ca- pacitors and resistors should be used since these compo- nents can add distortion. npo and silvermica type dielectric capacitors have excellent linearity. carbon surface mount resistors can generate distortion from self heating and from damage that may occur during soldering. metal film surface mount resistors are much less susceptible to both problems. when high amplitude unwanted signals are close in frequency to the desired signal frequency, a multiple pole filter is required. high external source resis- tance, combined with the 13pf of input capacitance, will reduce the rated 50mhz bandwidth and increase acquisi- tion time beyond 39ns. figure 1. rc input filter figure 2. overdriving v ref pin with an external reference input range the analog inputs of the ltc2355 -12/ltc2355-14 may be driven fully differentially with a single supply. each input may swing up to 2.5v p-p individually. when using the internal reference, the noninverting input should never be more than 2.5v more positive than the inverting input. the 0v to 2.5v range is also ideally suited for single-ended input use with single supply applications. the common mode range of the inputs extend from ground to the supply voltage v dd . if the difference between the a in + and a in C inputs exceeds 2.5v, the output code will stay fixed at all ones and if this difference goes below 0v, the ouput code will stay fixed at all zeros. internal reference the ltc2355 -12/ltc2355-14 has an on-chip, tempera- ture compensated, bandgap reference that is factory trimmed to 2.5v to obtain a unipolar 0v to 2.5v input span. the reference amplifier output v ref , (pin 3) must be bypassed with a capacitor to ground. the reference ampli- fier is stable with capacitors of 1 f or greater. for the best noise performance, a 10 f ceramic or a 10 f tantalum in parallel with a 0.1 f ceramic is recommended. the v ref pin can be overdriven with an external reference as shown in figure 2. the voltage of the external reference must be higher than the 2.5v output of the internal reference. the recommended range for an external reference is 2.55v to v dd . an external reference at 2.55v will see a dc quiescent load of 0.75ma and as much as 3ma during conversion. applicatio s i for atio wu u u 10 f 11 3 a in C LTC2355-12/ ltc2355-14 a in + 47pf 2 1 51 ? gnd v ref 2355 f01 gnd LTC2355-12/ ltc2355-14 lt1790-3 v ref 10 f 3.5v to 18v 11 3 3v 2355 f02
11 LTC2355-12/ltc2355-14 2355f input span versus reference voltage the differential input range has a 0v to v ref unipolar voltage span that equals the difference between the volt- age at the reference buffer output v ref at pin 3, and the voltage at the ground (exposed pad ground). the differ- ential input range of the adc is 0v to 2.5v when using the internal reference. the internal adc is referenced to these two nodes. this relationship also holds true with an external reference. differential inputs the ltc2355 -12/ltc2355-14 has a unique differential sample-and-hold circuit that measures input voltages from ground to v dd . the adc will always convert the unipolar difference of a in + C a in C , independent of the common mode voltage at the inputs. the common mode rejection holds up at extremely high frequencies, see figure 3. the only requirement is that both inputs not go below ground or exceed v dd . integral nonlinearity errors (inl) and differential nonlinearity errors (dnl) are largely independent of the common mode voltage. however, the offset error will vary. the change in offset error is typically less than 0.1% of the common mode voltage. figure 4 shows the ideal input/output characteristics for the ltc2355 -12/ltc2355-14. the code transitions occur midway between successive integer lsb values (i.e., 0.5lsb, 1.5lsb, 2.5lsb, fs C 1.5lsb). the output code is straight binary with 1lsb = 2.5v/16384 = 153 v for the ltc2355-14, and 1lsb = 2.5v/4096 = 610 v for the LTC2355-12. the ltc2355-14 has 1lsb rms of random white noise. figure 3. cmrr vs frequency figure 4. LTC2355-12/ltc2355-14 transfer characteristic applicatio s i for atio wu u u frequency (hz) 100 cmrr (db) 0 C20 C40 C60 C80 C100 C120 1k 10k 100k 1m 2355 f03 10m 100m input voltage (v) unipolar output code 2355 f05 111...111 111...110 111...101 000...000 000...001 000...010 fs C 1lsb 0
12 LTC2355-12/ltc2355-14 2355f board layout and bypassing wire wrap boards are not recommended for high resolu- tion and/or high speed a/d converters. to obtain the best performance from the ltc2355 -12/ltc2355-14, a printed circuit board with ground plane is required. layout for the printed circuit board should ensure that digital and analog signal lines are separated as much as possible. in particu- lar, care should be taken not to run any digital track alongside an analog signal track. if optimum phase match between the inputs is desired, the length of the two input wires should be kept matched. high quality tantalum and ceramic bypass capacitors should be used at the v dd and v ref pins as shown in the block diagram on the first page of this data sheet. for optimum performance, a 10 f surface mount tantalum capacitor with a 0.1 f ceramic is recommended for the v dd and v ref pins. alternatively, 10 f ceramic chip ca- figure 5. recommended layout pacitors such as murata grm235y5v106z016 may be used. the capacitors must be located as close to the pins as possible. the traces connecting the pins and the bypass capacitors must be kept short and should be made as wide as possible. figure 5 shows the recommended system ground connec- tions. all analog circuitry grounds should be terminated at the ltc2355 -12/ltc2355-14 gnd (pins 4, 5, 6 and exposed pad). the ground return from the ltc2355 -12/ ltc2355-14 (pins 4, 5, 6 and exposed pad) to the power supply should be low impedance for noise free operation. in applications where the adc data outputs and control signals are connected to a continuously active micropro- cessor bus, it is possible to get errors in the conversion results. these errors are due to feedthrough from the microprocessor to the successive approximation com- parator. the problem can be eliminated by forcing the microprocessor into a wait state during conversion or by using three-state buffers to isolate the adc data bus. power-down modes upon power-up, the ltc2355 -12/ltc2355-14 is initial- ized to the active state and is ready for conversion. the nap and sleep mode waveforms show the power-down modes for the ltc2355 -12/ltc2355-14. the sck and conv inputs control the power-down modes (see timing diagrams). two rising edges at conv, without any intervening rising edges at sck, put the ltc2355 -12/ ltc2355-14 in nap mode and the power consumption drops from 18mw to 4mw. the internal reference re- mains powered in nap mode. one or more rising edges at sck wake up the ltc2355 -12/ltc2355-14 very quickly, and conv can start an accurate conversion within a clock cycle. four rising edges at conv, without any intervening rising edges at sck, put the ltc2355 -12/ltc2355-14 in applicatio s i for atio wu u u v ref bypass 0805 size v dd bypass 0805 size optional input filtering
13 LTC2355-12/ltc2355-14 2355f sleep mode and the power consumption drops from 18mw to 13 w. one or more rising edges at sck wake up the ltc2355 -12/ltc2355-14 for operation. the internal reference (v ref ) takes 2ms to slew and settle with a 10 f load. note that, using sleep mode more frequently than every 2ms, compromises the settled accuracy of the internal reference. note that, for slower conversion rates, the nap and sleep modes can be used for substantial reductions in power consumption. digital interface the ltc2355 -12/ltc2355-14 has a 3-wire spi-compatible (serial protocol interface) interface. the sck and conv inputs and sdo output implement this interface. the sck and conv inputs accept swings from 3.3v logic and are ttl compatible, if the logic swing does not exceed v dd . a detailed description of the three serial port signals follows. conversion start input (conv) the rising edge of conv starts a conversion, but subsequent rising edges at conv are ignored by the ltc2355 -12/ltc2355-14 until the following 16 sck rising edges have occurred. it is necessary to have a minimum of 16 rising edges of the clock input sck between rising edges of conv. but to obtain maximum conversion speed (with a 63mhz sck), it is necessary to allow two more clock periods between conversions to allow 39ns of acqui- sition time for the internal adc sample-and-hold circuit. with 16 clock periods per conversion, the maximum conversion rate is limited to 3.5msps to allow 39ns for acquisition time. in either case, the output data stream comes out within the first 16 clock periods to ensure compatibility with processor serial ports. the duty cycle of conv can be arbitrarily chosen to be used as a frame sync signal for the processor serial port. a simple approach to generate conv is to create a pulse that is one sck wide to drive the ltc2355 -12/ltc2355-14 and then buffer this signal with the appropriate number of inverters to ensure the correct delay driving the frame sync input of the processor serial port. it is good practice to drive the ltc2355 -12/ltc2355-14 conv input first to avoid digital noise interference during the sample-to-hold transition triggered by conv at the start of conversion. it is also good practice to keep the width of the low portion of the conv signal greater than 15ns to avoid introducing glitches in the front end of the adc just before the sample-and-hold goes into hold mode at the rising edge of conv. minimizing jitter on the conv input in high speed applications where high amplitude sine waves above 100khz are sampled, the conv signal must have as little jitter as possible (10ps or less). the square wave output of a common crystal clock module usually meets this requirement. the challenge is to generate a conv signal from this crystal clock without jitter corrup- tion from other digital circuits in the system. a clock divider and any gates in the signal path from the crystal clock to the conv input should not share the same integrated circuit with other parts of the system. as shown in figure 6, the sck and conv inputs should be driven first, with digital buffers used to drive the serial port interface. also note that the master clock in the dsp may already be corrupted with jitter, even if it comes directly from the dsp crystal. another problem with high speed processor clocks is that they often use a low cost, low speed crystal (i.e., 10mhz) to generate a fast, but jittery, phase-locked-loop system clock (i.e., 40mhz). the jitter in these pll-generated high speed clocks can be several nanoseconds. note that if you choose to use the frame sync signal generated by the dsp port, this signal will have the same jitter of the dsps master clock. applicatio s i for atio wu u u
14 LTC2355-12/ltc2355-14 2355f the typical application figure on page 16 shows a circuit for level-shifting and squaring the output from an rf signal generator or other low-jitter source. a single d-type flip flop is used to generate the conv signal to the LTC2355-12/ltc2355-14. re-timing the master clock signal eliminates clock jitter introduced by the controlling device (dsp, fpga, etc.) both the inverter and flip flop must be treated as analog components and should be powered from a clean analog supply. serial clock input (sck) the rising edge of sck advances the conversion process and also udpates each bit in the sdo data stream. after conv rises, the third rising edge of sck starts clocking out the 12/14 data bits with the msb sent first. a simple approach is to generate sck to drive the ltc2355 -12/ ltc2355-14 first and then buffer this signal with the appropriate number of inverters to drive the serial clock input of the processor serial port. use the falling edge of the clock to latch data from the serial data output (sdo) into your processor serial port. the 14-bit serial data will be received right justified, in a 16-bit word with 16 or more clocks per frame sync. it is good practice to drive the ltc2355 -12/ltc2355-14 sck input first to avoid digital noise interference during the internal bit comparison decision by the internal high speed comparator. unlike the conv input, the sck input is not sensitive to jitter because the input signal is already sampled and held constant. serial data output (sdo) upon power-up, the sdo output is automatically reset to the high impedance state. the sdo output remains in high impedance until a new conversion is started. sdo sends out 12/14 bits in the output data stream beginning at the third rising edge of sck after the rising edge of conv. sdo is always in high impedance mode when it is not sending out data bits. please note the delay specification from sck to a valid sdo. sdo is always guaranteed to be valid by the next rising edge of sck. the 16-bit output data stream is compatible with the 16-bit or 32-bit serial port of most processors. loading on the sdo line must be minimized. sdo can directly drive most fast cmos logic inputs directly. how- ever, the general purpose i/o pins on many programmable logic devices (fpgas, cplds) and dsps have excessive capacitance. in these cases, a 100 ? resistor in series with sdo can isolate the input capacitance of the receiving device. if the receiving device has more than 10pf of input capacitance or is located far from the LTC2355-12/ ltc2355-14, an nc7svu04p5x inverter can be used to provide more drive. applicatio s i for atio wu u u
15 LTC2355-12/ltc2355-14 2355f u package descriptio msop (mse) 0603 0.53 0.152 (.021 .006) seating plane 0.18 (.007) 1.10 (.043) max 0.17 C 0.27 (.007 C .011) typ 0.127 0.076 (.005 .003) 0.86 (.034) ref 0.50 (.0197) bsc 12 3 45 4.90 0.152 (.193 .006) 0.497 0.076 (.0196 .003) ref 8 9 10 10 1 7 6 3.00 0.102 (.118 .004) (note 3) 3.00 0.102 (.118 .004) (note 4) note: 1. dimensions in millimeter/(inch) 2. drawing not to scale 3. dimension does not include mold flash, protrusions or gate burrs. mold flash, protrusions or gate burrs shall not exceed 0.152mm (.006") per side 4. dimension does not include interlead flash or protrusions. interlead flash or protrusions shall not exceed 0.152mm (.006") per side 5. lead coplanarity (bottom of leads after forming) shall be 0.102mm (.004") max 0.254 (.010) 0 C 6 typ detail a detail a gauge plane 5.23 (.206) min 3.20 C 3.45 (.126 C .136) 0.889 0.127 (.035 .005) recommended solder pad layout 0.305 0.038 (.0120 .0015) typ 2.083 0.102 (.082 .004) 2.794 0.102 (.110 .004) 0.50 (.0197) bsc bottom view of exposed pad option 1.83 0.102 (.072 .004) 2.06 0.102 (.081 .004) mse package 10-lead plastic msop (reference ltc dwg # 05-08-1663) 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 represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
16 LTC2355-12/ltc2355-14 2355f typical applicatio u lt/lwi 0207 ? printed in usa ? linear technology corporation 2007 linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com related parts low-jitter clock timing with rf sine generator using clock squaring/level shifting circuit and re-timing flip-flop part number description comments adcs ltc1402 12-bit, 2.2msps serial adc 5v or 5v supply, 4.096v or 2.5v span ltc1403/ltc1403a 12-/14-bit, 2.8msps serial adc 3v, 15mw, unipolar inputs, msop package ltc1403-1/ltc1403a-1 12-/14-bit, 2.8msps serial adc 3v, 15mw, bipolar inputs, msop package ltc1405 12-bit, 5msps parallel adc 5v, selectable spans, 115mw ltc1407/ltc1407a 12-/14-bit, 3msps simultaneous sampling adc 3v, 2-channel differential, unipolar inputs, 14mw, msop package ltc1407-1/ltc1407a-1 12-/14-bit, 3msps simultaneous sampling adc 3v, 2-channel differential, bipolar inputs, 14mw, msop package ltc1411 14-bit, 2.5msps parallel adc 5v, selectable spans, 80db sinad ltc1412 12-bit, 3msps parallel adc 5v supply, 2.5v span, 72db sinad lct1414 14-bit, 2.2msps parallel adc 5v supply, 2.5v span, 78db sinad ltc1420 12-bit, 10msps parallel adc 5v, selectable spans, 72db sinad ltc1604 16-bit, 333ksps parallel adc 5v supply, 2.5v span, 90db sinad ltc1608 16-bit, 500ksps parallel adc 5v supply, 2.5v span, 90db sinad ltc1609 16-bit, 250ksps serial adc 5v, configurable bipolar/unipolar inputs ltc1864/ltc1865 16-bit, 250ksps serial adcs 5v supply, 1 and 2 channel, 4.3mw, msop package ltc2356-12/ltc2356-14 12-/14-bit, 3.5msps serial adc 3.3v supply, 1.25v span, msop package dacs ltc1666/ltc1667/ltc1668 12-/14-/16-bit, 50msps dacs 87db sfdr, 20ns settling time ltc1592 16-bit, serial softspan tm i out dac 1lsb inl/dnl, software selectable spans references lt1790-2.5 micropower series reference in sot-23 0.05% initial accuracy, 10ppm drift lt1461-2.5 precision voltage reference 0.04% initial accuracy, 3ppm drift lt1460-2.5 micropower series voltage reference 0.1% initial accuracy, 10ppm drift softspan is a trademark of linear technology corporation. pre v cc 1k 1k 50 ? v cc nl17sz74 convert enable nc7svu04p5x master clock 0.1 f conv ltc2355 control logic (fpga, cpld, dsp, etc.) dq q conv sck sdo 100 ? nc7svu04p5x clr 2355 ta03

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