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1 for more information www.linear.com/ltc2668 block diagram features description 16-channel 16-/12-bit 10v v out softspan dacs with 10ppm/c max reference the lt c ? 2668 is a family of 16-channel, 16-/12-bit 10v digital-to-analog converters with integrated precision references. they are guaranteed monotonic and have built-in rail-to-rail output buffers. these softspan? dacs offer five output ranges up to 10v. the range of each channel is independently programmable, or the part can be hardware-configured for operation in a fixed range. the integrated 2.5 v reference is buffered separately to each channel; an external reference can be used for additional range options. the ltc2668 also includes a/b toggle capability via a dedicated pin or software toggle command . the spi/ microwire-compatible 3 -wire serial interface operates on logic levels as low as 1.71v at clock rates up to 50mhz. integral nonlinearity (ltc2668-16) a pplications n precision reference 10ppm/c max n independently programmable output ranges: 0v to 5v, 0v to 10v, 2.5v, 5v, 10v n full 16-bit/12-bit resolution at all ranges n maximum inl error: 4lsb at 16 bits n a/b toggle via software or dedicated pin n 16:1 analog multiplexer n guaranteed monotonic over temperature n internal or external reference n outputs drive 10ma guaranteed n 1.8v to 5v spi serial interface n 6mm 6mm 40-lead qfn package n optical networking n instrumentation n data acquisition n automatic test equipment n process control and industrial automation l, lt , lt c , lt m , linear technology and the linear logo are registered trademarks and softspan is a trademark of linear technology corporation. all other trademarks are the property of their respective owners. rgstr a internal reference rgstr b rgstr b control logic decode toggle select register monitor mux power-on reset rgstr a refcomp ovrtmp 2668 ta01a gnd 14, 37 reflo 13, 35 mux register span span dac 0 v ref v ref mux register span span dac 15 v ref rgstr a rgstr b rgstr b rgstr a mux register span span dac 7 ????? ????? ????? ????? ????? ????? ????? ????? mux register span span dac 8 v ref 34 38 v out0 v out1 v out2 v out3 2 3 4 5 v out4 v out5 v out6 v out7 6 7 8 9 cs/ld sck sdi sdo 16 17 19 18 mux 12 ref avp v + 10, 31 v ? 11, 32 33 36 v out13 v out12 v out11 v out10 28 27 v out15 v out14 30 29 26 25 v out9 v out8 clr ldac 24 23 21 15 tgp msp0 msp1 msp2 20 39 40 1 ovp 22 32-bit shift register code 10v range 0 ?4 inl (lsb) 0 ?1 ?2 ?3 1 2 3 4 16384 32768 49152 2668 ta01b 65535 ltc 2668 2668fa
2 for more information www.linear.com/ltc2668 p in c on f iguration ab solute m aximum r atings analog supply voltage ( avp ) ....................... C 0.3 v to 6v digital i/o voltage ( ovp ) .............................. C 0.3 v to 6v reflo ....................................................... C 0.3 v to 0.3 v v + ............................................................ C0.3 v to 16.5 v v C .. .......................................................... C 16.5 v to 0.3 v cs / ld , sck , sdi , ldac , clr , tgp .............. C 0.3 v to 6v msp 0, msp 1, msp 2 ....... C 0.3 v to min ( avp + 0.3 v, 6v) v out 0 to v out 15 , mux ... v C C 0.3 v to v + + 0.3 v ( max 16.5 v) ref , refcomp ............... C 0.3 v to min ( avp + 0.3 v, 6v) sdo ................................ C 0.3 v to min ( ovp + 0.3 v, 6v) ovrtmp ....................................................... C 0.3 v to 6v operating temperature range ltc 26 68 c ................................................ 0 c to 70 c ltc 26 68 i ............................................. C 40 c to 85 c ltc 26 68 h .......................................... C 4 0 c to 125 c maximum junction temperature .......................... 15 0 c storage temperature range .................. C 65 c to 150 c (notes 1, 2) v out15 v out14 v out13 v out12 v out11 v out10 v out9 v out8 ovp clr v ? mux reflo gnd ldac cs/ld sck sdo sdi tgp 39 40 38 37 36 35 34 33 32 31 11 20 12 13 14 15 top view 41 v ? uj package 40-lead (6mm 6mm) plastic qfn 16 17 18 19 22 23 24 25 26 27 28 29 9 8 7 6 5 4 3 2 msp2 v out0 v out1 v out2 v out3 v out4 v out5 v out6 v out7 v + msp1 msp0 ovrtmp gnd avp reflo refcomp ref v ? v + 21 30 10 1 t jmax = 150c, ja = 33c/w, jc = 2c/w exposed pad is v C , must be soldered to pcb ltc 2668 2668fa
3 for more information www.linear.com/ltc2668 p ro d uct s election g ui d e o r d er i n f ormation lead free finish tape and reel part marking package description temperature range ltc2668cuj-16#pbf ltc2668cuj-16#trpbf ltc2668uj-16 40-lead (6mm 6mm) qfn 0c to 70c ltc2668iuj-16#pbf ltc2668iuj-16#trpbf ltc2668uj-16 40-lead (6mm 6mm) qfn C40c to 85c ltc2668huj-16#pbf ltc2668huj-16#trpbf ltc2668uj-16 40-lead (6mm 6mm) qfn C40c to 125c ltc2668cuj-12#pbf ltc2668cuj-12#trpbf ltc2668uj-12 40-lead (6mm 6mm) qfn 0c to 70c ltc2668iuj-12#pbf ltc2668iuj-12#trpbf ltc2668uj-12 40-lead (6mm 6mm) qfn C40c to 85c ltc2668huj-12#pbf ltc2668huj-12#trpbf ltc2668uj-12 40-lead (6mm 6mm) qfn C40c to 125c ltc2668 c uj 16 #tr pbf lead free designator pbf = lead free tape and reel tr = 2000-piece tape and reel resolution 16 = 16-bit 12 = 12-bit p ackage t ype uj = 40-lead qfn temperature grade c = commercial temperature range (0c to 70c) i = industrial temperature range (C40c to 85c) h = automotive temperature range (C40c to 125c) product p ar t number consult ltc marketing for information on nonstandard lead based finish parts. for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ ltc 2668 2668fa
4 for more information www.linear.com/ltc2668 e lectrical c haracteristics the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25c. avp = 5v, ovp = 5v, v + = 15v, v C = C15v, v ref = 2.5v, v out unloaded unless otherwise specified. symbol parameter conditions min typ max units v out output voltage swing to v C (unloaded, v C = gnd) to v + (unloaded, v + = 5v) to v C (C10ma i out 10ma) to v + (C10ma i out 10ma) l l v + C 1.4 v C + 0.004 v + C 0.004 v C + 1.4 v v v v load regulation C10 ma i out 10ma (note 4) l 78 150 v/ma r out dc output impedance C10ma i out 10ma (note 4) l 0.078 0.15 dc crosstalk (note 5) 0v to 5v range due to full-scale output change due to load current change due to powering down (per channel) 1 2 4 v v/ma v i sc v + / v C short-circuit output current (note 6) avp = 5.5v, v + / v C = 15.75v, v ref = 2.5v, 10v output range code : zero-scale; forcing output to gnd code: full-scale; forcing output to gnd l l 16 C40 42 C14.5 ma ma reference reference output voltage 2.495 2.5 2.505 v reference temperature coefficient ( note 7) 2 10 ppm/c reference line regulation avp 10% 50 v/v reference short-circuit current avp = 5.5v, forcing output to gnd l 5 ma ltc2668-16/ltc2668-12 symbol parameter conditions ltc2668-12 ltc2668-16 units min typ max min typ max dc performance resolution l 12 16 bits monotonicity all ranges (note 3) l 12 16 bits dnl differential nonlinearity all ranges (note 3) l 0.05 0.5 0.2 1 lsb inl integral nonlinearity all ranges (note 3) v + / v C = 15v v C = gnd (note 3) c-grade, i-grade h-grade l l l 0.2 0.2 0.2 1 1 1 2.2 2.2 2.2 4 4 5 lsb lsb lsb v os unipolar offset error 0v to 5v range 0v to 10v range l l 1 2 2 4 1 2 2 4 mv mv v os temperature coefficient all unipolar ranges 1 1 ppm/c zse single-supply zero-scale error all unipolar ranges, v C = gnd l 2 4 2 4 mv bze bipolar zero error all bipolar ranges l 0.02 0.08 0.02 0.08 %fsr bze temperature coefficient all bipolar ranges 1 1 ppm/c ge gain error all ranges, external reference l 0.02 0.08 0.02 0.08 %fsr gain temperature coefficient 2 2 ppm/c psr power supply rejection all ranges avp = 5v, 10% v + /v C = 15v, 5% 0.1 0.001 1 0.01 lsb/v lsb/v ltc 2668 2668fa
5 for more information www.linear.com/ltc2668 e lectrical c haracteristics the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25c. avp = 5v, ovp = 5v, v + = 15v, v C = C15v, v ref = 2.5v, v out unloaded unless otherwise specified. symbol parameter conditions min typ max units refcomp pin short-circuit current avp = 5.5v, forcing output to gnd l 200 a reference load regulation avp = 5v 10%, i out = 100a sourcing 140 mv/ma reference output voltage noise density c refcomp = c ref = 0.1f, at f = 10khz 32 nv/ hz reference input range external reference mode (note 8) l 0.5 avp C 1.75 v reference input current external reference l 0.001 1 a reference input capacitance ( note 9) l 40 pf power supply avp analog supply voltage l 4.5 5.5 v v + analog positive supply l 4.5 15.75 v v C analog negative supply v C not tied to gnd v C tied to gnd l C15.75 0 C4.5 v v ovp digital i /o supply voltage l 1.71 avp + 0.3 v i avp supply current avp avp = 5v, unipolar ranges (note 10) avp = 5v, bipolar ranges (note 10) l l 5.4 9.4 6.5 12 ma ma i s supply current v + / v C unipolar ranges (code = 0) bipolar ranges (note 11) l l 4.6 8 6.5 9.5 ma ma i ovp supply current ovp (note 12) ovp = 5v l 0.02 1 a avp shutdown supply current ovp = avp = 5v, v + / v C = 15v l 1 3 a v + shutdown supply current ovp = avp = 5v, v + / v C = 15v l 35 70 a v C shutdown supply current ovp = avp = 5v, v + / v C = 15v l C60 C27 a monitor mux monitor mux dc output impedance 2.2 k monitor mux leakage current monitor mux disabled (high impedance) l 0.02 1 a monitor mux output voltage range monitor mux selected to dac channel l v C v + C 1.4 v monitor mux continuous current (note 9) l 1 ma ac performance t set settling time (notes 9, 13) 0v to 5v or 2.5v span, 5v step 0.024% (1lsb at 12 bits) 0.0015% (1lsb at 16 bits) 4.5 9 s s settling time (notes 9, 13) 0v to 10v or 5v span, 10v step 0.024% (1lsb at 12 bits) 0.0015% (1lsb at 16 bits) 8 9 s s settling time (notes 9, 13) 10v span, 20v step 0.024% (1lsb at 12 bits) 0.0015% (1lsb at 16 bits) 15.5 20.5 s s sr voltage output slew rate 5 v/s capacitive load driving 1000 pf glitch impulse (note 14) at mid-scale transition, 0v to 5v range 8 nv ? s dac-to-dac crosstalk (note 15) due to full-scale output change 6 nv ? s e n output voltage noise 0v to 5v output span, internal reference density at f = 1khz density at f = 10khz 0.1hz to 10hz, internal reference 0.1hz to 200khz, internal reference 90 80 1.7 55 nv / hz nv/ hz v rms v rms ltc 2668 2668fa
6 for more information www.linear.com/ltc2668 e lectrical c haracteristics t iming c haracteristics the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25c. avp = 5v, ovp = 5v, v + = 15v, v C = C15v, v ref = 2.5v, v out unloaded unless otherwise specified. the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25c. digital input low and high voltages are 0v and ovp, respectively. ltc2668-16/ltc2668-12 symbol parameter conditions min typ max units avp = 4.5v to 5.5v, ovp = 2.7v to avp t1 sdi valid to sck setup l 6 ns t2 sdi valid to sck hold l 6 ns t3 sck high time l 9 ns t4 sck low time l 9 ns t5 cs/ld pulse width l 10 ns t6 lsb sck high to cs/ld high l 7 ns t7 cs/ld low to sck high l 7 ns t8 sdo propagation delay from sck falling edge c load = 10pf ovp = 4.5v to avp ovp = 2.7v to 4.5v l l 20 30 ns ns t 9 clr pulse width l 20 ns t10 cs/ld high to sck positive edge l 7 ns t12 ldac pulse width l 15 ns t13 cs/ld high to ldac high or low transition l 15 ns sck frequency 50% duty cycle l 50 mhz t14 tgp high time (note 9) l 1 s t15 tgp low time (note 9) l 1 s symbol parameter conditions min typ max units digital i/o v oh digital output high voltage sdo pin. load current = C100a l ovp C 0.2 v v ol digital output low voltage sdo pin. load current = 100a ovrtmp pin. load current = 100a l l 0.2 0.2 v v i oz digital hi-z output leakage sdo pin leakage current ( cs/ld high) ovrtmp pin leakage current (not asserted) l l 1 1 a a i lk digital input leakage v in = gnd to ovp l 1 a c in digital input capacitance (note 9) l 8 pf ovp = 2.7v to avp v ih digital input high voltage l 0.8 ? ovp v v il digital input low voltage l 0.5 v ovp = 1.71v to 2.7v v ih digital input high voltage l 0.8 ? ovp v v il digital input low voltage l 0.3 v ltc 2668 2668fa
7 for more information www.linear.com/ltc2668 ltc2668-16/ltc2668-12 symbol parameter conditions min typ max units avp = 4.5v to 5.5v, ovp = 1.71v to 2.7v t1 sdi valid to sck setup l 7 ns t2 sdi valid to sck hold l 7 ns t3 sck high time l 30 ns t4 sck low time l 30 ns t5 cs/ld pulse width l 15 ns t6 lsb sck high to cs/ld high l 7 ns t7 cs/ld low to sck high l 7 ns t8 sdo propagation delay from sck falling edge c load = 10pf l 60 ns t9 clr pulse width l 30 ns t10 cs/ld high to sck positive edge l 7 ns t12 ldac pulse width l 15 ns t13 cs/ld high to ldac high or low transition l 15 ns sck frequency 50% duty cycle l 15 mhz t14 tgp high time (note 9) l 1 s t15 tgp low time (note 9) l 1 s t iming c haracteristics the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25c. digital input low and high voltages are 0v and ovp, respectively. 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 voltages are with respect to gnd note 3: for v C = gnd, linearity is defined from code k l to code 2 n C 1, where n is the resolution and k l is the lower end code for which no output limiting occurs. for v ref = 2.5v and n = 16, k l = 128 and linearity is defined from code 128 to code 65,535. for v ref = 2.5v and n = 12, k l = 8 and linearity is defined from code 8 to code 4095. note 4: 4.5v v + 16.5v; C16.5v v C C4.5v or v C = gnd. v out is at least 1.4v below v + and 1.4v above v C . note 5: dc crosstalk is measured with avp = 5v, using the internal reference. the conditions of one dac channel are changed as specified, and the output of an adjacent channel (at mid-scale) is measured before and after the change. note 6: this ic includes current limiting that is intended to protect the device during momentary overload conditions. junction temperature can exceed the rated maximum during current limiting. continuous operation above the specified maximum operating junction temperature may impair device reliability. note 7: temperature coefficient is calculated by first computing the ratio of the maximum change in output voltage to the nominal output voltage. the ratio is then divided by the specified temperature range. note 8: gain-error and bipolar zero error specifications may be degraded for reference input voltages less than 1.25v. see the gain error vs reference input and bipolar zero vs reference input curves in the typical performance characteristics section. note 9: guaranteed by design and not production tested. note 10: internal reference on. note 11: i(v + ) measured in 10v span; outputs unloaded; all channels at full scale. i(v C ) measured in 10v span; outputs unloaded; all channels at negative full scale. each dac amplifier is internally loaded by a 40k? feedback network, so supply currents increase as output voltages diverge from 0v . note 12: digital inputs at 0v or ovp. note 13: internal reference mode. load is 2k in parallel with 100pf to gnd. note 14: avp = 5v, 0v to 5v range, internal reference mode. dac is stepped 1lsb between half-scale and half-scale C 1lsb. load is 2k in parallel with 200pf to gnd. note 15: dac-to-dac crosstalk is the glitch that appears at the output of one dac due to full-scale change at the output of another dac. 0v to 10v range with internal reference. the measured dac is at mid-scale. ltc 2668 2668fa
8 for more information www.linear.com/ltc2668 t ypical p er f ormance c haracteristics dnl vs temperature gain error vs temperature bipolar zero error vs temperature settling 5v step settling 10v step t a = 25c, unless otherwise noted. ltc2668-16 integral nonlinearity (inl) differential nonlinearity (dnl) inl vs temperature code 10v range 0 ?4 inl (lsb) 0 ?1 ?2 ?3 1 2 3 4 16384 32768 49152 2668 g01 65535 inl vs output range code 0 ?1 dnl (lsb) ?0.4 ?0.2 ?0.6 ?0.8 0.4 0.2 0.6 0.8 1 16384 32768 49152 2668 g02 65535 0 10v range temperature (c) ?40 ?4 inl (lsb) 0 ?1 ?2 ?3 2 1 3 4 ?20 0 100 2668 g03 120 60 80 20 40 inl (neg) inl (pos) 10v range temperature (c) ?40 ?0.08 bze (%fs) 0 ?0.04 ?0.02 0.02 ?0.06 0.06 0.04 0.08 ?20 0 100 2668 g06 120 60 80 20 40 5v range 10v range 2.5v range 5s/div voltage 2668 g09 0v to 10v range; internal reference rising 10v step; average of 64 events. falling settling is similar or better. subtract 100ns fixture delay from settling waveform v out residual 500v/div t settle = 8.9s v out = 2v/div cs/ld temperature (c) ?40 ?1.0 dnl (lsb) 0 ?0.4 ?0.2 0.2 ?0.6 ?0.8 0.6 0.4 0.8 1.0 ?20 0 100 2668 g04 120 60 80 20 40 dnl (neg) dnl (pos) 10v range temperature (c) ?40 ?0.08 ge (%fs) 0 ?0.04 ?0.02 0.02 ?0.06 0.06 0.04 0.08 ?20 0 100 2668 g05 120 60 80 20 40 0v to 5v range 0v to 10v range 5v range 10v range 2.5v range output range (v) ?4 inl (lsb) 0 ?1 ?2 ?3 2 1 3 4 2668 g07 2.5v 5v 10v 0v to 5v 0v to 10v 2s/div voltage 2668 g08 cs/ld v out residual 500v/div t settle = 9s v out = 1v/div 0v to 5v range; internal reference rising 5v step; average of 64 events. falling settling is similar or better. subtract 100ns fixture delay from settling waveform ltc 2668 2668fa
9 for more information www.linear.com/ltc2668 t ypical p er f ormance c haracteristics integral nonlinearity (inl) (ltc2668-12) differential nonlinearity (dnl) (ltc2668-12) single-supply zero-scale error vs temperature gain error vs reference input bipolar zero error vs reference input settling 20v step ltc2668-16/ltc2668-12 t a = 25c, unless otherwise noted. reference output vs temperature unipolar offset vs temperature headroom to v C rail vs output current 10s/div voltage 2668 g10 10v range; internal reference rising 20v step; average of 64 events. falling settling is similar or better. subtract 100ns fixture delay from settling waveform. t settle = 20.2s v out residual 500v/div v out = 5v/div cs/ld code 10v range 0 ?1.0 inl (lsb) 0 ?0.2 ?0.4 ?0.6 ?0.8 0.4 0.2 0.6 0.8 1.0 1024 2048 3072 2668 g11 4095 code 0 ?0.5 dnl (lsb) 0 ?0.1 ?0.2 ?0.3 ?0.4 0.2 0.1 0.3 0.4 0.5 1024 2048 3072 2668 g12 4095 10v range v ref (v) 0.5 ?0.1 ge (%fs) 0 0.05 ?0.05 0.10 0.15 1 1.5 2668 g13 3.53 2 2.5 10v range 16 channels v ref (v) 0.5 ?0.15 bze (%fs) ?0.05 0 ?0.10 0.10 0.10 1 1.5 2668 g14 3.53 2 2.5 10v range 16 channels temperature (c) ?40 0 zse (mv) 1 3 2 4 ?20 0 100 2668 g15 120 60 80 20 40 0v to 5v range v avp = 5v v(v + ) = 5v v(v ? ) = 0v temperature (c) ?40 2.494 v ref (v) 2.496 2.504 2.502 2.500 2.498 2.506 ?20 0 100 2668 g16 120 60 80 20 40 temperature (c) ?40 ?4 v os (mv) ?3 3 2 1 0 ?1 ?2 4 ?20 0 100 2668 g17 120 60 80 20 40 0v to 10v range 0v to 5v range output current sinking (ma) 0 0 v out delta above v ? (v) 0.2 0.8 0.6 0.4 1.0 2 4 2668 g18 10 6 8 0v to 5v range; v + = 4.5v, v ? = gnd 5v range; v + /v ? = 4.5v code: zero-scale ltc 2668 2668fa
10 for more information www.linear.com/ltc2668 t ypical p er f ormance c haracteristics v + /v C shutdown current vs symmetric supplies i ovp supply current vs logic voltage hardware clr to mid-scale hardware clr to zero-scale headroom to v + rail vs output current unipolar offset vs reference input t a = 25c, unless otherwise noted. ltc2668-16/ltc2668-12 unipolar offset vs reference input avp supply current vs bipolar output voltage avp shutdown current vs avp output current sourcing (ma) 0 3.5 v out (v) 3.7 4.3 4.5 3.9 4.1 4.7 ?2 ?4 2668 g19 ?10 ?6 ?8 0v to 5v range; v + = 4.5v, v ? = gnd 5v range; v + /v ? = 4.5v code: full-scale v ref (v) 0.5 ?2.0 v os (mv) ?1.0 1.0 ?1.5 1.5 0 0.5 2.0 1 1.5 2668 g20 3.53 2 2.5 0v to 5v range 16 channels v ref (v) 0.5 ?4 ?3 v os (mv) ?1 2 ?2 3 0 1 4 1 1.5 2668 g20 3.53 2 2.5 0v to 10v range 16 channels v out (v) ?10 0 i avp (ma) 2 8 10 4 6 12 ?7.5 ?5 2.5 2668 g22 10 5 ?2.5 0 7.5 10 range 5v range 2.5v range all channels at same code outputs unloaded v avp (v) 4.4 0 i avp (a) 0.2 0.8 0.4 0.6 1.0 4.6 4.8 2668 g23 5.4 5 5.2 v + /v ? (v) 4 ?40 v + /v ? shutdown current (a) ?30 0 10 20 30 ?20 ?10 40 10 2668 g24 16 12 6 8 14 i(v + ) shutdown i(v ? ) shutdown input logic voltage (v) avp = 5v sck, sdi, cs/ld, clr, ldac, tgp tied together 0 ?0.1 i ovp (ma) 0 0.3 0.4 0.5 0.1 0.2 0.6 3 2668 g25 54 1 2 ovp = 1.8v ovp = 3.3v ovp = 5v 2s/div v out 5v/div clr 2668 g26 from full-scale from zero-scale 10v range 2s/div v out 1v/div clr 2668 g27 0v to 5v range ltc 2668 2668fa
11 for more information www.linear.com/ltc2668 t ypical p er f ormance c haracteristics noise density vs frequency output 0.1hz to 10hz voltage noise reference 0.1hz to 10hz voltage noise load regulation mid-scale glitch impulse dac-to-dac crosstalk large signal response t a = 25c, unless otherwise noted. ltc2668-16/ltc2668-12 1s/div ovp, avp: 5v v + , v ? : 15v 0v to 10v range internal reference c ref , c refcomp : 0.1f cs/ld v out 10mv/div 2668 g29 subject channel: v out0 aggressor channel: v out1 10v to 0v step v out1 rising is similar or better all channels are similar or better 6nv-s typ 10s/div ?15 v out (v) ?10 5 10 ?5 0 15 2668 g30 0v to 10v range 10v range 0v to 5v range frequency (hz) 10 0 noise density (nv/hz) 400 300 200 100 500 100 1k 10k 100k 2668 g31 1m 1s/div avp = 5v, v + , v ? = 15v 0v to 5v range code = mid-scale internal reference c ref = c refcomp = 0.1f v out 10v/div 2668 g32 1s/div avp = 5v, v + , v ? = 15v v ref = 2.5v c ref = c refcomp = 0.1f v ref 10v/div 2668 g33 v out load current (ma) ?30 ?10 ?v out (mv) 2 8 4 6 ?8 ?2 0 ?6 ?4 10 ?20 ?10 10 2668 g34 40 20 0 30 0v to 5v range, avp, v + = 5v, v ? = gnd 10v range, avp = 5v, v + /v ? = 15v 78v/ma typ code: mid-scale internal ref ltc 2668 2668fa ovp, avp: 5v v + /v ? : 15v 0v to 5v range internal reference c ref , c refcomp : 0.1f 8nv?s typ 1s/div v out 10mv/div cs /ld 2668 g28 falling major carry transition rising transition is similar or better all channels are similar or better
12 for more information www.linear.com/ltc2668 p in functions msp2 (pin 1): mspan bit 2. tie this pin to avp or gnd to select the power-on span and power-on-reset code for all 16 channels (see table 4). v out0 to v out15 (pins 2-9, 23-30): dac analog voltage outputs. v + (pins 10, 31): analog positive supply. typically 15v; 4.5v to 15.75v range. bypass to gnd with a 1f capacitor. v C (pins 11, 32, 41): analog negative supply. typically C15v; C4.5 v to C15.75v range, or can be tied to gnd. bypass to gnd with a 1 f capacitor unless v C is con - nected to gnd. mux ( pin 12): analog multiplexer output. any of the 16 dac outputs can be internally routed to the mux pin. when the mux is disabled, this pin becomes high impedance. reflo (pins 13, 35): reference low pins. signal ground for all dac channels and internal reference. these pins should be tied to gnd. gnd (pins 14, 37): analog ground. tie to a clean analog ground plane. ldac (pin 15): active-low asynchronous dac update pin. if cs/ld is high, a falling edge on ldac immediately updates all dac registers with the contents of the input registers ( similar to a software update). if cs/ld is low when ldac goes low, the dac registers are updated after cs/ld returns high. a low on the ldac pin powers up the dacs. a software power-down command is ignored if ldac is low. logic levels are determined by ovp. tie ldac high (to ovp) if not used. updates can then be performed through spi commands (see table 1). cs /ld (pin 16): serial interface chip select/load input. when cs/ld is low, sck is enabled for shifting data on sdi into the register . when cs/ ld is taken high, sck is disabled and the specified command ( see table 1) is executed. logic levels are determined by ovp. sck (pin 17): serial interface clock input . logic levels are determined by ovp. sdo (pin 18): serial interface data output . the serial output of the 32-bit shift register appears at the sdo pin. the data transferred to the device via the sdi pin is delayed 32 sck rising edges before being output at the next falling edge. can be used for data echo readback or daisy-chain operation (pull-up/down resistor required). the sdo pin becomes high impedance when cs/ld is high. logic levels are determined by ovp. sdi (pin 19): serial interface data input . data on sdi is clocked into the dac on the rising edge of sck. the ltc2668 accepts input word lengths of either 24 or 32 bits. logic levels are determined by ovp. tgp (pin 20): asynchronous toggle pin . a falling edge updates the dac register with data from input register ?a. a rising edge updates the dac register with data from input register b. toggle operations only affect those dac channels with their toggle select bit (tx) set to 1. tie the tgp pin to ovp if toggle operations are to be done through software. tie the tgp pin to gnd if not using toggle opera - tions. logic levels are determined by ovp. clr (pin 21): active-low asynchronous clear input. a logic low at this level-triggered input clears the part to the reset code and range determined by the hardwired option chosen using the mspan pins and specified in table 4. the control registers are cleared to zero. logic levels are determined by ovp. ovp ( pin 22): digital input/ output supply voltage . 1.71v ovp avp + 0.3v. bypass to gnd with a 0.1f capacitor. ref (pin 33): reference in/out. the voltage at the ref pin sets the full-scale range of all channels . by default, the internal reference is routed to this pin . must be buffered when driving external dc load currents . if the reference is disabled ( see reference modes in the operation section), its output is disconnected and the ref pin becomes a high impedance input to which you may apply a precision external reference . for low noise and reference stability , tie a capacitor from this pin to gnd. the value must be c refcomp , where c refcomp is the capacitance tied to the refcomp pin. the allow - able external reference input voltage range is 0.5 v to v avp C 1.75v. ltc 2668 2668fa
13 for more information www.linear.com/ltc2668 p in functions refcomp ( pin 34): internal reference compensation pin. for low noise and reference stability, tie a 0.1f capacitor to gnd. tying refcomp to gnd causes the part to power up with the internal reference disabled , allowing the use of an external reference at start-up. avp (pin 36): analog supply voltage input . 4.5v avp 5.5v. bypass to gnd with a 1f capacitor. ovrtmp (pin 38): thermal protection interrupt pin. this open-drain n-channel output pulls low when chip tem - perature exceeds 160c. this pin is released on the next cs/ld rising edge. a pull-up resistor is required. msp 0 (pin 39): mspan bit 0. tie this pin to avp or gnd to select the power-on span and power-on-reset code for all 16 channels (see table 4). msp1 (pin 40): mspan bit 1. tie this pin to avp or gnd to select the power-on span and power-on-reset code for all 16 channels (see table 4). exposed pad (pin 41): analog negative supply (v C ). must be soldered to pcb. ltc 2668 2668fa
14 for more information www.linear.com/ltc2668 block diagram rgstr a internal reference rgstr b rgstr b control logic decode toggle select register monitor mux power-on reset rgstr a refcomp ovrtmp 2668 bd gnd 14, 37 reflo 13, 35 mux register span span dac 0 v ref v ref mux register span span dac 15 v ref rgstr a rgstr b rgstr b rgstr a mux register span span dac 7 ????? ????? ????? ????? ????? ????? ????? ????? mux register span span dac 8 v ref 34 38 v out0 v out1 v out2 v out3 2 3 4 5 v out4 v out5 v out6 v out7 6 7 8 9 cs/ld sck sdi sdo 16 17 19 18 mux 12 ref avp v + 10, 31 v ? 11, 32 33 36 v out13 v out12 v out11 v out10 28 27 v out15 v out14 30 29 26 25 v out9 v out8 clr ldac 24 23 21 15 tgp msp0 msp1 msp2 20 39 40 1 ovp 22 32-bit shift register ltc 2668 2668fa
15 for more information www.linear.com/ltc2668 the ltc2668 is a family of 16-channel, 10v digital-to- analog converters with selectable output ranges and an integrated precision reference. the dacs operate on posi - tive 5 v and bipolar 15 v supplies. the bipolar supplies can operate as low as 4.5v, and need not be symmetrical. in addition , the negative v C supply can be operated at ground , making the parts compatible with single-supply systems . the outputs are driven by the bipolar supply rails. the output amplifiers offer true rail-to-rail operation. when drawing a load current from the v + or v C rail , the output voltage headroom with respect to that rail is limited by the 60 typical channel resistance of the output devices. see the graph, headroom at rails vs output current, in the typical performance characteristics section. the ltc2668 is controlled using a cascadable 3-wire spi/ microwire-compatible interface with echo readback. power-on reset the outputs reset when power is first applied , making system initialization consistent and repeatable. by tying the mspan pins (msp2, msp1, msp0) to gnd and/or avp , you can select the initial output range and reset code ( zero- or mid-scale), as well as selecting between a manual (fixed) range and softspan operation . see table ?4 for pin configurations and available options. t iming diagram o peration power supply sequencing and start-up the supplies ( avp , ovp, v + and v C ) may be powered up in any convenient order. if an external reference is used, the voltage at ref should be kept within the range C0.3v v ref avp + 0.3v (see the absolute maximum ratings section). particular care should be taken to observe these limits during power supply turn-on and turn-off sequences when the voltage at avp is in transition. supply bypassing is critical to achieving the best possible performance. we recommend at least 1f to ground on avp , v + and v C supplies, and at least 0.1f of low esr capacitance for each supply, as close to the device as possible. the larger capacitor may be omitted for ovp. hot-plugging or hard switching of supplies is not recom - mended, as power supply cable or trace inductances combined with bypass capacitances can cause supply voltage transients beyond absolute maximum ratings , even if the bench supply has been carefully current- / voltage-limited. during start-up, limit the supply inrush currents to no more than 5a and supply slew rates to no more than 5v/s. internal protection circuitry can be dam - aged and long-term reliability adversely affected if these requirements are not met . figure 1. serial interface timing sdi cs/ld sck 2668 f01 t 2 t 10 t 5 t 7 t 6 t 1 t 3 t 4 1 2 3 23 24 ltc 2668 2668fa
16 for more information www.linear.com/ltc2668 o peration serial interface when the cs/ld pin is taken low, the data on the sdi pin is loaded into the shift register on the rising edge of the clock (sck pin). the 4-bit command, c3-c0, is loaded first, followed by the 4-bit dac address, a3-a0, and finally the 16 -bit data word in straight binary format. for the ltc2668-16, the data word comprises the 16-bit input code, ordered msb-to-lsb. for the ltc2668-12, the data word comprises the 12-bit input code, ordered msb-to-lsb, followed by four dont-care bits. data can only be transferred to the ltc2668 when the cs/ld signal is low. the rising edge of cs/ld ends the data transfer and causes the device to carry out the action specified in the 24 -bit input word. the complete sequence is shown in figure 3a. table 1. command codes command c3 c2 c1 c0 0 0 0 0 write code to n 1 0 0 0 write code to all 0 1 1 0 write span to n 1 1 1 0 write span to all 0 0 0 1 update n (power up) 1 0 0 1 update all (power up) 0 0 1 1 write code to n , update n (power up) 0 0 1 0 write code to n , update all (power up) 1 0 1 0 write code to all , update all (power up) 0 1 0 0 power down n 0 1 0 1 power down chip (all dacs, mux and reference) 1 0 1 1 monitor mux 1 1 0 0 toggle select 1 1 0 1 global toggle 0 1 1 1 config 1 1 1 1 no operation table 2. dac addresses, n address a3 a2 a1 a0 0 0 0 0 dac 0 0 0 0 1 dac 1 0 0 1 0 dac 2 0 0 1 1 dac 3 0 1 0 0 dac 4 0 1 0 1 dac 5 0 1 1 0 dac 6 0 1 1 1 dac 7 1 0 0 0 dac 8 1 0 0 1 dac 9 1 0 1 0 dac 10 1 0 1 1 dac 11 1 1 0 0 dac 12 1 1 0 1 dac 13 1 1 1 0 dac 14 1 1 1 1 dac 15 data transfer functions the dac input-to-output transfer functions for all output ranges and resolutions are shown in figures 2a and 2b. the input code is in straight binary format for all ranges . straight binary code (decimal equivalent) 2.5v internal reference 0 ?10 v out (v) 0 ?2.5 ?5 ?7.5 2.5 5 7.5 10 16384 32768 49152 2668 f01a 65535 0v to 5v range 0v to 10v range 5v range 10v range 2.5v range straight binary code (decimal equivalent) 2.5v internal reference 0 ?10 v out (v) 0 ?2.5 ?5 ?7.5 2.5 5 7.5 10 1024 2048 3072 2668 f02b 4095 0v to 5v range 0v to 10v range 5v range 10v range 2.5v range figure 2a. ltc2668-16 transfer function figure 2b. ltc2668-12 transfer function ltc 2668 2668fa
17 for more information www.linear.com/ltc2668 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 c2 c1 c0 a3 a2 a1 a0 d15 d14 d13 d12 d11 d10 d9 d8 d7 d6 d5 d4 d3 d2 d1 d0 c3xxxxxxxx cs/ld sck sdi command word address word data word don?t care 32-bit input word c2 c1 c0 a3 a2 a1 a0 d15 d14 d13 d12 d11 d10 d9 d8 d7 d6 d5 d4 d3 d2 d1 d0 c3xxxxxxxx sdo current 32-bit input word (hi-z) (hi-z) x 2668 f03b previous 32-bit input word t 2 t 3 t 4 t 1 t 8 d15 17 sck sdi sdo previous d14 previous d15 18 d14 3a. ltc2668-16 24-bit load sequence (minimum input word). ltc2668-12 sdi data word is 12-bit input code + 4 dont-care bits 3b. ltc2668-16 32-bit load sequence. ltc2668-12 sdi/sdo data word is 12-bit input code + 4 dont-care bits figure 3. ltc2668 load sequences o peration 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 c2 c1 c0 a3 a2 a1 a0 d15 d14 d13 d12 d11 d10 d9 d8 d7 d6 d5 d4 d3 d2 d1 d0 c3 cs/ld sck sdi command word address word data word 24-bit input word 2668 f03a ltc 2668 2668fa
18 for more information www.linear.com/ltc2668 figure 4. write span syntax o peration while the minimum input word is 24 bits, it may option- ally be extended to 32 bits. to use the 32-bit word width, 8 don t-care bits must be transferred to the device first, followed by the 24-bit word, as just described. figure 3b shows the 32-bit sequence. the 32-bit word is required for echo readback and daisy-chain operation, and is also available to accommodate processors that have a minimum word width of 16 or more bits. input and dac registers the ltc2668 has five internal registers for each dac, in addition to the main shift register (see the block diagram). each dac channel has two sets of double-buffered reg - isters: one set for the code data, and one set for the span ( output range ) of the dac. double buffering provides the capability to simultaneously update the span and code, which allows smooth voltage transitions when changing output ranges. it also permits the simultaneous updating of multiple dacs. each set of double-buffered registers comprises an input register and a dac register: ? input register: the write operation shifts data from the sdi pin into a chosen input register. the input registers are holding buffers; write operations do not affect the dac outputs. in the code data path, there are two input registers, a and b, for each dac register. register b is an alternate input register used only in the toggle operation, while register a is the default input register ( see the block diagram). ? dac register : the update operation copies the contents of an input register to its associated dac register. the content of a dac register directly controls the dac output voltage or range. the update operation also powers up the selected dac if it had been in power- down mode. the data path and registers are shown in the block diagram. note that updates always refresh both code and span data, but the values held in the dac registers remain unchanged unless the associated input register values have been changed via a write operation . for example, if you write a new code and update the channel , the code is updated, while the span is refreshed unchanged. a channel update can come from a serial update com - mand, an ldac negative pulse, or a toggle operation. table 3. write span code s2 s1 s0 output range internal reference external reference 0 0 0 0v to 5v 0v to 2v ref 0 0 1 0v to 10v 0v to 4v ref 0 1 0 5v 2v ref 0 1 1 10v 4v ref 1 0 0 2.5v v ref output ranges the ltc2668 is a 16-channel dac with selectable output ranges. ranges can either be programmed in software or hardwired through pin strapping. softspan operation softspan operation (ranges controlled through the serial interface) is invoked by tying all three mspan pins (msp2, msp1 and msp0) to avp ( see table 4). in softspan con - figuration, all channels initialize to zero-scale in 0v to 5v range at power -on . the range and code of each channel are then fully programmable. each channel has a set of double-buffered registers for range information ( see the block diagram). program the span input register using the write span n or write span all commands (0110 b and 1110 b, respectively). figure 4 shows the syntax, and table 3 shows the span codes and ranges. as with the double-buffered code registers, update opera - tions copy the span input registers to the associated span dac registers . 2668 f04 0 1 1 0 a3 a2 a1 a0 x x x x x x x x x x x x x s2 s1 s0 don?t care address span code write span command ltc 2668 2668fa
19 for more information www.linear.com/ltc2668 manual span operation multiple output ranges are not needed in all applications. by tying the mspan pins ( msp2, msp1 and msp0) to gnd and/ or avp, any output range can be hardware-configured without additional operational overhead. zero-scale and mid-scale reset options are also available for the unipolar modes (see table 4). table 4. mspan pin configurations msp2 msp1 msp0 output range reset code manual span soft- span 0 0 0 10v mid-scale x 0 0 avp 5v mid-scale x 0 avp 0 2.5v mid-scale x 0 avp avp 0v to 10v zero-scale x avp 0 0 0v to 10v mid-scale x avp 0 avp 0v to 5v zero-scale x avp avp 0 0v to 5v mid-scale x avp avp avp 0v to 5v zero-scale x monitor mux the ltc2668 includes a high voltage multiplexer (mux) for surveying the channel outputs. the mux pin is intended for use with high impedance inputs only; the output impedance of the multiplexer is 2.2k. continuous dc output current at the mux pin must be limited to 1ma to avoid damaging internal circuits. the output voltage range of the multiplexer is from v C to v + C 1.4v. the output is disabled (high impedance) at power-up. the syntax and codes for the mux command are shown in figure 5 and table 5. table 5. monitor mux control codes m4 m3 m2 m1 m0 mux pin output 0 0 0 0 0 disabled (hi-z) 1 0 0 0 0 v out0 1 0 0 0 1 v out1 1 0 0 1 0 v out2 1 0 0 1 1 v out3 1 0 1 0 0 v out4 1 0 1 0 1 v out5 1 0 1 1 0 v out6 1 0 1 1 1 v out7 1 1 0 0 0 v out8 1 1 0 0 1 v out9 1 1 0 1 0 v out10 1 1 0 1 1 v out11 1 1 1 0 0 v out12 1 1 1 0 1 v out13 1 1 1 1 0 v out14 1 1 1 1 1 v out15 toggle operations some systems require that dac outputs switch repetitively between two voltage levels. examples include introducing a small ac bias, or independently switching between on and off states. the ltc2668 toggle function facilitates these kinds of operations by providing two input registers (a and b) per dac channel. toggling between a and b is controlled by three signals . the first of these is the toggle select command, which acts on a data field of 16 bits, each of which controls a single channel ( see figure 6). the second is the global toggle command, which controls all selected channels using the global toggle bit tgb (see figure 7). finally, the tgp pin o peration figure 5. mux command 2668 f05 1 0 1 1 x x x x x x x x x x x x x x x m4 m3 m2 m1 m0 don?t care mux control code mux command ltc 2668 2668fa
20 for more information www.linear.com/ltc2668 o peration figure 6. toggle select syntax figure 7. global toggle syntax allows the use of an external clock or logic signal to toggle the dac outputs between a and b. the signals from these controls are combined as shown in figure 8. if the toggle function is not needed, tie tgp (pin 20) to ground and leave the toggle select register in its power-on reset state ( cleared to zero). input registers a then function as the sole input registers, and registers b are not used. toggle select register (tsr) the toggle select command (1100b) syntax is shown in figure 6. each bit in the 16 -bit tsr data field controls the dac channel of the same name: t0 controls channel 0, t1 channel 1,, and tx controls channel x. the toggle select bits (t0, t1,..., t15) have a dual function. first, each toggle select bit controls which input register (a or b) receives data from a write-code operation. when the toggle select bit of a given channel is high , write-code operations are directed to input register b of the addressed channel. when the bit is low, write-code operations are directed to input register a. secondly, each toggle select bit enables the corresponding channel for a toggle operation. writing to input registers a and b having chosen channels to toggle, write the desired codes to input registers a for the chosen channels; then set the channels toggle select bits using the toggle select command; and finally, write the desired codes to input registers b. once these steps are completed, the channels are ready to toggle. for example, to set up channel 3 to toggle between codes 4096 and 4200: 1) write code channel 3 (code = 4096) to register a 00000011 00010000 00000000 2) toggle select (set bit t3) 11000000 00000000 00001000 3) write code channel 3 (code = 4200) to register b 00000011 00010000 01101000 the write code of step (3) is directed to register b because in step (2), bit t3 was set to 1. channel 3 now has input registers a and b holding the two desired codes, and is prepared for the toggle operation. toggling between registers a and b once input registers a and b have been written to for all desired channels and the corresponding toggle select bits are set high, as in the previous example, the channels are ready for toggling. 2668 f06 lsb msb 1 1 0 0 x x x x t15 t14 t13 t12 t11 t10 t9 t8 t7 t6 t5 t4 t3 t2 t1 t0 toggle select bits (16 bits, one for each channel) don?t care toggle select 2668 f07 1 1 0 1 x x x x x x x x x x x x x x x x x x x tgb global toggle bit don?t care global toggle command ltc 2668 2668fa
21 for more information www.linear.com/ltc2668 o peration the ltc2668 supports three types of toggle operations: a first in which all selected channels are toggled together using the spi port; a second in which all selected channels are toggled together using an external clock or logic signal; and a third in which any combination of channels can be instructed to update from either input register a or b. the internal toggle-update circuit is edge triggered , so only transitions (of tgb or tgp) trigger an update from the respective input register. to toggle all selected channels together using the spi port , ensure the tgp pin is high and that the bits in the toggle select register corresponding to the desired channels are also high. use the global toggle command (1101b) to alternate codes , sequentially changing the global toggle bit tgb (see figure 7). changing tgb from 1 to 0 updates the dac registers from their respective input registers a. changing tgb from 0 to 1 updates the dac registers from their respective input registers b . note that in this way up to 16 channels may be toggled with just one se - rial command. t o toggle all selected channels using an external logic signal, ensure that the tgb bit in the global toggle register is high and that in the toggle select register, the bits cor - responding to the desired channels are also high. apply a clock or logic signal to the tgp pin to alternate codes . tgp falling edges update the dac registers from their associated input registers a . tgp rising edges update the dac registers from their associated input registers b. note that once the input registers are set up, all toggling is triggered by the signal applied to the tgp pin, with no further spi instructions needed. to cause any combination of channels to update from either input register a or b, ensure the tgp pin is high and that the tgb bit in the global toggle register is also high. using the toggle select command, set the toggle select bits as needed to select the input register (a or b) with which each channel is to be updated. then update all channels, either by using the serial command (1001b) or by applying a negative pulse to the ldac pin. any channels whose toggle select bits are 0 update from input register figure 8. simplified toggle block diagram. conceptual only, actual circuit may differ 2668 f08 toggle select bit t15 16 16 0 1 16 16 upd wr cs/ld sck sdi ldac 15 tgp tgb 20 32-bit shift register logic 16-bit toggle select register one tx bit per channel t0 t1 t2 t3 t4 t5 t6 t7 t8 t9 t10 t11 t12 t13 t14 t15 global toggle bit (tgb) input register b (16 bit) input register a (16 bit) a /b mux dac register 16-bit softspan dac 16 17 19 ltc2668 channel 15 ltc 2668 2668fa
22 for more information www.linear.com/ltc2668 o peration a, while channels whose toggle select bits are 1 update from input register b (see figure 8). by alternating toggle- select and update operations, up to 16 channels can be simultaneously switched to a or b as needed. daisy-chain operation the serial output of the shift register appears at the sdo pin. data transferred to the device from the sdi input is delayed 32 sck rising edges before being output at the next sck falling edge , suitable for clocking into the mi - croprocessor on the next 32 sck rising edges. the sdo output can be used to facilitate control of multiple serial devices from a single 3 -wire serial port (i.e., sck, sdi and cs/ld). such a daisy-chain series is configured by connecting the sdo of each upstream device to the sdi of the next device in the chain. the shift registers of the devices are thus connected in series, effectively forming a single input shift register which extends through the entire chain. because of this, the devices can be addressed and controlled individually by simply concatenating their input words; the first instruction addresses the last device in the chain and so forth. the sck and cs/ld signals are common to all devices in the series. in use, cs/ld is first taken low. then, the concatenated input data is transferred to the chain , using the sdi of the first device as the data input. when the data transfer is complete, cs/ ld is taken high, completing the instruction sequence for all devices simultaneously. a single device can be controlled by using the no-operation command (1111) for all other devices in the chain. when cs/ ld is taken high, the sdo pin presents a high impedance output, so a pull-up resistor is required at the sdo of each device (except the last) for daisy-chain operation. echo readback the sdo pin can be used to verify data transfer to the device. during each 32-bit instruction cycle, sdo outputs the previous 32-bit instruction for verification. when cs/ld is high, sdo presents a high impedance output, releasing the bus for use by other spi devices. power-down mode for power-constrained applications , power-down mode can be used to reduce the supply current whenever less than sixteen dac outputs are needed. when in power-down, the output amplifiers and reference buffers are disabled. the dac outputs are put into a high impedance state , and the output pins are passively pulled to ground through individual 42k (minimum) resistors. register contents are not disturbed during power-down. any channel or combination of channels can be put into power-down mode by using command 0100b in combina - tion with the appropriate dac address. in addition, all the dac channels and the integrated reference together can be put into power -down mode using the power-down chip command, 0101b. the 16-bit data word is ignored for all power-down commands. normal operation resumes by executing any command which includes a dac updateeither in software, as shown in table 1, by taking the asynchronous ldac pin low, or by toggling ( see the types of toggle operations section). the selected dac is powered up as its voltage output is updated. when updating a powered-down dac, add wait time to accommodate the extra power-up delay . if the channels have been powered down (command 0100b) prior to the update command, the power-up delay time is 30s. if, on the other hand, the chip has been powered down (command 0101b), the power-up delay time is 35s. asynchronous dac update using ldac in addition to the update commands shown in table 1, the asynchronous, active-low ldac pin updates all 16 dac registers with the contents of the input registers. if cs/ld is high, a low on the ldac pin causes all dac registers to be updated with the contents of the input registers. if cs/ld is low, a low-going pulse on the ldac pin be - fore the rising edge of cs/ld powers up all dac outputs, but does not cause the outputs to be updated . if ldac remains low after the rising edge of cs/ld, then ldac is recognized, the command specified in the 24 -bit word is executed and the dac outputs are updated. ltc 2668 2668fa
23 for more information www.linear.com/ltc2668 o peration the dac outputs are powered up when ldac is taken low, independent of the state of cs/ld. if ldac is low at the time cs/ld goes high, any software power-down command (power down n , power-down chip, config/select external reference) that was specified in the input word is inhibited. reference modes the ltc2668 has two reference modes (internal and ex - ternal) with which the reference source can be selected. in either mode , the voltage at the ref pin and the output range settings determine the full-scale voltage of each of the channels. the device has a precision 2.5v integrated reference with a typical temperature drift of 2ppm/c. to use the internal reference, the refcomp pin should be left floating (no dc path to ground). in addition, the rd bit in the config register must have a value of 0. this value is reset to 0 at power-up, or it can be reset using the config command, 0111b. figure 9 shows the command syntax. a buffer is needed if the internal reference is to drive exter - nal circuitry . for reference stability and low noise, a 0.1f capacitor should be tied between refcomp and gnd . in this configuration , the internal reference can drive up to 0.1 f with excellent stability. in order to ensure stable operation, the capacitive load on the ref pin should not exceed that on the refcomp pin. to use an external reference , tie the refcomp pin to ground. this disables the output of the internal reference at start-up , so that the ref pin becomes a high impedance input. apply the desired reference voltage at the ref pin after powering up, and set the rd bit to 1 using the config command (0111b). this reduces avp supply current by approximately 200a. the acceptable external reference voltage range is: 0.5v v ref avp C 1.75v. integrated reference buffers each channel has its own integrated high performance reference buffer. the buffers have very high input imped - ance and do not load the reference voltage source . these b u ffers shield the reference voltage from glitches caused by dac switching and, thus, minimize dac-to-dac dynamic crosstalk. typically dac-to-dac crosstalk is less than 6nv ? s (0 v to 10v range). see the dac-to-dac crosstalk graph in the typical performance characteristics section . voltage outputs an amplifiers ability to maintain its rated voltage accuracy over a wide range of load conditions is characterized in its load regulation specification. the change in output voltage is measured per milliampere of forced load current change . each of the ltc2668's high voltage, rail-to-rail output amplifiers has guaranteed load regulation when sourcing or sinking up to 10ma with supply headroom as low as 1.4v. additionally, the amplifiers can drive up to 14ma if available headroom is increased to 2.2v or more. dc output impedance is equivalent to load regulation, and may be derived from it by simply calculating a change in units from v/ma to ohms. the amplifiers dc output impedance is typically 0.08 when driving a load well away from the rails. when drawing a load current from either rail, the output voltage headroom with respect to that rail is limited by the 60 typical channel resistance of the output devices e.g., when sinking 1ma, the minimum output voltage (above v C ) is 60 ? 1ma = 60mv. see the headroom at rails vs output current graphs in the typical performance characteristics section. figure 9. config command syntaxthermal shutdown (ts) and reference disable (rd) 2668 f09 0 1 1 1 x x x x x x x x x x x x x x x x x x ts rd config bits don?t care config command ltc 2668 2668fa
24 for more information www.linear.com/ltc2668 o peration the amplifiers are stable driving capacitive loads of up to 1000pf. thermal overload protection the ltc2668 protects itself if the die temperature exceeds 160c. all channels power down, and the open-drain ovrtmp interrupt pin pulls low. the reference and bias circuits stay powered on . once triggered, the device stays in shutdown even after the die cools. the temperature of the die must fall to approximately 150c before the channels can be returned to normal operation. once the part has cooled sufficiently, the shutdown can be cleared with any valid update operation, including ldac or a toggle operation. a cs/ld rising edge releases the ovrtmp pin regardless of the die temperature. since the total load current of the device can easily exceed 100ma, die heating potential of the system design should be evaluated carefully. grounded loads as low as 1k may be used and will not result in excessive heat. thermal protection can be disabled by using the config command to set the ts bit (see figure 9). board layout the excellent load regulation and dc crosstalk perfor - mance of these devices is achieved in part by minimizing common-mode resistance of signal and power grounds . as with any high resolution converter , clean board ground - ing is important. a low impedance analog ground plane is necessary , as are star-grounding techniques. keep the board layer used for star ground continuous to minimize ground resistances; that is, use the star-ground concept without using separate star traces. resistance from the reflo pin to the star point should be as low as possible. for best performance , stitch the ground plane with arrays of vias on 150 to 200 mil centers connecting it with the ground pours from the other board layers . this reduces overall ground resistance and minimizes ground loop area. using ltc2668 in 5v single-supply systems ltc2668 can be used in single-supply systems simply by connecting the v C pins to ground along with reflo and gnd, while v + and avp are connected to a 5v supply. ovp can be connected to the 5 v supply or to the logic supply voltage if lower than 5v. with the internal reference, use the 0v to 5v output range. as with any rail-to-rail device, the output is limited to voltages within the supply range. since the outputs of the device cannot go below ground, they may limit at the lowest codes, as shown in figure 10b. simi - larly, limiting can occur near full-scale if full-scale error (fse = v os + ge) is positive, or if v + < 2 ? v ref . see figure 10c. the multiplexer can be used and is fully functional. it can pull all the way to ground, but the upper headroom limita - tion means that it is useful for output voltages of 3.6v or below only (v + = 5v). more flexibility can be afforded by using an external refer - ence. for example, by using a 1.25v reference such as the ltc 6655, we can now select between 0x to 2x and 0x to 4 x ranges, which give full-scale voltages of 2.5v and 5v, respectively. furthermore, the part can be configured for reset to zero- or mid-scale codes (see the output ranges section). ltc 2668 2668fa
25 for more information www.linear.com/ltc2668 o peration figure 10. effects of 0v to 5v output range for single-supply operation. (10a) overall transfer function (10b) effect of negative offset for codes near zero-scale (10c) effect of positive full-scale error for codes near full-scale 2668 f10 input code (10b) output voltage negative offset 0v 32,768 0 65,535 input code output voltage (10a) v + = 2v ref = 5v v + = 2v ref = 5v (10c) input code output voltage positive fse v ? = v reflo = 0v ltc 2668 2668fa
26 for more information www.linear.com/ltc2668 p ackage description please refer to http://www .linear.com/designtools/packaging/ for the most recent package drawings. 6.00 0.10 (4 sides) note: 1. drawing is a jedec package outline variation of (wjjd-2) 2. drawing not to scale 3. all dimensions are in millimeters 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 top mark (see note 6) pin 1 notch r = 0.45 or 0.35 45 chamfer 0.40 0.10 4039 1 2 bottom view?exposed pad 4.50 ref (4-sides) 4.42 0.10 4.42 0.10 4.42 0.05 4.42 0.05 0.75 0.05 r = 0.115 typ 0.25 0.05 0.50 bsc 0.200 ref 0.00 ? 0.05 (uj40) qfn rev ? 0406 recommended solder pad pitch and dimensions apply solder mask to areas that are not soldered 0.70 0.05 4.50 0.05 (4 sides) 5.10 0.05 6.50 0.05 0.25 0.05 0.50 bsc package outline r = 0.10 typ uj package 40-lead plastic qfn (6mm 6mm) (reference ltc dwg # 05-08-1728 rev ?) uj package 40-lead plastic qfn (6mm 6mm) (reference ltc dwg # 05-08-1728 rev ?) ltc 2668 2668fa
27 for more information www.linear.com/ltc2668 r evision h istory rev date description page number a 7/15 updated v C and glitch impulse in the electrical characteristics section. replaced mid-scale glitch impulse graph. edited the power supply sequencing and start-up section. updated v out output voltage swing conditions. fixed avp pin on schematic (pin 36). 5 11 15 28 ltc 2668 2668fa
28 for more information www.linear.com/ltc2668 ? linear technology corporation 2014 lt 0715 rev a ? printed in usa linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax : (408) 434-0507 www.linear.com/ltc2668 r elate d p arts t ypical a pplication part number description comments ltc2704 quad serial 16-/14-/12-bit v out softspan dacs with 2lsb inl, 1lsb dnl software programmable output ranges up to 10v, spi interface, no external amps needed ltc 2754 quad serial 16-/12-bit i out softspan dacs with 1lsb inl, 1lsb dnl software programmable output ranges up to 10v, spi interface, 7mm 8mm qfn package ltc 2656 octal serial 16-/12- bit v out dacs with internal reference 10ppm/c internal reference, 4mm 5mm qfn package ltc2636 octal 12-/10-/8-bit spi v out dacs with internal reference 10ppm/c internal reference, 4mm 3mm dfn and 16-lead msop packages references ltc6655a low drift precision buffered reference 0.025% max tolerance, 2ppm/c max, 0.25ppm p-p 0.1hz to 10hz noise v out0 v out1 v out2 v out3 v out4 v out5 v out6 v out7 v out8 v out9 v out10 v out11 v out12 v out13 v out14 v out15 mux ovrtmp 2 3 4 5 6 7 8 9 23 24 25 26 27 28 29 30 msp0 msp1 msp2 ldac tgp clr sdi cs/ld sdo sck 39 40 1 15 20 21 19 16 18 17 ? + 0.1f dac outputs 0.1f 3 7 6 4 2 15 1 7 8 3, 16 5 2 15v ?15v 4 in+ in? 0.1f 0.1f 2668 ta02 47f v dd 5v ov dd v ddlbyp refbuf ltc2328-18 lt1468 ltc2668-16 refin gnd 0.1f 10f 2 10f 2 0.1f sdo sck rdl/sdi busy cnv chain 14 13 12 11 9 10 12 38 ovp avp reflo reflo gnd refcomp ref v ? pad v + 0.1f 33 0.1f 34 4132, 11 14, 37 1335 0.1f ?15v 1f 0.1f 5v 5v 1f 0.1f 15v 1f 22 36 31, 10 sdi cs1 cs2 sck sdo to microcontroller using the analog multiplexer to measure dac output voltages up to 10.24v. independent adc reference cross-checks ltc2668 internal reference ltc 2668 2668fa

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