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low noise, precision cmos amplifier ad8655/ad8656 rev. a information furnished by analog devices is believed to be accurate and reliable. however, no responsibility is assumed by analog devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. specifications subject to change without notice. no license is granted by implication or otherwise under any patent or patent ri ghts of analog devices. trademarks and registered trademarks are the property of their respective owners. one technology way, p.o. box 9106, norwood, ma 02062-9106, u.s.a. tel: 781.329.4700 www.analog.com fax: 781.461.3113 ? 2005 analog devices, inc. all rights reserved. features pin configurations low noise: 2.7 nv/ hz @ f = 10 khz nc 1 ? in 2 +in 3 v? 4 nc 8 v+ 7 out 6 nc 5 ad8655 top view (not to scale) 05304-048 nc = no connect out a 1 ?in a 2 +in a 3 v? 4 v+ 8 out b 7 ?in b 6 +in b 5 ad8656 top view (not to scale) 05304-059 low offset voltage: 250 v max over v cm offset voltage drift: 0.4 v/c typ and 2.3 v/c max bandwidth: 28 mhz rail-to-rail input/output unity gain stable figure 1. ad8655 8-lead msop (rm-8) figure 2. ad8656 8-lead msop (rm-8) 2.7 v to 5.5 v operation ?40c to +125c operation applications out a 1 ?in a 2 +in a 3 v? 4 v+ 8 out b 7 ?in b 6 +in b 5 ad8656 top view (not to scale) 05304-060 nc 1 ?in 2 +in 3 v? 4 nc 8 v+ 7 out 6 nc 5 nc = no connect ad8655 top view (not to scale) 05304-049 adc and dac buffers audio industrial controls precision filters digital scales figure 3. ad8655 8-lead soic (r-8) figure 4. ad8656 8-lead soic (r-8) strain gauges pll filters general description the ad8655/ad8656 are the industrys lowest noise, precision cmos amplifiers. they leverage the analog devices digitrim? technology to achieve high dc accuracy. the high precision performance of the ad8655/ad8656 improves the resolution and dynamic range in low voltage applications. audio applications, such as microphone pre-amps and audio mixing consoles, benefit from the low noise, low distortion, and high output current capability of the ad8655/ ad8656 to reduce system level noise performance and maintain audio fidelity. the high precision and rail-to-rail input and output of the ad8655/ad8656 benefit data acquisition, process controls, and pll filter applications. the ad8655/ad8656 provide low noise (2.7 nv/hz @ 10 khz), low thd + n (0.0007%), and high precision performance (250 v max over v cm ) to low voltage applications. the ability to swing rail-to-rail at the input and output enables designers to buffer analog-to-digital converters (adcs) and other wide dynamic range devices in single-supply systems. the ad8655/ad8656 are fully specified over the ?40c to +125c temperature range. the ad8655/ad8656 are available in pb-free, 8-lead msop and soic packages.
ad8655/ad8656 rev. a | page 2 of 20 table of contents specifications ..................................................................................... 3 absolute maximum ratings ............................................................ 5 esd caution .................................................................................. 5 typical performance characteristics ............................................. 6 theory of operation ...................................................................... 15 applications ..................................................................................... 16 input overvoltage protection ................................................... 16 input capacitance ....................................................................... 16 driving capacitive loads .......................................................... 16 layout, grounding, and bypassing considerations .................. 18 power supply bypassing ............................................................ 18 grounding ................................................................................... 18 leakage currents ........................................................................ 18 outline dimensions ....................................................................... 19 ordering guide .......................................................................... 19 revision history 6/05rev. 0 to rev. a added ad8656 ...................................................................universal added figure 2 and figure 4........................................................... 1 changes to specifications ................................................................ 3 changed caption of figure 12 and added figure 13 .................. 7 replaced figure 16 ........................................................................... 7 changed caption of figure 37 and added figure 38 ................ 11 replaced figure 47 ......................................................................... 13 added figure 55.............................................................................. 14 changes to ordering guide .......................................................... 18 4/05revision 0: initial version
ad8655/ad8656 rev. a | page 3 of 20 specifications v s = 5.0 v, v cm = v s /2, t a = 25c, unless otherwise specified. table 1. parameter symbol conditions min typ max unit input characteristics offset voltage v os v cm = 0 v to 5 v 50 250 v ?40c t a +125c 550 v offset voltage drift v os / t ?40c t a +125c 0.4 2.3 v/c input bias current i b 1 10 pa ?40c t a +125c 500 pa input offset current i os 10 pa ?40c t a +125c 500 pa input voltage range 0 5 v common-mode rejection ratio cmrr v cm = 0 v to 5 v 85 100 db large signal voltage gain a vo v o = 0.2 v to 4.8 v, r l = 10 k, v cm = 0 v 100 110 db ?40c t a +125c 95 db output characteristics output voltage high v oh i l = 1 ma; ?40c t a +125c 4.97 4.991 v output voltage low v ol i l = 1 ma; ?40c t a +125c 8 30 mv output current i out v out = 0.5 v 220 ma power supply power supply rejection ratio psrr v s = 2.7 v to 5.0 v 88 105 db supply current/amplifier i sy v o = 0 v 3.7 4.5 ma ?40c t a +125c 5.3 ma input capacitance c in differential 9.3 pf common-mode 16.7 pf noise performance input voltage noise density e n f = 1 khz 4 nv/ hz f = 10 khz 2.7 nv/ hz total harmonic distortion + noise thd + n g = 1, r l = 1 k, f = 1 khz, v in = 2 v p-p 0.0007 % frequency response gain bandwidth product gbp 28 mhz slew rate sr r l = 10 k 11 v/s settling time ts to 0.1%, v in = 0 v to 2 v step, g = +1 370 ns phase margin c l = 0 pf 69 degrees
ad8655/ad8656 rev. a | page 4 of 20 v s = 2.7 v, v cm = v s /2, t a = 25c, unless otherwise specified. table 2. parameter symbol conditions min typ max unit input characteristics offset voltage v os v cm = 0 v to 2.7 v 44 250 v ?40c t a +125c 550 v offset voltage drift v os / t ?40c t a +125c 0.4 2.0 v/c input bias current i b 1 10 pa ?40c t a +125c 500 pa input offset current i os 10 pa ?40c t a +125c 500 pa input voltage range 0 2.7 v common-mode rejection ratio cmrr v cm = 0 v to 2.7 v 80 98 db large signal voltage gain a vo v o = 0.2 v to 2.5 v, r l = 10 k, v cm = 0 v 98 db ?40c t a +125c 90 db output characteristics output voltage high v oh i l = 1 ma; ?40c t a +125c 2.67 2.688 v output voltage low v ol i l = 1 ma; ?40c t a +125c 10 30 mv output current i out v out = 0.5 v 75 ma power supply power supply rejection ratio psrr v s = 2.7 v to 5.0 v 88 105 db supply current/amplifier i sy v o = 0 v 3.7 4.5 ma ?40c t a +125c 5.3 ma input capacitance c in differential 9.3 pf common-mode 16.7 pf noise performance input voltage noise density e n f = 1 khz 4.0 nv/ hz f = 10 khz 2.7 nv/ hz total harmonic distortion + noise thd + n g = 1, r l = 1k, f = 1 khz, v in = 2 v p-p 0.0007 % frequency response gain bandwidth product gbp 27 mhz slew rate sr r l = 10 k 8.5 v/s settling time ts to 0.1%, v in = 0 to 1 v step, g = +1 370 ns phase margin c l = 0 pf 54 degrees
ad8655/ad8656 rev. a | page 5 of 20 absolute maximum ratings table 3. stresses above those listed under absolute maximum ratings may cause permanent damage to the device. this is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. parameter rating supply voltage 6 v input voltage vss ? 0.3 v to vdd + 0.3 v differential input voltage 6 v indefinite output short-circuit duration to gnd electrostatic discharge (hbm) 3.0 kv table 4. ?65c to +150c storage temperature range r, rm packages package type unit ja 1 jc junction temperature range r, rm packages ?65c to +150c 8-lead msop (rm) 210 45 c/w 8-lead soic (r) 158 43 c/w 260c lead temperature (soldering, 10 sec) 1 ja is specified for worst-case conditions; that is, ja is specified for a device soldered in the circuit boar d for surface-mount packages. esd caution esd (electrostatic discharge) sensiti ve device. electrostatic charges as hi gh as 4000 v readily accumulate on the human body and test equipment and can discharge without detection. although this product features proprietary esd protection circuitry, permanent dama ge may occur on devices subjected to high energy electrostatic discharges. therefore, proper esd precautions are recommended to avoid performance degradation or loss of functionality.
ad8655/ad8656 rev. a | page 6 of 20 typical performance characteristics 60 50 40 30 20 10 0 ?150 ?100 ?50 0 50 100 150 v os ( v) number of amplifiers 05304-001 v s = 2.5v 20 10 0 ?10 ?20 ?30 012 34 common-mode voltage (v) 56 v os ( v) 05304-004 v s = 2.5v figure 5. input offset voltage distribution figure 8. input offset voltage vs. common-mode voltage 150.0 100.0 50.0 0.0 ?50.0 ?100.0 ?150.0 ?50 0 50 temperature (c) 100 150 v os ( v) 05304-002 v s = 2.5v 250 200 150 100 50 0 0 20 40 60 80 100 120 140 temperature (c) ib (pa) v s = 2.5v 05304-005 figure 6. input offset voltage vs. temperature figure 9. input bias current vs. temperature 60 50 40 30 20 10 0 0 0.2 0.4 0.6 0.8 1.0 1.2 |tcv os | ( v/c) 1.4 1.6 number of amplifiers 05304-003 v s = 2.5v 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 01 2 3 4 supply voltage (v) 56 supply current (ma) 05304-006 v s = 2.5v figure 10. supply current vs. supply voltage figure 7. |tcv os | distribution
ad8655/ad8656 rev. a | page 7 of 20 4.5 4.0 3.5 3.0 2.5 2.0 ?50 0 50 temperature (c) 100 150 supply current (ma) v s = 2.5v 05304-007 4.996 4.990 4.988 4.986 4.984 4.982 ?50 0 50 temperature (c) 100 150 v oh (v) 05304-009 4.992 4.994 v s = 2.5v load current = 1ma figure 11. supply current vs. temperature figure 14. output voltage swing high vs. temperature 12 10 8 6 4 2 ?50 0 50 temperature (c) 100 150 v ol (mv) 05304-010 load current = 1ma v s = 2.5v v oh v ol 2500 2000 1500 1000 500 0 0 50 100 150 200 current load (ma) 250 delta swing from supply (mv) 05304-008 v s = 2.5v figure 15. output voltage swing low vs. temperature figure 12. ad8655 output voltage to supply rail vs. current load 80 60 0 100 1k 10k frequency (hz) 100k 10m cmrr (db) 120 100 v s = 2.5v v in = 28mv r l = 1m c l = 47pf 40 20 1m 05304-011 100 10 1 0.1 1 10 current load (ma) 100 1000 delta swing from supply (mv) 10000 1000 v s = 2.5v v ol v oh 05304-056 figure 13. ad8656 output swing vs. current load figure 16. cmrr vs. frequency
ad8655/ad8656 rev. a | page 8 of 20 100 10 1 1 10 100 1k frequency (hz) 10k 100k voltage noise density (nv/ hz 1/2) 05304-019 v s = 2.5v 110.00 107.00 104.00 101.00 98.00 95.00 92.00 ?50 0 50 temperature (c) 100 150 cmrr (db) 05304-012 v s = 2.5v v cm = 0v figure 20. voltage noise density vs. frequency figure 17. large signal cmrr vs. temperature 100 80 60 40 20 0 100 1k 10k 100k 1m frequency (hz) 10m 100m psrr (db) v s = 2.5v v in = 50mv r l = 1m c l = 47pf 05304-013 +psrr ?psrr 1 v s = 2.5v vn (p-p) = 1.23 v 05304-020 500nv/div 1s/div figure 21. low frequency noise (0.1 hz to 10 hz). figure 18. small signal pssr vs. frequency 110.00 108.00 106.00 104.00 102.00 100.00 ?50 0 50 temperature (c) 100 150 psrr (db) 05304-014 v s = 2.5v t 2 v s = 2.5v c l = 50pf gain = +1 05304-021 1v/div 20 s/div v in v out figure 22. no phase reversal figure 19. large signal pssr vs. temperature
ad8655/ad8656 rev. a | page 9 of 20 120 100 80 60 40 20 0 ?20 ?40 10k 100k 1m frequency (hz) 10m 100m gain (db) 05304-015 ?90 ?135 ?180 ?225 phase shift (degrees) ?45 v s = 2.5v c load = 11.5pf phase margin = 69 6 5 4 3 2 1 0 10k 100k 1m frequency (hz) 10m output (v) 05304-018 v s = 2.5v v in = 5v g = +1 figure 26. maximum output swing vs. frequency figure 23. open-loop gain and phase vs. frequency 140.00 130.00 120.00 110.00 90.00 ?50 0 50 temperature (c) 100 150 a vo (db) 05304-016 v s = 2.5v r l = 10k 100.00 t 2 v s = 2.5v c l = 100pf gain = +1 v in = 4v 05304-022 time (10 s/div) v out (1v/div) figure 24. large signal open-loop gain vs. temperature figure 27. large signal response 40 50 30 20 1k 10k 100k 1m frequency (hz) 10m 100m closed-loop gain (db) 05304-017 v s = 2.5v r l = 1m c l = 47pf 10 0 ?10 ?20 2 t v s = 2.5v c l = 100pf g = +1 05304-023 time (1 s/div) v out (100mv/div) figure 25. closed-loop gain vs. frequency figure 28. small signal response
ad8655/ad8656 rev. a | page 10 of 20 30 25 20 15 10 5 0 0 50 100 150 200 250 300 350 capacitance (pf) overshoot % v s = 2.5v v in = 200mv ?os +os 05304-024 100 10 1 0.1 frequency (hz) 100 1k 10k 100k 1m 10m 100m output impedance ( ) 05304-027 v s = 2.5v g = +100 g = +10 g = +1 figure 29. small signal overshoot vs. load capacitance figure 32. output impedance vs. frequency 80 70 60 50 40 30 20 10 0 ?150 ?125 ?100 ?75 ?50 ?25 0 v os ( v) 25 50 75 100 125 150 number of amplifiers 05304-028 v s = 1.35v t 2 1 v s = 2.5v v in = 300mv gain = ?10 recovery time = 240ns 05304-025 300mv 0v 0v ?2.5v v in v out 400ns/div figure 30. negative overload recovery time figure 33. input offset voltage distribution 60 40 20 0 ?20 ?40 ?50 0 50 100 150 temperature (c) v os ( v) 05304-029 v s = 1.35v 1 2 05304-026 400ns/div v s = 2.5v v in = 300mv gain = ?10 recovery time = 240ns t v in v out 0v 0v ? 300mv 2.5v figure 34. input offset voltage vs. temperature figure 31. positive overload recovery time
ad8655/ad8656 rev. a | page 11 of 20 80 70 60 50 40 30 20 10 0 0 0.2 0.4 0.6 0.8 |tcv os | ( v/ c) 1.0 1.2 1.4 1.6 number of amplifiers 05304-030 v s = 1.35v 100 10 1 0.1 1 10 current load (ma) 100 delta output from supply (mv) 10000 1000 v ol v oh v s = 1.35v 05304-057 figure 35. |tcv os | distribution figure 38. ad8656 output swing vs. current load 4.5 4.0 3.5 3.0 2.5 2.0 ?50 0 50 100 150 temperature (c) supply current (ma) 05304-031 v s = 1.35v 2.698 2.694 2.690 2.686 2.682 2.678 2.674 ?50 0 50 100 150 temperature (c) v oh (v) 05304-032 v s = 1.35v load current = 1ma figure 39. output voltage swing high vs. temperature figure 36. supply current vs. temperature 14 12 10 8 6 4 2 ?50 0 50 100 150 temperature (c) v ol (mv) 05304-033 v s = 1.35v load current = 1ma 1400 1200 1000 800 600 400 200 0 020406080 load current (ma) 100 120 (v sy -v out ) (mv) v s = 1.35v v oh v ol 05304-050 figure 40. output voltage swing low vs. temperature figure 37. ad8655 output voltage to supply rail vs. load current
ad8655/ad8656 rev. a | page 12 of 20 35 30 25 20 15 10 5 0 0 50 100 150 200 250 300 350 capacitance (pf) overshoot % v s = 1.35v v in = 200mv ?os +os 05304-044 t 2 v s = 1.35v g = +1 c l = 50pf 05304-047 1v/div v in v out 20 s/div figure 44. small signal overshoot vs. load capacitance figure 41. no phase reversal 400ns/div t 1 2 v s = 1.35v v in = 200mv gain = ?10 recovery time = 180ns 05304-045 200mv 0v 0v ?1.35v v in v out t 2 v s = 1.35v c l = 50pf gain = +1 05304-042 time (10 s/div) v out (500mv/div) figure 42. large signal response figure 45. negative overload recovery time t 1 2 v s = 1.35v v in = 200mv gain = ?10 recovery time = 200ns 05304-046 0v ? 200mv 1.35v 0v 400ns/div v in v out 2 t v s = 1.35v c l = 100pf gain = +1 05304-043 time (1 s/div) v out (100mv/div) figure 43. small signal response figure 46. positive overload recovery time
ad8655/ad8656 rev. a | page 13 of 20 120 100 80 60 40 20 ?20 ?40 10k 100k 1m frequency (hz) 10m 100m gain (db) 05304-036 ?90 ?135 ?180 ?225 phase shift (degrees) ?45 v s = 1.35v c load = 11.5pf phase margin = 54 0 40 20 0 100 1k 10k frequency (hz) 100k 1m cmrr (db) 120 80 100 60 v s = 1.35v v in = 28mv r l = 1m c l = 47pf 05304-034 figure 50. open-loop gain and phase vs. frequency figure 47. cmrr vs. frequency 130.00 120.00 110.00 100.00 90.00 80.00 ?50 0 50 temperature (c) 100 150 a vo (db) 05304-037 v s = 1.35v r l = 10k 102.00 98.00 94.00 90.00 86.00 ?50 0 50 temperature (c) 100 150 cmrr (db) 05304-035 v s = 1.35v figure 51. large signal open-loop gain vs. temperature figure 48. large signal cmrr vs. temperature 50 40 30 20 10 0 ?10 ?20 1k 10k 100k 1m frequency (hz) 10m 100m closed-loop gain (db) v s = 1.35v r l = 1m c l = 47pf 05304-038 100 80 60 40 20 0 100 1k 10k 100k frequency (hz) 1m 100m 10m psrr (db) v s = 1.35v v in = 50mv r l = 1m c l = 47pf 05304-040 +psrr ?psrr figure 49. small signal pssr vs. frequency figure 52. closed-loop gain vs. frequency
ad8655/ad8656 rev. a | page 14 of 20 ?40 ?60 ?140 10 100 1k frequency (hz) 10k channel seperation (db) 0 ?20 100k 1m 10m 100m ?80 ?100 ?120 v s = 2.5v v in = 50mv v+ v? +2.5v ?2.5v + ? v in 50mv p-p a r2 100 r1 10k v? v+ v out b 05304-058 3.0 2.5 2.0 1.5 1.0 0.5 0 10k 100k 1m frequency (hz) 10m output (v) 05304-039 v s = 1.35v v in = 2.7v g = +1 no load figure 53. maximum output swing vs. frequency figure 55. channel separation vs. frequency 1000 100 10 1 0.1 frequency (hz) 100 1k 10k 100k 1m 100m 10m output impedance ( ) 05304-041 v s = 1.35v g = +1 g = +100 g = +10 figure 54. output impedance vs. frequency
ad8655/ad8656 rev. a | page 15 of 20 theory of operation the ad8655/ad8656 amplifiers are voltage feedback, rail-to- rail input and output precision cmos amplifiers, which operate from 2.7 v to 5.0 v of power supply voltage. these amplifiers use the analog devices digitrim technology to achieve a higher degree of precision than is available from most cmos amplifiers. digitrim technology, used in a number of adi amplifiers, is a method of trimming the offset voltage of the amplifier after it is packaged. the advantage of post-package trimming is that it corrects any offset voltages caused by the mechanical stresses of assembly. the ad8655/ad8656 are available in standard op amp pinouts, making digitrim completely transparent to the user. the input stage of the amplifiers is a true rail-to-rail architecture, allowing the input common-mode voltage range of the amplifiers to extend to both positive and negative supply rails. the open- loop gain of the ad8655/ad8656 with a load of 10 k is typically 110 db. the ad8655/ad8656 can be used in any precision op amp application. the amplifier does not exhibit phase reversal for common-mode voltages within the power supply. the ad8655/ad8656 are great choices for high resolution data acquisition systems with voltage noise of 2.7 nv/hz and thd + noise of C103 db for a 2 v p-p signal at 10 khz. their low noise, sub-pa input bias current, precision offset, and high speed make them superb preamps for fast filter applications. the speed and output drive capability of the ad8655/ad8656 also make them useful in video applications.
ad8655/ad8656 rev. a | page 16 of 20 applications input overvoltage protection one simple technique for compensation is a snubber that consists of a simple rc network. with this circuit in place, output swing is maintained, and the amplifier is stable at all gains. the internal protective circuitry of the ad8655/ad8656 allows voltages exceeding the supply to be applied at the input. it is recommended, however, not to apply voltages that exceed the supplies by more than 0.3 v at either input of the amplifier. if a higher input voltage is applied, series resistors should be used to limit the current flowing into the inputs. the input current should be limited to less than 5 ma. figure 57 shows the implementation of a snubber, which reduces overshoot by more than 30% and eliminates ringing. using a snubber does not recover the loss of bandwidth incurred from a heavy capacitive load. time (2 s/div) v s = 2.5v a v = 1 c l = 500pf 05304-051 voltage (100mv/div) the extremely low input bias current allows the use of larger resistors, which allows the user to apply higher voltages at the inputs. the use of these resistors adds thermal noise, which contributes to the overall output voltage noise of the amplifier. for example, a 10 k resistor has less than 12.6 nv/hz of thermal noise and less than 10 nv of error voltage at room temperature. input capacitance along with bypassing and ground, high speed amplifiers can be sensitive to parasitic capacitance between the inputs and ground. for circuits with resistive feedback network, the total capacitance, whether it is the source capacitance, stray capacitance on the input pin, or the input capacitance of the amplifier, causes a breakpoint in the noise gain of the circuit. as a result, a capacitor must be added in para llel with the gain resistor to obtain stability. the noise gain is a function of frequency and peaks at the higher frequencies, assuming the feedback capaci- tor is selected to make the se cond-order system critically damped. a few picofarads of capacitance at the input reduce the input impedance at high frequencies, which increases the amplifiers gain, causing peaking in the frequency response or oscillations. with the ad 8655/ad8656, additional input damping is required for stability with capacitive loads greater than 200 pf with direct input to output feedback. see the figure 56. driving heavy capacitive loads without compensation v+ 200 500pf 500pf v? v cc v ee 200mv + ? 05304-052 + ? figure 57. snubber network v s = 2.5v a v = 1 r s = 200 c s = 500pf c l = 500pf time (10 s/div) 05304-053 voltage (100mv/div) driving capacitive loads section. driving capacitive loads although the ad8655/ad8656 can drive capacitive loads up to 500 pf without oscillating, a large amount of ringing is present when operating the part with input frequencies above 100 khz. this is especially true when the amplifiers are configured in positive unity gain (worst case). when such large capacitive loads are required, the use of external compensation is highly recommended. this reduces the overshoot and minimizes ringing, which, in turn, improves the stability of the ad8655/ad8656 when driving large capacitive loads. figure 58. driving heavy capacitive loads using a snubber network
ad8655/ad8656 rev. a | page 17 of 20 1.0 0.1 0.01 0.001 0.0001 % 20 100 1k 10k 80k 50 500 5k 50k 200 2k 20k hz 0.5 0.05 0.005 0.0005 0.2 0.02 0.002 0.0002 sweep 1: v in = 2v p-p r l = 10k sweep 2: v in = 2v p-p r l = 1k sweep 1 sweep 2 05304-055 thd readings vs. common-mode voltage total harmonic distortion of the ad8655/ad8656 is well below 0.0007% with a load of 1 k. this distortion is a function of the circuit configuration, the voltage applied, and the layout, in addition to other factors. + ? v in r l v out +2.5v ?2.5v ad8655 05304-054 figure 59. thd + n test circuit figure 60. thd + noise vs. frequency
ad8655/ad8656 rev. a | page 18 of 20 layout, grounding, and by passing considerations power supply bypassing power supply pins can act as inputs for noise, so care must be taken to apply a noise-free, stable dc voltage. the purpose of bypass capacitors is to create low impedances from the supply to ground at all frequencies, thereby shunting or filtering most of the noise. bypassing schemes are designed to minimize the supply impedance at all frequencies with a parallel combination of capacitors with values of 0.1 f and 4.7 f. chip capacitors of 0.1 f (x7r or npo) are critical and should be as close as possible to the amplifier package. the 4.7 f tantalum capacitor is less critical for high frequency bypassing, and, in most cases, only one is needed per board at the supply inputs. grounding a ground plane layer is important for densely packed pc boards to minimize parasitic inductances. this minimizes voltage drops with changes in current. however, an under- standing of where the current flows in a circuit is critical to implementing effective high speed circuit design. the length of the current path is directly proportional to the magnitude of parasitic inductances, and, therefore, the high frequency impedance of the path. large changes in currents in an inductive ground return create unwanted voltage noise. the length of the high frequency bypass capacitor leads is critical, and, therefore, surface-mount capacitors are recom- mended. a parasitic inductance in the bypass ground trace works against the low impedance created by the bypass capacitor. because load currents flow from the supplies, the ground for the load impedance should be at the same physical location as the bypass capacitor grounds. for larger value capacitors intended to be effective at lower frequencies, the current return path distance is less critical. leakage currents poor pc board layout, contaminants, and the board insulator material can create leakage currents that are much larger than the input bias current of the ad8655/ad8656. any voltage differential between the inputs and nearby traces creates leakage currents through the pc board insulator, for example, 1 v/100 g = 10 pa. similarly, any contaminants on the board can create significant leakage (skin oils are a common problem). to significantly reduce leakage, put a guard ring (shield) around the inputs and input leads that are driven to the same voltage potential as the inputs. this ensures there is no voltage potential between the inputs and the surrounding area to create any leakage currents. to be effective, the guard ring must be driven by a relatively low impedance source and should completely surround the input leads on all sides, above and below, by using a multilayer board. the charge absorption of the insulator material itself can also cause leakage currents. minimizing the amount of material between the input leads and the guard ring helps to reduce the absorption. also, using low absorption materials, such as teflon? or ceramic, may be necessary in some instances.
ad8655/ad8656 rev. a | page 19 of 20 outline dimensions 0.80 0.60 0.40 8 0 4 8 1 5 4.90 bsc pin 1 0.65 bsc 3.00 bsc seating plane 0.15 0.00 0.38 0.22 1.10 max 3.00 bsc coplanarity 0.10 0.23 0.08 compliant to jedec standards mo-187-aa 0.25 (0.0098) 0.17 (0.0067) 1.27 (0.0500) 0.40 (0.0157) 0.50 (0.0196) 0.25 (0.0099) 45 8 0 1.75 (0.0688) 1.35 (0.0532) seating plane 0.25 (0.0098) 0.10 (0.0040) 4 1 85 5.00 (0.1968) 4.80 (0.1890) 4.00 (0.1574) 3.80 (0.1497) 1.27 (0.0500) bsc 6.20 (0.2440) 5.80 (0.2284) 0.51 (0.0201) 0.31 (0.0122) coplanarity 0.10 controlling dimensions are in millimeters; inch dimensions (in parentheses) are rounded-off millimeter equivalents for reference only and are not appropriate for use in design compliant to jedec standards ms-012-aa figure 61. 8-lead standard small outline package [soic_n] narrow body (r-8) dimensions shown in millimeters and (inches) figure 62. 8-lead mini small outline package [msop] (rm-8) dimensions shown in millimeters ordering guide temperature range model package description package option branding ad8655arz ?40c to +125c 8-lead soic_n r-8 1 ad8655arz-reel ?40c to +125c 8-lead soic_n r-8 1 ad8655arz-reel7 ?40c to +125c 8-lead soic_n r-8 1 ad8655armz-reel ?40c to +125c 8-lead msop rm-8 a0d 1 ad8655armz-r2 ?40c to +125c 8-lead msop rm-8 a0d 1 ad8656arz ?40c to +125c 8-lead soic_n r-8 1 ad8656arz-reel ?40c to +125c 8-lead soic_n r-8 1 ad8656arz-reel7 ?40c to +125c 8-lead soic_n r-8 1 ad8656armz-reel ?40c to +125c 8-lead msop rm-8 a0s 1 AD8656ARMZ-R2 ?40c to +125c 8-lead msop rm-8 a0s 1 1 z = pb-free part.
ad8655/ad8656 rev. a | page 20 of 20 notes ? 2005 analog devices, inc. all rights reserved. trademarks and registered trademarks are the prop erty of their respective owners. d05304C0C6/05(a)

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