MHz, 2 V/μs, G =, 2, 5, i CMOS Programmable Gain Instrumentation Amplifier AD825 FEATURES Small package: -lead MSOP Programmable gains:, 2, 5, Digital or pin-programmable gain setting Wide supply: ±5 V to ±5 V Excellent dc performance High CMRR 98 db (min), G = Low gain drift: ppm/ C (max) Low offset drift:.7 μv/ C (max), G = Excellent ac performance Fast settling time: 65 ns to.% (max) High slew rate: 2 V/μs (min) Low distortion: db THD at khz High CMRR over frequency: 8 db to 5 khz (min) Low noise: 8 nv/ Hz, G = (max) Low power: 4 ma APPLICATIONS Data acquisition Biomedical analysis Test and measurement GENERAL DESCRIPTION The AD825 is an instrumentation amplifier with digitally programmable gains that has GΩ input impedance, low output noise, and low distortion making it suitable for interfacing with sensors and driving high sample rate analog-to-digital converters (ADCs). It has high bandwidth of MHz, low THD of db and fast settling time of 65 ns to.%. Offset drift and gain drift are guaranteed to.7 μv/ C and ppm/ C, respectively for G =. In addition to its wide input common voltage range, it boasts a high common-mode rejection of 8 db at G = from dc to 5 khz. The combination of precision dc performance coupled with high speed capabilities make the AD825 an excellent candidate for data acquisition. Furthermore, this monolithic solution simplifies design and manufacturing, and boosts performance of instrumentation by maintaining a tight match of internal resistors and amplifiers. The AD825 user interface consists of a parallel port that allows users to set the gain in one of two different ways (see Figure for the functional block diagram). A 2-bit word sent via a bus can be latched using the WR input. An alternative is to use transparent gain mode where the state of logic levels at the gain port determines the gain. GAIN (db) IN +IN 25 2 5 5 5 FUNCTIONAL BLOCK DIAGRAM LOGIC DGD WR 2 6 AD825 A 5 A 4 8 3 9 Figure. G = G = 5 G = 2 G = 7 OUT k k k M M M FREQUENCY (Hz) Figure 2. Gain vs. Frequency Table. Instrumentation and Difference Amplifiers by Category High Performance Low Cost High Voltage Mil Grade Low Power 6288-6288-23 Digital Gain AD822 AD623 AD628 AD62 AD627 AD823 AD822 AD8553 AD629 AD62 AD825 AD8222 AD524 AD8555 AD8224 AD526 AD8556 AD624 AD8557 Rail-to-rail output. The AD825 is available in a -lead MSOP package and is specified over the 4 C to +85 C temperature range, making it an excellent solution for applications where size and packing density are important considerations. Rev. 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 rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners. One Technology Way, P.O. Box 96, Norwood, MA 262-96, U.S.A. Tel: 78.329.47 www.analog.com Fax: 78.46.33 27 Analog Devices, Inc. All rights reserved.
AD825 TABLE OF CONTENTS Features... Applications... General Description... Functional Block Diagram... Specifications... 3 Timing Diagram... 5 Absolute Maximum Ratings... 6 Maximum Power Dissipation... 6 ESD Caution... 6 Pin Configuration and Function Descriptions... 7 Typical Performance Characteristics... 8 Theory of Operation... 5 Gain Selection... 5 Input Bias Current Return Path... 7 Input Protection... 7 Reference Terminal... 8 Common-Mode Input Voltage Range... 8 Layout... 8 RF Interference... 8 Driving an Analog-to-Digital Converter... 9 Applications... 2 Differential Output... 2 Setting Gains with a Microcontroller... 2 Data Acquisition... 2 Outline Dimensions... 22 Ordering Guide... 22 Power Supply Regulation and Bypassing... 7 REVISION HISTORY /7 Revision : Initial Version Rev. Page 2 of 24
AD825 SPECIFICATIONS +VS = +5 V, VS = 5 V, V = V @ TA = 25 C, G =, RL = 2 kω, unless otherwise noted. Table 2. Parameter Conditions Min Typ Max Unit COMMON-MODE REJECTION RATIO (CMRR) CMRR to 6 Hz with kω Source Imbalance +IN = IN = V to + V G = 8 94 db G = 2 86 4 db G = 5 94 6 db G = 98 6 db CMRR to 5 khz +IN = IN = V to + V G = 8 db G = 2 86 db G = 5 9 db G = 9 db NOISE Voltage Noise, khz, RTI G = 4 nv/ Hz G = 2 27 nv/ Hz G = 5 2 nv/ Hz G = 8 nv/ Hz. Hz to Hz, RTI G = 2.5 μv p-p G = 2 2.5 μv p-p G = 5.5 μv p-p G =. μv p-p Current Noise, khz 5 pa/ Hz Current Noise,. Hz to Hz 6 pa p-p VOLTAGE OFFSET Offset RTI VOS G =, 2, 5, ±2 + 6/G μv Over Temperature T = 4 C to +85 C ±26 + 9/G μv Average TC T = 4 C to +85 C ±.2 + 5/G μv/ C Offset Referred to the Input vs. Supply (PSR) VS = ±5 V to ±5 V ±6 + 2/G μv/v INPUT CURRENT Input Bias Current 5 3 na Over Temperature T = 4 C to +85 C 4 na Average TC 4 pa/ C Input Offset Current 5 3 na Over Temperature T = 4 C to +85 C 3 na Average TC 6 pa/ C DYNAMIC RESPONSE Small Signal 3 db Bandwidth G = MHz G = 2 MHz G = 5 MHz G = 3 MHz Settling Time.% ΔOUT = V step G = 585 ns G = 2 65 ns G = 5 65 ns G = 648 ns Rev. Page 3 of 24
AD825 Parameter Conditions Min Typ Max Unit Settling Time.% ΔOUT = V step G = 65 ns G = 2 635 ns G = 5 635 ns G = 685 ns Slew Rate G = 2 V/μs G = 2 25 V/μs G = 5 25 V/μs G = 25 V/μs Total Harmonic Distortion f = khz, RL = kω, G = db GAIN Gain Range G =, 2, 5, V/V Gain Error OUT = ± V G =.3 % G = 2, 5,.4 % Gain Nonlinearity OUT = V to + V G = RL = kω, 2 kω, 6 Ω 6 ppm G = 2 RL = kω, 2 kω, 6 Ω 8 ppm G = 5 RL = kω, 2 kω, 6 Ω 8 ppm G = RL = kω, 2 kω, 6 Ω ppm Gain vs. Temperature All gains ppm/ C INPUT Input Impedance Differential GΩ pf Common Mode GΩ pf Input Operating Voltage Range VS = ±5 V to ±5 V VS +. +VS. V Over Temperature T = 4 C to +85 C VS +. +VS.4 V OUTPUT Output Swing 3.5 +3.5 V Over Temperature T = 4 C to +85 C 3.5 +3.5 V Short-Circuit Current 37 ma ERENCE INPUT RIN 2 kω IIN +IN, IN, = μa Voltage Range VS +VS V Gain to Output ±. V/V DIGITAL LOGIC Digital Ground Voltage, Referred to GND VS + 4.25 +VS 2.7 V Digital Input Voltage Low Referred to GND 2. V Digital Input Voltage High Referred to GND 2.8 +VS V Digital Input Current μa Gain Switching Time 325 ns tsu See Figure 3 timing diagram 2 ns thd ns t WR -LOW t WR -HIGH 2 4 ns ns Rev. Page 4 of 24
AD825 Parameter POWER SUPPLY Operating Range Conditions Min ±5 Typ Max ±5 Unit V Quiescent Current, +IS 4. 4.5 ma Quescent Current, IS 3.7 4.5 ma Over Temperature T = 4 C to +85 C 4.5 ma TEMPERATURE RANGE Specified Performance 4 +85 C Add time for the output to slew and settle to calculate the total time for a gain change. TIMING DIAGRAM t WR-HIGH t WR-LOW WR t SU t HD A, A Figure 3. Timing Diagram for Latched Gain Mode (See the Timing for Latched Gain Mode Section) 6288-57 Rev. Page 5 of 24
AD825 ABSOLUTE MAXIMUM RATINGS Table 3. Parameter Rating Supply Voltage ±7 V Power Dissipation See Figure 2 Output Short-Circuit Current Indefinite Common-Mode Input Voltage ±VS Differential Input Voltage ±VS Digital Logic Inputs ±VS Storage Temperature Range 65 C to +25 C Operating Temperature Range 2 4 C to +85 C Lead Temperature (Soldering sec) 3 C Junction Temperature 4 C θja (4-Layer JEDEC Standard Board) 2 C/W Package Glass Transition Temperature 4 C Assumes the load is referenced to mid supply. 2 Temperature for specified performance is 4 C to +85 C. For performance to +25 C, see the Typical Performance Characteristics section. 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. MAXIMUM POWER DISSIPATION The maximum safe power dissipation in the AD825 package is limited by the associated rise in junction temperature (TJ) on the die. The plastic encapsulating the die locally reaches the junction temperature. At approximately 4 C, which is the glass transition temperature, the plastic changes its properties. Even temporarily exceeding this temperature limit may change the stresses that the package exerts on the die, permanently shifting the parametric performance of the AD825. Exceeding a junction temperature of 4 C for an extended period can result in changes in silicon devices, potentially causing failure. The still-air thermal properties of the package and PCB (θja), the ambient temperature (TA), and the total power dissipated in the package (PD) determine the junction temperature of the die. The junction temperature is calculated as T = T + J A ( P θ ) D JA power is the voltage between the supply pins (VS) times the quiescent current (IS). Assuming the load (RL) is referenced to midsupply, the total drive power is VS/2 IOUT, some of which is dissipated in the package and some in the load (VOUT IOUT). The difference between the total drive power and the load power is the drive power dissipated in the package. PD = Quiescent Power + (Total Drive Power Load Power) P D = ( V I ) S S V V + S OUT 2 R L V R 2 OUT In single-supply operation with RL referenced to VS, worst case is VOUT = VS/2. Airflow increases heat dissipation, effectively reducing θja. In addition, more metal directly in contact with the package leads from metal traces, through holes, ground, and power planes reduces the θja. Figure 4 shows the maximum safe power dissipation in the package vs. the ambient temperature on a 4-layer JEDEC standard board. MAXIMUM POWER DISSIPATION (W) 2..75.5.25..75.5.25 4 2 2 4 6 8 2 AMBIENT TEMPERATURE ( C) Figure 4. Maximum Power Dissipation vs. Ambient Temperature ESD CAUTION L 6288-4 The power dissipated in the package (PD) is the sum of the quiescent power dissipation and the power dissipated in the package due to the load drive for all outputs. The quiescent Rev. Page 6 of 24
AD825 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS IN A A 2 3 4 5 AD825 TOP VIEW (Not to Scale) +IN 9 8 7 OUT 6 WR Figure 5. -Lead MSOP (RM-), Pin Configuration 6288-5 Table 4. Pin Function Descriptions Pin No. Name Description IN Inverting Input Terminal. True differential input. 2 Digital Ground. 3 VS Negative Supply Terminal. 4 A Gain Setting Pin (LSB). 5 A Gain Setting Pin (MSB). 6 WR Write Enable. 7 OUT Output Terminal. 8 +VS Positive Supply Terminal. 9 Reference Voltage Terminal. +IN Noninverting Input Terminal. True differential input. Rev. Page 7 of 24
AD825 TYPICAL PERFORMANCE CHARACTERISTICS TA @ 25 C, +VS = +5 V, VS = 5 V, RL = kω, unless otherwise noted. 4 5 NUMBER OF UNITS 2 8 6 4 2 NUMBER OF UNITS 4 3 2 2 9 6 3 3 6 9 2 CMRR (µv/v) Figure 6. Typical Distribution of CMRR, G = 6288-6 3 2 2 3 INPUT BIAS (na) Figure 9. Typical Distribution of Input Offset Current 6288-9 35 9 3 8 NUMBER OF UNITS 25 2 5 5 2 5 5 5 5 2 OFFSET VOLTAGE RTI (µv) Figure 7. Typical Distribution of Offset Voltage, VOSI 6288-7 NOISE (nv/ Hz) 7 6 5 G = 4 G = 2 3 G = 5 2 G = FREQUENCY (Hz) Figure. Voltage Spectral Density vs. Frequency 6288-6 5 NUMBER OF UNITS 4 3 2 3 2 2 3 INPUT BIAS (na) 6288-8 2µV/DIV s/div 6288- Figure 8. Typical Distribution of Input Bias Current Figure.. Hz to Hz RTI Voltage Noise, G = Rev. Page 8 of 24
AD825 5 3 G = G = 5 PSRR (db) 9 7 G = 2 G = 5 3 µv/div s/div Figure 2.. Hz to Hz RTI Voltage Noise, G = 6288-2 k k k M FREQUENCY (Hz) Figure 5. Positive PSRR vs. Frequency, RTI 6288-6 8 5 6 3 CURRENT NOISE (pa/ Hz) 4 2 8 6 4 PSRR (db) 9 7 5 G = 2 G = G = G = 5 2 FREQUENCY (Hz) 6288-3 3 k k k M FREQUENCY (Hz) 6288-7 Figure 3. Current Noise Spectral Density vs. Frequency Figure 6. Negative PSRR vs. Frequency, RTI 5 BIAS CURRENT (na) 5 5 I B I B + I OS 4pA/DIV Figure 4.. Hz to Hz Current Noise s/div 6288-4 5 4 25 5 2 35 5 65 8 85 25 TEMPERATURE ( C) Figure 7. Input Bias Current and Offset Current vs. Temperature 6288-9 Rev. Page 9 of 24
AD825 4 25 2 G = G = 5 2 G = CMRR (db) 8 G = 2 G = GAIN (db) 5 5 G = 5 G = 2 6 G = 4 5 2 k k k M FREQUENCY (Hz) Figure 8. CMRR vs. Frequency 6288-2 k k k M M M FREQUENCY (Hz) Figure 2. Gain vs. Frequency 6288-23 4 2 G = G = 5 4 3 f = khz CMRR (db) 8 6 4 G = 2 G = 2 k k k M FREQUENCY (Hz) Figure 9. CMRR vs. Frequency, kω Source Imbalance 6288-2 NONLINEARITY (ppm/div) 2 2 3 4 8 6 4 2 2 4 6 8 OUTPUT VOLTAGE (V) Figure 22. Gain Nonlinearity, G =, RL = kω, 2 kω, 6 Ω 6288-24 ΔCMRR (µv/v) 8 6 4 2 2 4 6 8 5 3 3 5 7 9 3 TEMPERATURE ( C) Figure 2. CMRR vs. Temperature, G = 6288-49 NONLINEARITY (ppm/div) 4 f = khz 3 2 2 3 4 8 6 4 2 2 4 6 8 OUTPUT VOLTAGE (V) Figure 23. Gain Nonlinearity, G = 2, RL = kω, 2 kω, 6 Ω 6288-25 Rev. Page of 24
AD825 NONLINEARITY (ppm/div) 4 f = khz 3 2 2 3 4 8 6 4 2 2 4 6 8 OUTPUT VOLTAGE (V) Figure 24. Gain Nonlinearity, G = 5, RL = kω, 2 kω, 6 Ω 6288-26 INPUT COMMON-MODE VOLTAGE (V) 6 4.V, +3.6V V, +3.8V +3.6V, +3.V 2 V S = ±5V 8 +V, +3.5V 4 4.2V, +2.2V +4.3V, +2.V V S = ±5V 4 4.2V, 2.V +4.3V, 2.V V, 4.V 8 2 4.V, 3.6V V, 4V +3.6V, 3.V 6 6 2 8 4 4 8 2 6 OUTPUT VOLTAGE (V) Figure 27. Input Common-Mode Voltage Range vs. Output Voltage, G = 6288-29 NONLINEARITY (ppm/div) 4 f = khz 3 2 2 3 4 8 6 4 2 2 4 6 8 OUTPUT VOLTAGE (V) Figure 25. Gain Nonlinearity, G =, RL = kω, 2 kω, 6 Ω 6288-27 INPUT VOLTAGE FERED TO SUPPLY VOLTAGES (V) 2 +2 + +25 C +85 C +85 C +25 C +25 C 4 C 4 C +25 C 4 6 8 2 4 6 SUPPLY VOLTAGE (±V S ) Figure 28. Input Voltage Limit vs. Supply Voltage, G =, V = V, RL = kω 6288-3 INPUT COMMON-MODE VOLTAGE (V) 6 V, +3.8V 2 3.8V, +6.9V V S = ±5V +3.8V, +6.9V 8 4 V, +3.7V 3.8V, +.9V +3.9V, +.9V V S = ±5V 4 3.8V,.9V +3.8V, 2.V V, 4.V 8 3.8V, 6.9V +3.8V, 6.9V 2 6 V, 4V 6 2 8 4 4 8 2 6 OUTPUT VOLTAGE (V) Figure 26. Input Common-Mode Voltage Range vs. Output Voltage, G = 6288-28 CURRENT (ma) 5 5 +IN IN 5 5 6 2 8 4 4 8 2 6 DIFFERENTIAL INPUT VOLTAGE (V) Figure 29. Fault Current Draw vs. Input Voltage, G =, RL = kω 6288-3 Rev. Page of 24
AD825 OUTPUT VOLTAGE SWING FERED TO SUPPLY VOLTAGES (V).2.4.6.8. +. +.8 +.6 +.4 +.2 +25 C +85 C +85 C +25 C +25 C +25 C 4 6 8 2 4 6 SUPPLY VOLTAGE (±V S ) 4 C 4 C Figure 3. Output Voltage Swing vs. Supply Voltage, G =, RL = 2 kω 6288-32 OUTPUT VOLTAGE SWING ERRED TO SUPPLY VOLTAGES (V).4 +85 C +25 C.8.2.6 2. +2. +.6 +.2 +25 C 4 C 4 C +25 C +.8 +.4 2 +85 C 4 6 +25 C 8 2 4 6 OUTPUT CURRENT (ma) Figure 33. Output Voltage Swing vs. Output Current 6288-35 OUTPUT VOLTAGE SWING FERED TO SUPPLY VOLTAGES (V).2.4.6.8. +. +.8 +.6 +.4 +.2 +85 C +25 C +25 C +25 C +25 C +85 C 4 6 8 2 4 6 SUPPLY VOLTAGE (±V S ) 4 C 4 C Figure 3. Output Voltage Swing vs. Supply Voltage, G =, RL = kω 6288-33 V OUT (V) NO LOAD 2mV/DIV 47pF pf TIME (µs) 2µs/DIV Figure 34. Small Signal Pulse Response for Various Capacitive Loads 6288-36 5 +25 C OUTPUT VOLTAGE SWING (V) 5 5 +25 C 4 C +85 C +85 C +25 C 4 C +25 C 5 k k LOAD RSISTANCE (Ω) Figure 32. Output Voltage Swing vs. Load Resistance 6288-34 5V/DIV.2%/DIV 585ns TO.% 65ns TO.% TIME (µs) 2µs/DIV Figure 35. Large Signal Pulse Response and Settling Time, G =, RL = kω 6288-37 Rev. Page 2 of 24
AD825 5V/DIV.2%/DIV 65ns TO.% 635ns TO.% V OUT (V) TIME (µs) 2µs/DIV Figure 36. Large Signal Pulse Response and Settling Time G = 2, RL = kω 6288-38 2mV/DIV TIME (µs) Figure 39. Small Signal Response G =, RL = 2 kω, CL = pf 2µs/DIV 6288-42 5V/DIV.2%/DIV 65ns TO.% 635ns TO.% V OUT (V) TIME (µs) 2µs/DIV Figure 37. Large Signal Pulse Response and Settling Time G = 5, RL = kω 6288-39 2mV/DIV TIME (µs) Figure 4. Small Signal Response G = 2, RL = 2 kω, CL = pf 2µs/DIV 6288-43 5V/DIV.2%/DIV 648ns TO.% 685ns TO.% V OUT (V) TIME (µs) 2µs/DIV Figure 38. Large Signal Pulse Response and Settling Time G =, RL = kω 6288-4 2mV/DIV TIME (µs) Figure 4. Small Signal Response G = 5, RL = 2 kω, CL = pf 2µs/DIV 6288-44 Rev. Page 3 of 24
AD825 2 8 SETTLED TO.% V OUT (V) TIME (ns) 6 SETTLED TO.% 4 2 2mV/DIV 2µs/DIV TIME (µs) Figure 42. Small Signal Response, G =, RL = 2 kω, CL = pf 6288-45 2 4 6 8 2 4 6 8 2 STEP SIZE (V) Figure 45. Settling Time vs. Step Size, G = 5, RL = kω 6288-5 2 2 8 SETTLED TO.% 8 SETTLED TO.% TIME (ns) 6 SETTLED TO.% TIME (ns) 6 SETTLED TO.% 4 4 2 2 2 4 6 8 2 4 6 8 2 STEP SIZE (V) Figure 43. Settling Time vs. Step Size, G =, RL = kω 6288-48 2 4 6 8 2 4 6 8 2 STEP SIZE (V) Figure 46. Settling Time vs. Step Size, G =, RL = kω 6288-46 2 8 SETTLED TO.% TIME (ns) 6 4 SETTLED TO.% 2 2 4 6 8 2 4 6 8 2 STEP SIZE (V) Figure 44. Settling Time vs. Step Size, G = 2, RL = kω 6288-47 Rev. Page 4 of 24
AD825 THEORY OF OPERATION A A 2.2kΩ IN 2.2kΩ A kω kω DIGITAL GAIN CONTROL A3 OUTPUT A2 kω kω +IN 2.2kΩ 2.2kΩ WR Figure 47. Simplified Schematic 6288-54 The AD825 is a monolithic instrumentation amplifier based on the classic, three-op amp topology as shown in Figure 47. It is fabricated on the Analog Devices, Inc. proprietary icmos process that provides precision, linear performance and a robust digital interface. A parallel interface allows users to digitally program gains of, 2, 5, and. Gain control is achieved by switching resistors in an internal, precision, resistor array (as shown in Figure 47). Although the AD825 has a voltage feedback topology, gain bandwidth product increases for gains of, 2, and 5 because each gain has its own frequency compensation. This results in maximum bandwidth at higher gains. All internal amplifiers employ distortion cancellation circuitry and achieve high linearity and ultralow THD. Laser trimmed resistors allow for a maximum gain error of less than.3% for G =, and minimum CMRR of 98 db for G =. A pinout optimized for high CMRR over frequency enables the AD825 to offer a guaranteed minimum CMRR over frequency of 8 db at 5 khz (G = ). The balanced input reduces the parasitics that, in the past, had adversely affected CMRR performance. GAIN SELECTION This section shows users how to configure the AD825 for basic operation. Logic low and Logic high voltage limits are listed in the Specifications section. Typically, Logic low is V and Logic high is 5 V; both voltages are measured with respect to. Refer to the specifications table (see Table 2) for the permissible voltage range of. The gain of the AD825 can be set using two methods. Transparent Gain Mode The easiest way to set the gain is to program it directly via a Logic high or Logic low voltage applied to A and A. Figure 48 shows an example of this gain setting method, referred to throughout the data sheet as transparent gain mode. Tie WR to the negative supply to engage transparent gain mode. In this mode, any change in voltage applied to A and A from Logic low to Logic high, or vice versa, immediately results in a gain change. Table 5 is the truth table for transparent gain mode and Figure 48 shows the AD825 configured in transparent gain mode. μf +IN IN μf.µf.µf +5V WR A A AD825 5V +5V +5V G = 5V NOTE:. IN TRANSPARENT GAIN MODE, WR IS TIED TO V S. THE VOLTAGE LEVELS ON A AND A DETERMINE THE GAIN. IN THIS EXAMPLE, BOTH A AND A ARE SET TO LOGIC HIGH, RESULTING IN A GAIN OF. Figure 48. Transparent Gain Mode, A and A = High, G = 6288-55 Rev. Page 5 of 24
AD825 Table 5. Truth Table Logic Levels for Transparent Gain Mode WR A A Gain VS Low Low VS Low High 2 VS High Low 5 VS High High LATCHED GAIN MODE Some applications have multiple programmable devices such as multiplexers or other programmable gain instrumentation amplifiers on the same PCB. In such cases, devices can share a data bus. The gain of the AD825 can be set using WR as a latch, allowing other devices to share A and A. Figure 49 shows a schematic using this method, known as latched gain mode. The AD825 is in this mode when WR is held at Logic high or Logic low, typically 5 V and V, respectively. The voltages on A and A are read on the downward edge of the WR signal as it transitions from Logic high to Logic low. This latches in the logic levels on A and A, resulting in a gain change. See the truth table listing in Table 6 for more on these gain changes. μf +IN IN μf.µf.µf +5V + WR AD825 5V NOTE:. ON THE DOWNWARD EDGE OF WR, AS IT TRANSITIONS FROM LOGIC HIGH TO LOGIC LOW, THE VOLTAGES ON A AND A ARE READ AND LATCHED IN, RESULTING IN A GAIN CHANGE. IN THIS EXAMPLE, THE GAIN SWITCHES TO G =. A A A G = PREVIOUS STATE WR A Figure 49. Latched Gain Mode, G = +5V V +5V V +5V V G = 6288-56 Table 6. Truth Table Logic Levels for Latched Gain Mode WR A A Gain High to Low Low Low Change to High to Low Low High Change to 2 High to Low High Low Change to 5 High to Low High High Change to Low to Low X X No Change Low to High X X No Change High to High X X No Change X = don t care. Upon power up, the AD825 defaults to a gain of when in latched gain mode. In contrast, if the AD825 is configured in transparent gain mode, it starts at the gain indicated by the voltage levels on A and A upon power-up. Timing for Latched Gain Mode In latched gain mode, logic levels at A and A have to be held for a minimum setup time, tsu, before the downward edge of WR latches in the gain. Similarly, they must be held for a minimum hold time of thd after the downward edge of WR to ensure that the gain is latched in correctly. After thd, A and A may change logic levels but the gain does not change (until the next downward edge of WR). The minimum duration that WR can be held high is t WR-HIGH, and t WR-LOW is the minimum duration that WR can be held low. Digital timing specifications are listed in Table 2. The time required for a gain change is dominated by the settling time of the amplifier. A timing diagram is shown in Figure 5. When sharing a data bus with other devices, logic levels applied to those devices can potentially feed through to the output of the AD825. Feedthrough can be minimized by decreasing the edge rate of the logic signals. Furthermore, careful layout of the PCB also reduces coupling between the digital and analog portions of the board. t WR-HIGH t WR-LOW WR t SU t HD A, A Figure 5. Timing Diagram for Latched Gain Mode 6288-57 Rev. Page 6 of 24
AD825 POWER SUPPLY REGULATION AND BYPASSING The AD825 has high PSRR. However, for optimal performance, a stable dc voltage should be used to power the instrumentation amplifier. Noise on the supply pins can adversely affect performance. As in all linear circuits, bypass capacitors must be used to decouple the amplifier. Place a. μf capacitor close to each supply pin. A μf tantalum capacitor can be used further away from the part (see Figure 5) and, in most cases, it can be shared by other precision integrated circuits. INCORRECT AD825 TRANSFORMER CORRECT AD825 TRANSFORMER.µF µf +IN WR A A AD825 AD825 MΩ AD825 V OUT IN LOAD THERMOCOUPLE THERMOCOUPLE.µF µf C C Figure 5. Supply Decoupling,, and Output Referred to Ground 6288-58 C AD825 f HIGH-PASS = 2πRC C R AD825 INPUT BIAS CURRENT RETURN PATH R The AD825 input bias current must have a return path to its local analog ground. When the source, such as a thermocouple, cannot provide a return current path, one should be created (see Figure 52). CAPACITIVELY COUPLED CAPACITIVELY COUPLED Figure 52. Creating an IBIAS Path INPUT PROTECTION All terminals of the AD825 are protected against ESD. Note that 2.2 kω series resistors precede the ESD diodes as shown in Figure 47. They limit current into the diodes and allow for dc overload conditions 3 V above the positive supply and 3 V below the negative supply. An external resistor should be used in series with each of the inputs to limit current for voltages greater than3 V beyond either supply rail. In either scenario, the AD825 safely handles a continuous 6 ma current at room temperature. For applications where the AD825 encounters extreme overload voltages, external series resistors and low leakage diode clamps such as BAV99Ls, FJHs, or SP72s should be used. 6288-59 Rev. Page 7 of 24
AD825 ERENCE TERMINAL The reference terminal,, is at one end of a kω resistor (see Figure 47). The instrumentation amplifier output is referenced to the voltage on the terminal; this is useful when the output signal needs to be offset to voltages other than its local analog ground. For example, a voltage source can be tied to the pin to level shift the output so that the AD825 can interface with a single-supply ADC. The allowable reference voltage range is a function of the gain, common-mode input, and supply voltages. The pin should not exceed either +VS or VS by more than.5 V. For best performance, especially in cases where the output is not measured with respect to the terminal, source impedance to the terminal should be kept low because parasitic resistance can adversely affect CMRR and gain accuracy. V INCORRECT AD825 V CORRECT + OP77 AD825 Figure 53. Driving the Reference Pin COMMON-MODE INPUT VOLTAGE RANGE The three-op amp architecture of the AD825 applies gain and then removes the common-mode voltage. Therefore, internal nodes in the AD825 experience a combination of both the gained signal and the common-mode signal. This combined signal can be limited by the voltage supplies even when the individual input and output signals are not. Figure 26 and Figure 27 show the allowable common-mode input voltage ranges for various output voltages, supply voltages, and gains. LAYOUT Grounding In mixed-signal circuits, low level analog signals need to be isolated from the noisy digital environment. Designing with the AD825 is no exception. Its supply voltages are referenced to an analog ground. Its digital circuit is referenced to a digital ground. Although it is convenient to tie both grounds to a single ground plane, the current traveling through the ground wires and PC board can cause a large error. Therefore, use separate analog and digital ground planes. Only at one point, star ground, should analog and digital ground meet. 6288-6 Coupling Noise To prevent coupling noise onto the AD825, follow these guidelines: Do not run digital lines under the device. Run the analog ground plane under the AD825. Shield fast switching signals with digital ground to avoid radiating noise to other sections of the board, and never run them near analog signal paths. Avoid crossover of digital and analog signals. Power supply lines should use large traces to ensure a low impedance path. Decoupling is necessary; follow the guidelines listed in the Power Supply Regulation and Bypassing section. Common-Mode Rejection The AD825 has high CMRR over frequency giving it greater immunity to disturbances, such as line noise and its associated harmonics, in contrast to typical in-amps whose CMRR falls off around 2 Hz. Those in-amps often need common-mode filters at the inputs to compensate for this shortcoming. The AD825 is able to reject CMRR over a greater frequency range, reducing the need for input common-mode filtering. Careful board layout maximizes system performance. To maintain high CMRR over frequency, lay out the input traces symmetrically. Ensure that the traces maintain resistive and capacitive balance; this holds for additional PCB metal layers under the input pins and traces. Source resistance and capacitance should be placed as close to the inputs as possible. Should a trace cross the inputs (from another layer), it should be routed perpendicular to the input traces. RF INTERFERENCE RF rectification is often a problem when amplifiers are used in applications where there are strong RF signals. The disturbance can appear as a small dc offset voltage. High frequency signals can be filtered with a low-pass, RC network placed at the input of the instrumentation amplifier, as shown in Figure 54. The filter limits the input signal bandwidth according to the following relationship: FilterFreqDIFF = 2 π R(2C + C ) FilterFreqCM = where CD CC. 2π RCC D C The output voltage of the AD825 develops with respect to the potential on the reference terminal. Take care to tie to the appropriate local analog ground or to connect it to a voltage that is referenced to the local analog ground. Rev. Page 8 of 24
AD825 R R C C C D C C.µF +IN IN +5V AD825 µf.µf µf 5V Figure 54. RFI Suppression V OUT Values of R and CC should be chosen to minimize RFI. Mismatch between the R CC at the positive input and the R CC at negative input degrades the CMRR of the AD825. By using a value of CD that is times larger than the value of CC, the effect of the mismatch is reduced and performance is improved. DRIVING AN ANALOG-TO-DIGITAL CONVERTER An instrumentation amplifier is often used in front of an analog-todigital converter to provide CMRR. Usually, instrumentation amplifiers require a buffer to drive an ADC. However, the low output noise, low distortion, and low settle time of the AD825 make it an excellent ADC driver. 6288-6 In this example, a nf capacitor and a 49.9 Ω resistor create an antialiasing filter for the AD762. The nf capacitor also serves to store and deliver necessary charge to the switched capacitor input of the ADC. The 49.9 Ω series resistor reduces the burden of the nf load from the amplifier and isolates it from the kickback current injected from the switched capacitor input of the AD762. Selecting too small a resistor improves the correlation between the voltage at the output of the AD825 and the voltage at the input of the AD762, but may destabilize the AD825. A trade off must be made between selecting a resistor small enough to maintain accuracy and large enough to maintain stability. μf +IN IN μf.µf.µf +5V 5V WR A A AD825 49.9Ω nf Figure 55. Driving an ADC.μF +2V 2V AD762 +5V ADR435.μF 6288-62 Rev. Page 9 of 24
AD825 APPLICATIONS DIFFERENTIAL OUTPUT In certain applications, it is necessary to create a differential signal. High resolution analog-to-digital converters often require a differential input. In other cases, transmission over a long distance can require differential signals for better immunity to interference. Figure 57 shows how to configure the AD825 to output a differential signal. An op amp, the AD87, is used in an inverting topology to create a differential voltage. V sets the output midpoint according to the equation shown in the figure. Errors from the op amp are common to both outputs and are thus common mode. Likewise, errors from using mismatched resistors cause a common-mode dc offset error. Such errors are rejected in differential signal processing by differential input ADCs or instrumentation amplifiers. When using this circuit to drive a differential ADC, V can be set using a resistor divider from the ADC reference to make the output ratiometric with the ADC. SETTING GAINS WITH A MICROCONTROLLER μf +IN IN μf.µf.µf +5V + 5V WR A A AD825 MICRO- CONTROLLER Figure 56. Programming Gain Using a Microcontroller 6288-63 +2V.μF +5V 5V AMPLITUDE V IN +IN + WR A AD825 G = A 4.99kΩ V OUT A = V IN + V 2 AMPLITUDE +2.5V 2.5V V TIME +2V 2V μf.μf μf 2V 4.99kΩ 2V pf.µf + AD87 +2V.µF V OUT B = V IN + V 2 Figure 57. Differential Output with Level Shift V V AMPLITUDE +2.5V 2.5V V TIME 6288-64 Rev. Page 2 of 24
AD825 DATA ACQUISITION The AD825 makes an excellent instrumentation amplifier for use in data acquisition systems. Its wide bandwidth, low distortion, low settling time, and low noise enable it to condition signals in front of a variety of 6-bit ADCs. Figure 59 shows a schematic of the AD825x data acquisition demonstration board. The quick slew rate of the AD825 allows it to condition rapidly changing signals from the multiplexed inputs. An FPGA controls the AD762, AD825, and ADG29. In addition, mechanical switches and jumpers allow users to pin strap the gains when in transparent gain mode. This system achieved db of THD at khz and a signal-tonoise ratio of 9 db during testing as shown in Figure 58. AMPLITUDE (db) 2 3 4 5 6 7 8 9 2 3 4 5 5 2 25 3 35 4 45 5 FREQUENCY (khz) Figure 58. FFT of the AD825x DAQ Demo Board Using the AD825, khz Signal 6288-66 JMP.µF +2V +2V 2V + + µf µf JMP +5V 2kΩ +CH +CH2 +CH3 +CH4 CH4 CH3 CH2 CH 86Ω 86Ω 86Ω 86Ω 86Ω 86Ω 86Ω 86Ω 4 V DD 2 4 SA EN 5 S2A 6 S3A 7 S4A 8 ADG29 S4B S3B 9 2 S2B 5 A SB A V SS 6 3 3 GND Ω Ω C D Ω Ω C C +IN C C IN C3.µF 2 JMP WR + 4 A A AD825 V V 9 S 8 6 3 5 +2V 2V C4.µF +5V 2kΩ VOUT 7 Ω 49.9Ω +IN nf AD762 ADR435 ALTERA EPF6ATC44-3.µF 2V JMP +5V 2kΩ JMP +5V R8 2kΩ Figure 59. Schematic of ADG29, AD825, and AD762 in the AD825x DAQ Demo Board 6288-65 Rev. Page 2 of 24
AD825 OUTLINE DIMENSIONS 3. 3. 2.9 3. 3. 2.9 6 5 5.5 4.9 4.65 PIN.5 BSC.95.85.75.5.5.33.7 COPLANARITY.. MAX SEATING PLANE.23.8 8.8.6.4 COMPLIANT TO JEDEC STANDARDS MO-87-BA Figure 6. -Lead Mini Small Outline Package [MSOP] (RM-) Dimensions shown in millimeters ORDERING GUIDE Model Temperature Range Package Description Package Option Branding AD825ARMZ 4 C to +85 C -Lead MSOP RM- H AD825ARMZ-RL 4 C to +85 C -Lead MSOP RM- H AD825ARMZ-R7 4 C to +85 C -Lead MSOP RM- H AD825-EVALZ Evaluation Board Z = Pb-free part. Rev. Page 22 of 24
AD825 NOTES Rev. Page 23 of 24
AD825 NOTES 27 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D6288--/7() Rev. Page 24 of 24