Preliminary Technical Data Wide Supply Range, Rail-to-Rail Output Instrumentation Amplifier AD8420 FEATURES Gain set with 2 external resistors Gain range: 1 to 1000 Input voltage goes below ground Very wide power supply range Single supply: 2.7V to 36V Dual supply: +/-2.7V to+/-18V Bandwidth (G=100): 2.5 kHz Input noise: 50 nV/√Hz Max supply current: 90 µA Max offset voltage: 200 uV Max differential input voltage: 1V Min CMRR: 100 dB MSOP-8 package APPLICATIONS Bridge amplifiers Pressure Measurement Medical instrumentation Portable data acquisition Multichannel systems PIN CONFIGURATION NC 1 8 +IN 2 7 FB –IN 3 6 REF 5 +VS –VS 4 AD8420 V OUT TOP VIEW (Not to Scale) Figure 1. Table 1. Instrumentation Amplifiers by Category1 General Purpose AD8221/2 AD8220/4 AD8228 AD8295 1 Zero Drift AD8231 AD8290 AD8293 AD8553 AD8556 AD8557 Military Grade AD620 AD621 AD524 AD526 AD624 Low Power AD8420 AD8235/6 AD627 AD8226/7 AD623 AD8223 Digital Gain AD8250 AD8251 AD8253 AD8231 See www.analog.com for the latest instrumentation amplifiers. GENERAL DESCRIPTION The AD8420 is a low cost, wide supply range amplifier that uses two resistors to set any gain between 1 and 1000. It is optimized to amplify small differential voltages in the presence of large common mode signals. The AD8420 is based on a current mode architecture that gives it excellent input common mode range. Unlike conventional instrumentation amplifiers, the AD8420 can easily amplify signals at or even slightly below ground without requiring dual supplies. The AD8420 has a full rail to rail output, and the output voltage is completely independent of the input common mode voltage. The AD8420 can operate off both single or dual supplies. It works well for a portable system with a limited single supply voltage and equally well for a system using large dual supplies. Gain is set using the ratio of two resistors. A reference pin allows the user to offset the output voltage. This feature is useful when the output signal needs to be centered around a specific voltage, such as mid-supply. The AD8420 is available in an 8 pin MSOP package. Performance is specified over the full temperature range of −40°C to +85°C. Part is operational from −40°C to +125°C Rev. PrD 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 9106, Norwood, MA 02062-9106, U.S.A. www.analog.com Tel: 781.329.4700 Fax: 781.461.3113 ©2011 Analog Devices, Inc. All rights reserved. AD8420 Preliminary Technical Data SPECIFICATIONS +VS = +5V, −VS = 0V, VREF = 0 V, VIN+=0V, VIN-=0V, TA = 25°C, G = 1 to 1000, RL = 20 kΩ, specifications referred to input, unless otherwise noted Table 2. Parameter COMMON-MODE REJECTION RATIO (CMRR) CMRR DC to 60 Hz CMRR at 1 kHz NOISE Voltage Noise Spectral Density Peak to Peak Current Noise Spectral Density Peak to Peak VOLTAGE OFFSET Offset Average Temperature Coefficient Offset RTI vs. Supply (PSR) INPUTS Input Bias Current 1 Average Temperature Coefficient Input Offset Current Average Temperature Coefficient Input Impedance Differential Common Mode Differential Input Operating Voltage Input Operating Voltage (+IN, -IN, or REF) DYNAMIC RESPONSE Small Signal –3 dB Bandwidth G=1 G = 10 G = 100 G =1000 Settling Time 0.01% G = 10 G = 100 G = 1000 Slew Rate Conditions VCM = 0 V to 2.7 V Min Typ Max 100 dB dB f = 1 kHz, VDIFF ≤100 mV f = 0.1 Hz to 10 Hz, VDIFF ≤ 100 mV 50 1.5 nV/√Hz µV p-p f = 1 kHz f = 0.1 Hz to 10 Hz fA/√Hz pA p-p TA = −40°C to +85°C Valid for REF & FB pair, as well as +IN & -IN TA = +25°C TA = +85°C TA = −40°C TA = −40°C to +85°C TA = +25°C TA = +85°C TA = −40°C TA = −40°C to +85°C 25 200 1 µV µV/°C dB 40 nA nA nA pA/°C nA nA nA pA/°C 1 130||2 1000||2 TA = –40°C to +85°C TA = +25°C TA = +85°C TA = –40°C Unit -1 −VS – 0.15 −VS – 0.05 −VS – 0.2 1 +VS −2.2 +VS – 1.8 +VS – 2.7 250 25 2.5 0.25 MΩ||pF MΩ||pF V V V V kHz kHz kHz kHz 4 V step Exceeds Bandwidth Limit Rev. PrD | Page 2 of 2 µs µs µs V/µs Preliminary Technical Data GAIN 2 Gain Range Gain Error Gain vs. Temperature OUTPUT Output Swing RL = 10 kΩ to mid supply AD8420 G = 1 + (R2/R1) 1 1000 0.05 10 V/V % ppm/°C TA = +25°C TA = +85°C TA = –40°C −VS + 0.15 +VS – 0.15 V V V TA = +25°C TA = +85°C TA = –40°C −VS + 0.1 +VS – 0.1 V V V mA 36 90 V µA µA µA µA +85 +125 °C °C VOUT = 0.2V to 4.8V TA = −40°C to +85°C RL = 100 kΩ to mid supply Short-Circuit Current POWER SUPPLY Operating Range Quiescent Current 10 Single supply operation 3 TA = +25°C TA = –40°C TA = +85°C TA = +85°C TEMPERATURE RANGE Specified Operational 4 2.7 75 100 −40 −40 The input stage uses pnp transistors, so input bias current always flows out of the part. For G>1, errors from external resistors R1 and R2 should be added to these specifications, including error from FB pin bias current. 3 Minimum supply voltage indicated for V+IN, V-IN, VREF= 0V. . 4 See Typical Performance Curves for operation between 85°C and 125°C 1 2 Rev. PrD | Page 3 of 3 AD8420 Preliminary Technical Data +VS = +15 V, −VS = −15 V, VREF = 0 V, TA = 25°C, G = 1 to 1000, RL = 20 kΩ, specifications referred to input, unless otherwise noted Table 3. Parameter COMMON-MODE REJECTION RATIO (CMRR) CMRR DC to 60 Hz CMRR at 1 kHz NOISE Voltage Noise Spectral Density Peak to Peak Current Noise Spectral Density Peak to Peak VOLTAGE OFFSET Offset Average Temperature Coefficient Offset RTI vs. Supply (PSR) INPUTS Input Bias Current 1 Average Temperature Coefficient Input Offset Current Average Temperature Coefficient Input Impedance Differential Common Mode Differential Input Operating Voltage Input Operating Voltage (+IN, -IN, or REF) DYNAMIC RESPONSE Small Signal –3 dB Bandwidth G=1 G = 10 G = 100 G =1000 Settling Time 0.01% G=1 G = 10 G = 100 G = 1000 Slew Rate Conditions VCM = –10 V to +10 V Min Typ Max 100 dB dB f = 1 kHz, VDIFF ≤ 100 mV f = 0.1 Hz to 10 Hz, VDIFF ≤ 100 mV 50 1.5 nV/√Hz µV p-p f = 1 kHz f = 0.1 Hz to 10 Hz fA/√Hz pA p-p VS = ±5 V to ±15 V TA = −40°C to +85°C Valid for REF & FB pair, as well as +IN & -IN TA = +25°C TA = +85°C TA = −40°C TA = −40°C to +85°C TA = +25°C TA = +85°C TA = −40°C TA = −40°C to +85°C 25 200 1 µV µV/°C dB 40 nA nA nA pA/°C nA nA nA pA/°C 1 130||3 1000||3 TA = –40°C to +85°C TA = +25°C TA = +85°C TA = –40°C Check voltage differential Unit -1 −VS – 0.15 −VS – 0.05 −VS – 0.2 1 +VS −2.2 +VS – 1.8 +VS – 2.7 250 25 2.5 0.25 MΩ||pF MΩ||pF V V V V kHz kHz kHz kHz 10 V step Exceeds Bandwidth Limit Rev. PrD | Page 4 of 4 µs µs µs µs V/µs Preliminary Technical Data GAIN 2 Gain Range Gain Error Gain Nonlinearity G = 1 to 10 G = 100 G = 1000 Gain vs. Temperature OUTPUT Output Swing RL = 20 kΩ to ground AD8420 G = 1 + (R2/R1) 1 VOUT ±10 V VOUT = –10 V to +10 V RL ≥ 20 kΩ RL ≥ 20 kΩ RL ≥ 20 kΩ TA = −40°C to +85°C 1000 0.05 V/V % 10 ppm ppm ppm ppm/°C TA = +25°C TA = +85°C TA = –40°C −VS + 0.15 +VS – 0.15 V V V TA = +25°C TA = +85°C TA = –40°C −VS + 0.1 +VS – 0.1 V V V mA ±18V 90 V µA µA µA +85 +125 °C °C RL = 100 kΩ to ground Short-Circuit Current POWER SUPPLY Operating Range Quiescent Current 10 Dual supply operation 3 TA = +25°C TA = –40°C TA = +85°C TEMPERATURE RANGE Specified Operational 4 ±2.7 75 100 −40 −40 The input stage uses pnp transistors, so input bias current always flows out of the part. For G>1, errors from external resistors R1 and R2 should be added to these specifications, including error from FB pin bias current Minimum positive supply voltage indicated for V+IN, V-IN, VREF= 0V. With V+IN, V-IN, VREF= -VS, minimum supply is ±1.35V. 4 See Typical Performance Curves for operation between 85°C and 125°C 1 2 3 Rev. PrD | Page 5 of 5 AD8420 Preliminary Technical Data ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE Table 4. Parameter Supply Voltage Output Short-Circuit Current Maximum Voltage at −IN or +IN Minimum Voltage at −IN or +IN Maximum Voltage at REF Minimum Voltage at REF Storage Temperature Range ESD Human Body Model Charge Device Model Machine Model θJA is specified for a device in free air. Rating ±18 V Indefinite –Vs + 40V +Vs – 40V +Vs + 0.2V –Vs – 0.2V −65°C to +150°C Table 5. Package 8-Lead MSOP, 4-Layer JEDEC Board ESD CAUTION 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. Rev. PrD | Page 6 of 6 θJA 135 Unit °C/W Preliminary Technical Data AD8420 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS NC 1 8 +IN 2 7 FB –IN 3 6 REF 5 +VS –VS 4 AD8420 V OUT TOP VIEW (Not to Scale) Figure 2. Pin Configuration Table 6. Pin Function Descriptions Pin No. 1 Mnemonic NC 2 3 4 5 6 7 8 +IN −IN −VS +VS REF FB VOUT Description This pin not connected internally. For best CMRR vs. frequency and leakage performance, connect this pin to negative supply. Positive Input. Negative Input Negative Supply. Positive Supply. Reference. Feedback. Output. Rev. PrD | Page 7 of 7 AD8420 Preliminary Technical Data THEORY OF OPERATION A3 +IN ESD AND OVERVOLTAGE PROTECTION ESD AND OVERVOLTAGE PROTECTION R2 +VS VBIAS FB V –IN VOUT V I –VS I R1 +VS REF A1 A2 –VS Figure 3. Simplified Schematic ARCHITECTURE Table 7. Suggested Resistors for Various Gains - 1% Resistors The AD8420 consists of three amplifiers: two matched transconductance amplifiers that convert voltage to current and one integrator amplifier that converts current to voltage. R1 (kΩ) none 49.9 20 10 5 2 1 1 1 1 The AD8420 works as follows: assume a differential voltage is applied across inputs +IN and -IN. This input voltage is converted into a current by Amplifier A1. This will create a difference in current between A1 and A2, which is fed into A3. A3’s output voltage will change until A2 sinks all the current A1 is generating. Because the gain of A1 and A2 are matched, this means the differential input voltage across A1 will appear across the inputs of A2. Gain is set by the ratio of R2 to R1. Because the AD8420 converts the input differential signals to a current, there are no internal headroom issues as with traditional instrumentation amplifier architectures. This is particularly important when amplifying a signal with a common mode voltage near one of the supply rails. To improve robustness and ease of use, the AD8420 includes overvoltage protection on its inputs. This protection scheme allows input voltages well beyond the supply rails (as well as wide differential input voltages) without damaging the part. R2 (kΩ) short 49.9 80.6 90.9 95.3 97.6 100 200 499 1000 While the ratio of R2 to R1 sets the gain, the absolute value of the resistors is up to the designer. Larger values reduce power consumption and output loading; smaller values limit FB input bias current error. A method that allows large value feedback resistors while limiting FB bias current error is to place a resistor of value R1||R2 in series with the REF terminal as shown in Figure 4. At higher gains, this resistor can simply be the same value as R1. AD8420 VOUT SETTING THE GAIN +IN FB The transfer function of the AD8420 is –IN REF VOUT = G(VIN+ − VIN−) + VREF R2 R1 R1||R2 where: G =1+ Gain 1.00 2.00 5.03 10.09 20.06 49.8 101 201 500 1001 R2 R1 G = 1+ R2 R1 VREF Figure 4. Cancelling Out Error from FB Input Bias Current Rev. PrD | Page 8 of 8 Preliminary Technical Data AD8420 INPUT VOLTAGE RANGE DRIVING THE REFERENCE PIN Unlike traditional instrumentation amplifier architectures, the allowed input range of the AD8420 is simplicity itself. For the AD8420’s transfer function to be valid, the input voltage should follow two rules: Traditional instrumentation amplifier architectures require the reference pin to be driven with a low impedance source. In traditional architectures, impedance at the reference pin degrades both CMRR and gain accuracy. With the AD8420 architecture, resistance at the reference pin has no effect on CMRR. 1) Keep differential input voltage within ±1V. 2) Keep voltage on +IN, -IN, and REF pins in specified input voltage range AD8420 No hexagonal figures. No complicated formulas. INPUT PROTECTION The AD8420 has very robust inputs and typically does not need additional input protection. Input voltages can be up to 40 V from the opposite supply rail. For example, with a +5 V positive supply and a −8 V negative supply, the part can safely withstand voltages from −35 V to 32 V. The part can handle large differential input voltages, even when the part is in high gain, without damage. The rest of the AD8420 terminals should be kept within the supplies. All terminals of the AD8426 are protected against ESD. For applications that require protection beyond the AD8420’s limits, place diodes at the AD8420 inputs to limit voltage and resistors in series with the inputs to limit the current into these diodes. To keep input bias current at minimum, low leakage diode clamps such as the BAV199 should be used. The AD8420 also combines well with TVS diodes such as the PTVSxS1UR. VOUT R2 +IN FB –IN REF R1 RREF VREF G = 1+ R2+RREF R1 Figure 5. Calculating Gain with Reference Resistance Resistance at the reference pin does affect the AD8420’s gain, but if this resistance is constant, the gain setting resistors can be adjusted to compensate. For example, the AD8420 can be driven with a voltage divider as shown in Figure 6. AD8420 VOUT FB +IN R2 VS R1 REF –IN R3 R4 G = 1+ Optional capacitor filters noise from Vs R2+R3||R4 R1 Figure 6. Using Resistor Divider to Set Reference Voltage Rev. PrD | Page 9 of 9 AD8420 Preliminary Technical Data OUTLINE DIMENSIONS 3.20 3.00 2.80 8 3.20 3.00 2.80 1 5 5.15 4.90 4.65 4 PIN 1 0.65 BSC 0.95 0.85 0.75 1.10 MAX 0.15 0.00 0.38 0.22 COPLANARITY 0.10 0.23 0.08 8° 0° 0.80 0.60 0.40 SEATING PLANE COMPLIANT TO JEDEC STANDARDS MO-187-AA Figure 7. 8-Lead Mini Small Outline Package [MSOP] (RM-8) Dimensions shown in millimeters ORDERING GUIDE Model 1 AD8420ARMZ AD8420ARMZ-R7 AD8420ARMZ-RL AD8420BRMZ AD8420BRMZ-R7 AD8420BRMZ-RL 1 Temperature Range −40°C to +125°C −40°C to +125°C −40°C to +125°C −40°C to +125°C −40°C to +125°C −40°C to +125°C Package Description Standard grade, tube Standard grade, 7 inch Tape and Reel Standard grade, 13 inch Tape and Reel High performance grade, tube High performance grade, 7 inch Tape and Reel High performance grade, 13 inch Tape and Reel Z = RoHS Compliant Part. Rev. PrD | Page 10 of 10 Package MSOP MSOP MSOP MSOP MSOP MSOP Branding Y3Y Y3Y Y3Y Y3Z Y3Z Y3Z