AD8420

Preliminary Technical Data 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 Wide Supply Range, Rail-to-Rail Output Instrumentation Amplifier AD8420 PIN CONFIGURATION NC 1 +IN 2 –IN 3 –VS 4 8 7 6 V OUT FB REF +VS AD8420 TOP VIEW (Not to Scale) 5 Figure 1. Table 1. Instrumentation Amplifiers by Category1 General Purpose AD8221/2 AD8220/4 AD8228 AD8295 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 APPLICATIONS Bridge amplifiers Pressure Measurement Medical instrumentation Portable data acquisition Multichannel systems 1 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 SPECIFICATIONS Preliminary Technical Data +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 Conditions VCM = 0 V to 2.7 V Min 100 Typ Max Unit 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 200 1 fA/√Hz pA p-p µV µV/°C dB nA nA nA pA/°C nA nA nA pA/°C MΩ||pF MΩ||pF V V V 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 40 Average Temperature Coefficient Input Offset Current 1 Average Temperature Coefficient Input Impedance Differential Common Mode Differential Input Operating Voltage Input Operating Voltage (+IN, -IN, or REF) 130||2 1000||2 TA = –40°C to +85°C TA = +25°C TA = +85°C TA = –40°C -1 −VS – 0.15 −VS – 0.05 −VS – 0.2 1 +VS −2.2 +VS – 1.8 +VS – 2.7 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 250 25 2.5 0.25 4 V step kHz kHz kHz kHz µs µs µs V/µs Exceeds Bandwidth Limit Rev. PrD | Page 2 of 2 Preliminary Technical Data GAIN 2 Gain Range Gain Error Gain vs. Temperature OUTPUT Output Swing RL = 10 kΩ to mid supply G = 1 + (R2/R1) 1 VOUT = 0.2V to 4.8V TA = −40°C to +85°C 1000 0.05 10 AD8420 V/V % ppm/°C TA = +25°C TA = +85°C TA = –40°C RL = 100 kΩ to mid supply TA = +25°C TA = +85°C TA = –40°C Short-Circuit Current POWER SUPPLY Operating Range Quiescent Current −VS + 0.15 +VS – 0.15 V V V V V V mA V µA µA µA µA °C °C −VS + 0.1 +VS – 0.1 10 Single supply operation 3 TA = +25°C TA = –40°C TA = +85°C TA = +85°C 2.7 75 36 90 100 −40 −40 +85 +125 TEMPERATURE RANGE Specified Operational 4 1 2 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 Rev. PrD | Page 3 of 3 AD8420 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 Conditions VCM = –10 V to +10 V Min 100 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 Typ Max Unit 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 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 200 1 fA/√Hz pA p-p µV µV/°C dB nA nA nA pA/°C nA nA nA pA/°C MΩ||pF MΩ||pF V V V V 25 40 Average Temperature Coefficient Input Offset Current 1 Average Temperature Coefficient Input Impedance Differential Common Mode Differential Input Operating Voltage Input Operating Voltage (+IN, -IN, or REF) 130||3 1000||3 TA = –40°C to +85°C TA = +25°C TA = +85°C TA = –40°C Check voltage differential -1 −VS – 0.15 −VS – 0.05 −VS – 0.2 1 +VS −2.2 +VS – 1.8 +VS – 2.7 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 250 25 2.5 0.25 10 V step kHz kHz kHz kHz µs µs µs µs V/µs Exceeds Bandwidth Limit Rev. PrD | Page 4 of 4 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 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 AD8420 V/V % ppm ppm ppm ppm/°C 10 TA = +25°C TA = +85°C TA = –40°C RL = 100 kΩ to ground TA = +25°C TA = +85°C TA = –40°C Short-Circuit Current POWER SUPPLY Operating Range Quiescent Current −VS + 0.15 +VS – 0.15 V V V V V V mA V µA µA µA °C °C −VS + 0.1 +VS – 0.1 10 Dual supply operation 3 TA = +25°C TA = –40°C TA = +85°C ±2.7 75 100 −40 −40 +85 +125 ±18V 90 TEMPERATURE RANGE Specified Operational 4 1 2 3 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 Rev. PrD | Page 5 of 5 AD8420 ABSOLUTE MAXIMUM RATINGS 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 Rating ±18 V Indefinite –Vs + 40V +Vs – 40V +Vs + 0.2V –Vs – 0.2V −65°C to +150°C Preliminary Technical Data THERMAL RESISTANCE θJA is specified for a device in free air. Table 5. Package 8-Lead MSOP, 4-Layer JEDEC Board θJA 135 Unit °C/W 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 Preliminary Technical Data PIN CONFIGURATION AND FUNCTION DESCRIPTIONS NC 1 +IN 2 –IN 3 –VS 4 8 7 6 AD8420 V OUT FB REF +VS AD8420 TOP VIEW (Not to Scale) 5 Figure 2. Pin Configuration Table 6. Pin Function Descriptions Pin No. 1 2 3 4 5 6 7 8 Mnemonic NC +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 THEORY OF OPERATION Preliminary Technical Data A3 VBIAS +VS VOUT R2 FB +IN ESD AND OVERVOLTAGE PROTECTION V –IN ESD AND OVERVOLTAGE PROTECTION I A1 V A2 I –VS +VS R1 REF –VS Figure 3. Simplified Schematic ARCHITECTURE The AD8420 consists of three amplifiers: two matched transconductance amplifiers that convert voltage to current and one integrator amplifier that converts current to voltage. 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. Table 7. Suggested Resistors for Various Gains - 1% Resistors R1 (kΩ) none 49.9 20 10 5 2 1 1 1 1 R2 (kΩ) short 49.9 80.6 90.9 95.3 97.6 100 200 499 1000 Gain 1.00 2.00 5.03 10.09 20.06 49.8 101 201 500 1001 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 +IN –IN VOUT SETTING THE GAIN The transfer function of the AD8420 is VOUT = G(VIN+ − VIN−) + VREF where: G =1+ R2 R1 FB R2 R1 R1||R2 REF G = 1+ R2 R1 VREF Figure 4. Cancelling Out Error from FB Input Bias Current Rev. PrD | Page 8 of 8 Preliminary Technical Data INPUT VOLTAGE RANGE 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: 1) 2) Keep differential input voltage within ±1V. Keep voltage on +IN, -IN, and REF pins in specified input voltage range AD8420 DRIVING THE REFERENCE PIN 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. No hexagonal figures. No complicated formulas. AD8420 +IN –IN VOUT 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. FB R2 R1 REF 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 +IN –IN VOUT FB R2 VS R1 R3 R4 Optional capacitor filters noise from Vs REF G = 1+ R2+R3||R4 R1 Figure 6. Using Resistor Divider to Set Reference Voltage Rev. PrD | Page 9 of 9 AD8420 OUTLINE DIMENSIONS 3.20 3.00 2.80 5.15 4.90 4.65 Preliminary Technical Data 3.20 3.00 2.80 8 5 1 4 PIN 1 0.65 BSC 0.95 0.85 0.75 0.15 0.00 0.38 0.22 SEATING PLANE 1.10 MAX 8° 0° 0.80 0.60 0.40 0.23 0.08 COPLANARITY 0.10 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 Package MSOP MSOP MSOP MSOP MSOP MSOP Branding Y3Y Y3Y Y3Y Y3Z Y3Z Y3Z Z = RoHS Compliant Part. Rev. PrD | Page 10 of 10