Datasheet

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3 nV/√Hz, Low Power

Instrumentation Amplifier

Data Sheet

AD8421

Rev. 0

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FEATURES

Low power

2.3 mA maximum supply current

Low noise

3.2 nV/√Hz maximum input voltage noise at 1 kHz

200 fA/√Hz current noise at 1 kHz

Excellent ac specifications

10 MHz bandwidth (G = 1)

2 MHz bandwidth (G = 100)

0.6 μs settling time to 0.001% (G = 10)

80 dB CMRR at 20 kHz (G = 1)

35 V/μs slew rate

High precision dc performance (AD8421BRZ)

94 dB CMRR minimum (G = 1)

0.2 μV/°C maximum input offset voltage drift

1 ppm/°C maximum gain drift (G = 1)

500 pA maximum input bias current

Inputs protected to 40 V from opposite supply

±2.5 V to ±18 V dual supply (5 V to 36 V single supply)

Gain set with a single resistor (G = 1 to 10,000)

APPLICATIONS

Medical instrumentation

Precision data acquisition

Microphone preamplification

Vibration analysis

Multiplexed input applications

ADC driver

PIN CONNECTION DIAGRAM

TOP VIEW

(Not to Scale)

–IN

+IN

OUT

REF

–V

AD8421

10123-001

Figure 1.

10µ

100 1M

TOTAL NOISE DENSITY AT 1kHz (V/√Hz)

SOURCE RESISTANCE, R

(Ω)

10123-078

1µ

100n

10n

1k 10k 100k

G = 100

AD8421

BEST AVAILABLE

1mA LOW POWER IN-AMP

BEST AVAILABLE

7mA LOW NOISE IN-AMP

NOISE ONLY

Figure 2. Noise Density vs. Source Resistance

GENERAL DESCRIPTION

The AD8421 is a low cost, low power, extremely low noise, ultralow

bias current, high speed instrumentation amplifier that is ideally

suited for a broad spectrum of signal conditioning and data

acquisition applications. This product features extremely high

CMRR, allowing it to extract low level signals in the presence of

high frequency common-mode noise over a wide temperature

range.

The 10 MHz bandwidth, 35 V/µs slew rate, and 0.6 µs settling

time to 0.001% (G = 10) allow the AD8421 to amplify high speed

signals and excel in applications that require high channel count,

multiplexed systems. Even at higher gains, the current feedback

architecture maintains high performance; for example, at G = 100,

the bandwidth is 2 MHz and the settling time is 0.8 µs. The

AD8421 has excellent distortion performance, making it suitable

for use in demanding applications such as vibration analysis.

The AD8421 delivers 3 nV/√Hz input voltage noise and

200 fA/√Hz current noise with only 2 mA quiescent current,

making it an ideal choice for measuring low level signals. For

applications with high source impedance, the AD8421 employs

innovative process technology and design techniques to provide

noise performance that is limited only by the sensor.

The AD8421 uses unique protection methods to ensure robust

inputs while still maintaining very low noise. This protection

allows input voltages up to 40 V from the opposite supply rail

without damage to the part.

A single resistor sets the gain from 1 to 10,000. The reference

pin can be used to apply a precise offset to the output voltage.

The AD8421 is specified from −40°C to +85°C and has typical

performance curves to 125°C. It is available in 8-lead MSOP

and SOIC packages.

Summary of content (28 pages)

PAGE 1
3 nV/√Hz, Low Power Instrumentation Amplifier AD8421 Data Sheet Medical instrumentation Precision data acquisition Microphone preamplification Vibration analysis Multiplexed input applications ADC driver AD8421 –IN 1 8 +VS RG 2 7 VOUT RG 3 6 REF +IN 4 5 –VS TOP VIEW (Not to Scale) Figure 1.
PAGE 2
AD8421 Data Sheet TABLE OF CONTENTS Features .............................................................................................. 1 Gain Selection............................................................................. 20 Applications....................................................................................... 1 Reference Terminal .................................................................... 21 Pin Connection Diagram .....................................................
PAGE 3
Data Sheet AD8421 SPECIFICATIONS VS = ±15 V, VREF = 0 V, TA = 25°C, G = 1, RL = 2 kΩ, unless otherwise noted. AR AND BR GRADES Table 1.
PAGE 4
AD8421 Parameter DYNAMIC RESPONSE Small Signal Bandwidth G=1 G = 10 G = 100 G = 1000 Settling Time to 0.01% G=1 G = 10 G = 100 G = 1000 Settling Time to 0.001% G=1 G = 10 G = 100 G = 1000 Slew Rate G = 1 to 100 GAIN 3 Gain Range Gain Error G=1 G = 10 to 1000 Gain Nonlinearity G=1 G = 10 to 1000 Gain vs.
PAGE 5
Data Sheet Parameter POWER SUPPLY Operating Range Quiescent Current Over Temperature TEMPERATURE RANGE For Specified Performance Operational 5 AD8421 Test Conditions/ Comments Min Dual supply Single supply ±2.5 5 AR Grade Typ Max 2 TA = −40°C to +85°C −40 −40 Min ±18 36 2.3 2.6 ±2.5 5 +85 +125 −40 −40 BR Grade Typ 2 Max Unit ±18 36 2.3 2.6 V V mA mA +85 +125 °C °C Total voltage noise = √(eni2 + (eno/G)2 + eRG2). See the Theory of Operation section for more information.
PAGE 6
AD8421 Parameter Offset RTI vs. Supply (PSR) G=1 G = 10 G = 100 G = 1000 INPUT CURRENT Input Bias Current Over Temperature Average TC Input Offset Current Over Temperature Average TC DYNAMIC RESPONSE Small Signal Bandwidth G=1 G = 10 G = 100 G = 1000 Settling Time 0.01% G=1 G = 10 G = 100 G = 1000 Settling Time 0.001% G=1 G = 10 G = 100 G = 1000 Slew Rate G = 1 to 100 GAIN 3 Gain Range Gain Error G=1 G = 10 to 1000 Gain Nonlinearity G=1 G = 10 to 1000 Gain vs.
PAGE 7
Data Sheet Parameter OUTPUT Output Swing Over Temperature Short-Circuit Current REFERENCE INPUT RIN IIN Voltage Range Reference Gain to Output POWER SUPPLY Operating Range Quiescent Current Over Temperature TEMPERATURE RANGE For Specified Performance Operational 5 AD8421 Test Conditions/ Comments RL = 2 kΩ VS = ±2.5 V to ±18 V TA = −40°C to +85°C Min ARM Grade Typ Max −VS + 1.2 −VS + 1.2 +VS − 1.6 +VS − 1.6 Min −VS + 1.2 −VS + 1.2 65 20 20 VIN+, VIN− = 0 V −VS ±2.
PAGE 8
AD8421 Data Sheet ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE Table 3. Parameter Supply Voltage Output Short-Circuit Current Duration Maximum Voltage at −IN or +IN1 Minimum Voltage at −IN or +IN Maximum Voltage at REF2 Minimum Voltage at REF Storage Temperature Range Operating Temperature Range Maximum Junction Temperature ESD Human Body Model Charged Device Model Machine Model θJA is specified for a device in free air using a 4-layer JEDEC printed circuit board (PCB).
PAGE 9
Data Sheet AD8421 –IN 1 RG AD8421 8 +VS 2 7 VOUT RG 3 6 REF +IN 4 5 –VS TOP VIEW (Not to Scale) 10123-002 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS Figure 3. Pin Configuration Table 5. Pin Function Descriptions Pin No. 1 2, 3 4 5 6 7 8 Mnemonic −IN RG +IN −VS REF VOUT +VS Description Negative Input Terminal. Gain Setting Terminals. Place resistor across the RG pins to set the gain. G = 1 + (9.9 kΩ/RG). Positive Input Terminal. Negative Power Supply Terminal.
PAGE 10
AD8421 Data Sheet TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, VS = ±15 V, VREF = 0 V, RL = 2 kΩ, unless otherwise noted. 600 600 500 500 300 300 200 200 100 100 0 –60 –40 –20 0 20 40 60 INPUT OFFSET VOLTAGE (µV) 0 –400 –200 –100 0 100 200 300 400 OUTPUT OFFSET VOLTAGE (µV) Figure 7. Typical Distribution of Output Offset Voltage Figure 4. Typical Distribution of Input Offset Voltage 1800 1200 1500 1000 1200 800 UNITS 900 600 600 400 300 0 –2.0 –1.5 –1.0 –0.
PAGE 11
Data Sheet AD8421 4 15 G=1 G = 100 VS = ±15V 3 COMMON-MODE VOLTAGE (V) VS = ±12V 5 0 –5 –10 VS = ±2.5V 0 –1 0 5 10 15 –3 –4 –3 –2 –1 0 1 2 3 4 OUTPUT VOLTAGE (V) Figure 10. Input Common-Mode Voltage vs. Output Voltage; VS = ±12 V and ±15 V (G = 1) 10123-012 –5 10123-009 –10 OUTPUT VOLTAGE (V) Figure 13. Input Common-Mode Voltage vs. Output Voltage; VS = ±2.5 V and ±5 V (G = 100) 4 40 G=1 VS = ±5V 3 30 VS = 5V G=1 20 2 INPUT CURRENT (mA) VS = ±2.
PAGE 12
AD8421 Data Sheet 160 40 GAIN = 1000 140 GAIN = 100 POSITIVE PSRR (dB) 120 GAIN = 10 10 0 –10 100 GAIN = 1 80 60 –20 40 –30 20 –40 –35 –30 –25 –20 –15 –10 –5 0 5 10 15 20 25 30 35 40 INPUT VOLTAGE (V) 0 0.1 10123-015 INPUT CURRENT (mA) 20 1 Figure 16. Input Overvoltage Performance; +VS = 5 V, −VS = 0 V, G = 100 100 1k FREQUENCY (Hz) 10k 100k 1M 100k 1M Figure 19. Positive PSRR vs.
PAGE 13
Data Sheet 160 6 GAIN = 1000 REPRESENTATIVE SAMPLES GAIN = 100 140 4 BIAS CURRENT (nA) GAIN = 10 120 GAIN = 1 100 80 60 2 0 –2 –4 –6 1 10 100 1k 10k 100k FREQUENCY (Hz) –8 –40 10123-021 40 0.1 –25 –10 5 20 35 50 65 80 95 110 125 110 125 110 125 TEMPERATURE (°C) 10123-024 CMRR (dB) AD8421 Figure 25. Input Bias Current vs. Temperature Figure 22. CMRR vs.
PAGE 14
AD8421 Data Sheet 40 3.0 –SR 35 VS = ±15V 30 2.0 SLEW RATE (V/µs) VS = ±5V 1.5 1.0 20 15 10 0.5 –10 5 20 35 50 65 80 95 110 125 TEMPERATURE (°C) 0 –40 10123-026 –25 –10 5 20 35 50 65 80 95 110 125 TEMPERATURE (°C) Figure 31. Slew Rate vs. Temperature, VS = ±5 V (G = 1) Figure 28. Supply Current vs. Temperature (G = 1) +VS 80 –0.5 ISHORT+ 40 20 0 –20 –40 –60 –80 ISHORT– –120 –40 –25 –10 5 20 35 –1.5 –2.0 –2.5 +2.5 +2.0 +1.
PAGE 15
AD8421 +VS 5 –0.5 4 GAIN = 1 –1.0 3 –1.5 NONLINEARITY (ppm) –2.0 –40°C +25°C +85°C +105°C +125°C –2.5 +2.5 +2.0 2 1 0 –1 –2 +1.5 –3 +1.0 0 2 4 6 8 10 12 14 16 18 20 SUPPLY VOLTAGE (±VS) –5 –10 10123-032 –VS RL = 2kΩ RL = 10kΩ –4 +0.5 –8 –6 –4 –2 0 2 4 6 8 10 OUTPUT VOLTAGE (V) Figure 34. Output Voltage Swing vs. Supply Voltage, RL = 600 Ω 10123-035 OUTPUT VOLTAGE (V) REFERRED TO SUPPLY VOLTAGES Data Sheet Figure 37.
PAGE 16
AD8421 Data Sheet 10k 100 GAIN = 1000 80 CURRENT NOISE (fA/√Hz) NONLINEARITY (ppm) 60 40 20 RL = 600Ω 0 –20 –40 1k 100 –60 –4 –3 –2 –1 0 1 2 3 4 5 OUTPUT VOLTAGE (V) 10 0.1 10123-073 –100 –5 1 10 100 1k 10k 10123-039 –80 100k FREQUENCY (Hz) Figure 40. Gain Nonlinearity (G = 1000), RL = 600 Ω, VOUT = ±5 V Figure 43. Current Noise Spectral Density vs.
PAGE 17
Data Sheet AD8421 5V/DIV 5V/DIV 720ns TO 0.01% 1.12µs TO 0.001% 3.8µs TO 0.01% 5.76µs TO 0.001% 1µs/DIV Figure 46. Large Signal Pulse Response and Settling Time (G = 1), 10 V Step, VS = ±15 V, RL = 2 kΩ, CL = 100 pF 4µs/DIV 10123-044 0.002%/DIV 10123-041 0.002%/DIV Figure 49. Large Signal Pulse Response and Settling Time (G = 1000), 10 V Step, VS = ±15 V, RL = 2 kΩ, CL = 100 pF 2500 SETTLING TIME (ns) 2000 420ns TO 0.01% 604ns TO 0.001% 0.002%/DIV 1500 SETTLED TO 0.
PAGE 18
AD8421 Data Sheet 1µs/DIV 10123-047 50mV/DIV 20pF 50pF NO LOAD 100pF G=1 50mV/DIV Figure 52. Small Signal Pulse Response (G = 10), RL = 600 Ω, CL = 100 pF 1µs/DIV 10123-053 GAIN = 10 Figure 55. Small Signal Response with Various Capacitive Loads (G = 1), RL = Infinity –40 GAIN = 100 –50 RL ≥ 600Ω VOUT = 10V p-p –60 AMPLITUDE (dBc) –70 –80 –90 –100 –110 –120 1µs/DIV –140 –150 10 100 1k 10k FREQUENCY (Hz) 10123-055 20mV/DIV 10123-048 –130 Figure 56.
PAGE 19
Data Sheet –50 NO LOAD RL = 2kΩ RL = 600Ω –20 VOUT = 10V p-p –30 –40 –70 –80 –90 –60 –70 –80 –90 –100 –120 –110 100 1k 10k FREQUENCY (Hz) 10123-075 –130 –120 10 Figure 58. Second Harmonic Distortion vs. Frequency (G = 1000) –40 VOUT = 10V p-p RL ≥ 600Ω –60 –70 –80 –90 –100 1k 10k FREQUENCY (Hz) 10123-076 –110 100 –140 10 100 1k FREQUENCY (Hz) Figure 60. THD vs.
PAGE 20
AD8421 Data Sheet THEORY OF OPERATION +VS I VB I A1 IB COMPENSATION A2 C1 10kΩ +VS C2 10kΩ NODE 1 –IN R1 Q1 4.95kΩ superβ ESD AND OVERVOLTAGE PROTECTION NODE 3 +VS +VS RG +VS 10kΩ R2 4.95kΩ Q2 superβ ESD AND OVERVOLTAGE PROTECTION –VS 10kΩ REF +IN NODE 4 I OUTPUT A3 NODE 2 –VS I –VS 10123-057 IB COMPENSATION DIFFERENCE AMPLIFIER STAGE GAIN STAGE Figure 61. Simplified Schematic ARCHITECTURE The AD8421 is based on the classic 3-op-amp topology.
PAGE 21
Data Sheet AD8421 REFERENCE TERMINAL Common-Mode Rejection Ratio over Frequency The output voltage of the AD8421 is developed with respect to the potential on the reference terminal. This can be used to sense the ground at the load, thereby taking advantage of the CMRR to reject ground noise or to introduce a precise offset to the signal at the output. For example, a voltage source can be tied to the REF pin to level shift the output, allowing the AD8421 to drive a singlesupply ADC.
PAGE 22
AD8421 Data Sheet Reference Pin The output voltage of the AD8421 is developed with respect to the potential on the reference terminal. Ensure that REF is tied to the appropriate local ground. INPUT BIAS CURRENT RETURN PATH For applications where the AD8421 encounters voltages beyond the limits in the Absolute Maximum Ratings table, external protection is required. This external protection depends on the duration of the overvoltage event and the noise performance that is required.
PAGE 23
Data Sheet AD8421 Overvoltage performance is shown in Figure 14, Figure 15, Figure 16, and Figure 17. The AD8421 inputs can withstand a current of 40 mA at room temperature for at least a day. This time is cumulative over the life of the device. If long periods of overvoltage are expected, the use of an external protection method is recommended. Under extreme input conditions, the output of the amplifier may invert.
PAGE 24
AD8421 Data Sheet Voltage Noise of the Instrumentation Amplifier The voltage noise of the instrumentation amplifier is calculated using three parameters: the device output noise, the input noise, and the RG resistor noise. It is calculated as follows: For example, if the R1 source resistance in Figure 69 is 4 kΩ, and the R2 source resistance is 1 kΩ, the total effect from the current noise is calculated as follows: (4 × 0.2 )2 + (1 × 0.
PAGE 25
Data Sheet AD8421 APPLICATIONS INFORMATION DIFFERENTIAL OUTPUT CONFIGURATION Figure 70 shows an example of how to configure the AD8421 for differential output. +IN AD8421 Because this circuit is susceptible to instability, a capacitor is included to limit the effective op amp bandwidth. This capacitor can be omitted if the amplifier pairing is stable. +OUT –IN 10kΩ 12pF 10kΩ VBIAS The open-loop gain and phase of any amplifier may vary with process variation and temperature.
PAGE 26
AD8421 Data Sheet frequency, and set the filter cutoff to settle to ½ LSB in one sampling period for a full-scale step. For additional considerations, refer to the data sheet of the ADC in use. DRIVING AN ADC The Class AB output stage, low noise and distortion, and high bandwidth and slew rate make the AD8421 a good choice for driving an ADC in a data acquisition system that requires frontend gain, high CMRR, and dc precision.
PAGE 27
Data Sheet AD8421 OUTLINE DIMENSIONS 5.00 (0.1968) 4.80 (0.1890) 8 4.00 (0.1574) 3.80 (0.1497) 5 1 4 1.27 (0.0500) BSC 0.25 (0.0098) 0.10 (0.0040) 6.20 (0.2441) 5.80 (0.2284) 1.75 (0.0688) 1.35 (0.0532) 0.51 (0.0201) 0.31 (0.0122) COPLANARITY 0.10 SEATING PLANE 0.50 (0.0196) 0.25 (0.0099) 45° 8° 0° 0.25 (0.0098) 0.17 (0.0067) 1.27 (0.0500) 0.40 (0.