Three Op Amp Instrumentation Amplifier Circuit

Design Goals

Input V_idiff (V_i2 – V_i1)		Common-Mode Voltage	Output		Supply
V_{i diff Min}	V_{i diff Max}	V_cm	V_oMin	V_oMax	V_cc	V_ee	V_ref
-0.5 V	+0.5 V	±7 V	–5 V	+5 V	+15 V	–15 V	0 V

Design Description

This design uses 3 op amps to build a discrete instrumentation amplifier. The circuit converts a differential signal to a single-ended output signal. Linear operation of an instrumentation amplifier depends upon linear operation of its building block: op amps. An op amp operates linearly when the input and output signals are within the device’s input common-mode and output swing ranges, respectively. The supply voltages used to power the op amps define these ranges.

Design Notes

Use precision resistors to achieve high DC CMRR performance
R₁₀ sets the gain of the circuit.
Add an isolation resistor to the output stage to drive large capacitive loads.
High-value resistors can degrade the phase margin of the circuit and introduce additional noise in the circuit.
Linear operation is contingent upon the input common-mode and the output swing ranges of the discrete op amps used. The linear output swing ranges are specified under the A_ol test conditions in the op amps data sheets.

Design Steps

Transfer function of this circuit:
When V_ref = 0, the transfer function simplifies to the following equation:
where
Select the feedback loop resistors R₅ and R₆:
Select R₁, R₂, R₃, R₄. To set the Vref gain at 1 V/V and avoid degrading the instrumentation amplifier's CMRR, ratios of R₄/R₃ and R₂/R₁ must be equal.
Calculate R₁₀ to meet the desired gain:
To check the common-mode voltage range, download and install the program from reference [5]. Edit the INA_Data.txt file in the installation directory by adding the code for a 3 op amp INA whose internal amplifiers have the common-mode range, output swing, and supply voltage range as defined by the amplifier of choice (TLV172, in this case). There is no V_be shift in this design and the gain of the output stage difference amplifeir is 1 V/V. The default supply voltage and reference voltages are ±15 V and 0 V, respectively. Run the program and set the gain and reference voltage accordingly. The resulting V_CM vs. V_OUT plot approximates the linear operating region of the discrete INA.

Design Simulations

DC Simulation Results

Transient Simulation Results

References:

Analog Engineer's Circuit Cookbooks
SPICE Simulation File SBOMAU8
TI Precision Labs
Instrumentation Amplifier V_CM vs. V_OUT Plots
Common-mode Range Calculator for Instrumentation Amplifiers

Design Featured Op Amp

TLV171
V_ss	4.5 V to 36 V
V_inCM	(V–) – 0.1 V < Vin < (V+) – 2 V
V_out	Rail–to–rail
V_os	0.25 mV
I_q	475 µA
I_b	8 pA
UGBW	3 MHz
SR	1.5 V/µs
#Channels	1,2, and4
TLV171

Design Alternate Op Amp

	OPA172	OPA192
V_ss	4.5 V to 36 V	4.5 V to 36 V
V_inCM	(V–) – 0.1 V < Vin < (V+) – 2 V	V_ee–0.1 V to V_cc+0.1 V
V_out	Rail–to–rail	Rail–to–rail
V_os	0.2 mV	±5 µV
I_q	1.6 mA	1 mA/Ch
I_b	8 pA	5 pA
UGBW	10 MHz	10 MHz
SR	10 V/µs	20 V/µs
#Channels	1, 2, and 4	1, 2, and 4
	OPA172	OPA192

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