Single Supply Instrumentation Amplifier

The OP284 is a low noise dual op amp with a bandwidth of 4MHz and rail-to-rail input/output operation. These properties make it ideal for low supply voltage applications such as in a two op amp instrumentation amplifier as shown in the diagram. The circuit uses the classic two op amp instrumentation topology with four resistors to set the gain. The transfer equation of the circuit is identical to that of a non-inverting amplifier. Resistors R2 and R3 should be closely matched to each other as well as to resistors (R1+P1) and R4 to ensure good common-mode rejection (CMR) performance. It is advisable to use resistor networks for R2 an and R3, because these exhibit the necessary relative tolerance matching for good performance.

Potentiometer P1 is used for optimum d.c. CMR adjustment, and capacitor C1 is used to optimize a.c. CMR. With circuit values as shown, circuit CMR is better than 80 dB over the frequency range of 20 Hz to 20 kHz. Circuit referred-to-input (RTI) noise in the 0.1 Hz to 10 Hz band is exemplary at 0.45 µVpp. Resistors R5 and R6 protect the inputs of the op amps against over-voltages. Capacitor C2 may be included to limit the bandwidth. Its value should be adjusted depending on the required closed-loop bandwidth of the circuit. The R4-C2 time constant creates a pole at a frequency, f3dB, equal to f3dB=1/2πR4C2. With a value of C2 of 12 pF, the bandwidth is about 500 kHz. The amplifier draws a current of about 2mA.

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