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.
