The circuit in Fig 1 shows an ordinary, off-the-shelf, general-purpose op amp configured as an instrumentation amplifier. Such a configuration yields an instrumentation amplifier having higher bandwidth than currently available dedicated instrumentation amplifiers have.
The circuit in Fig 1 has a gain of 9 with a 3-dB bandwidth of 5 MHz for Ò5V signals. The CMRR is >80 dB at 1 MHz, and the large-signal transient response is good. The circuit is very stable. The offset voltage and temperature coefficient of this circuit depends entirely on the specs of the op amp you choose.
The circuit is more subtle that in first appears. Fig 2 shows that the offset-null pins of an op amp are, in fact, low-impedance inverting and noninverting inputs. The op amp in Fig 1 is one of many such op amps that come with internal resistors in series with their null lines (Rand Rin Fig 2). The circuit in Fig 1 uses these resistors in an unusual way.
Keeping the null-line resistors in mind and noting that the connection from Pin 1 to Pin 6 in Fig 1 closes the loop, any signal applied between Pins 2 and 3 causes an output voltage:
where R 1 is the series resistor in the inverting input and Ris the total degeneration at the input stage. The resistor in the noninverting null line enables a reference-voltage output.