Instrumentation amp turns into 600-MHz gain-bandwidth op amp

Oct. 1, 1997
An interesting but brief mention is made in Analog Devices’ AN-245 application note (by Scott Wurcer and Walt Jung) that instrumentation amplifiers can be configured as op amps with very high gainbandwidth (GBW) products. In applications that use...

An interesting but brief mention is made in Analog Devices’ AN-245 application note (by Scott Wurcer and Walt Jung) that instrumentation amplifiers can be configured as op amps with very high gainbandwidth (GBW) products. In applications that use wide-band transducers, it’s necessary to use high-GBW-product amplifiers.

Presented here is an instrumentation amp that’s configured as an op amp with a GBW = 600 MHz (gain of 800 and a −3-dB bandwidth of 750 kHz) (see the figure). To achieve such a large GBW, the output stage of the instrumentation amp is configured as a modified Howland-current pump (see the figure, insert), giving it high conductance and large gain. The overall open-loop gain of the composite amplifier is approximately 90 dB at 100 Hz. The inherently high GBW = 100 MHz of the INA103 makes it an ideal candidate for this unusual application of using this device as an op amp. It’s difficult to find a low-cost op amp with comparable specifications to this op-amp-configured instrumentation amp with a GBW = 600 MHz.

In the insert, the current IO = (E1 − E2)(1/6 kΩ + 1/R), but as R tends toward zero, which is the case in the opamp-connected instrumentation amp circuit, the conductance will tend to become very large. This will provide a large voltage gain that’s bounded by the CMRR of the output stage. The maximum output voltage swing of the amplifier with a bipolar 12-V supply is 16 V p-p (1-kΩ load) at 500 kHz.

The overall gain is equal to AV = (R2 + R1)/R1, and in order to prevent instability due to excessive front-end phase, the gain set by R1 and R2 should be 10 times greater than the front-end gain, which in this case is strapped to 100. The front-end phase makes the op-amp-connected instrumentation amp only viable with large closed-loop gain settings.

Resistor R3 ensures that the source resistances presented to the instrumentation-amp inputs are equal. The dc offset at the output Vout can be nulled by adjusting RV1. The LM6361N highspeed op amp, which has a GBW = 50 MHz, can be used as a buffer amplifier or to provide additional gain. The LM 6361N should be replaced with a wideband JFET type if the source impedance of the transducer used is high.

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