PH4705/ET4305:Instrumentation Amp

PH4705/ET4305:Instrumentation

Amp

Our sensor will be connected to some kind of measurement system either directly, diag. 1, or as a bridge circuit diag 2. In either case the first stage of the measurement system is critical. It must not load the sensor by drawing too much current, nor must it itself introduce distortion or noise. Circuits based on Operational Amplifiers are commonly used.

PH4705/ET4305:Instrumentation

Amp

An “idea” OP Amp will have infinite input impedance thus presenting no load on the input. It will have zero output impedance, so as to drive any following circuit with ease. It will have infinite gain so that any required gain can be set easily with a couple of resistors. It will have zero offset, so that a zero input signal results in a zero output. It will have an infinite frequency response accurately amplifying the fastest input signal. Finally, it will work form a wide range of power supply voltages.

Real Op Amps easily achieve: Rin≥109Ω: Rout≤10Ω:Gain500k:Response to 100MHz: Vs±3-18v

PH4705/ET4305:Instrumentation R1

Amp

Practical Op Amp circuits for our two sensor connections are a shown. The gain (to a first approximation ) is given by R2/R1. To minimize the load on the sensor R1+R2 should be as high as practical. Circuit 2 is preferred because it effectively cancels noise picked up in the bridge and input leads. Any signal present on BOTH Op Amp inputs cancels. The degree of cancellation is termed Common Mode Rejection Ratio, CMRR. Typical values are -90dB (i.e.30k smaller than the signal)

PH4705/ET4305:Instrumentation

Amp

This modification improves the input impedance of our circuit. The sensor is now connected via “Unity Gain Buffer”, an Op Amp with its –ve input connected directly to its output. This configuration has gain of 1 and an input impedance ≥109Ω so presenting virtually no load to the sensor. Gain and CMRR are achieved by the second part of the circuit, however, for a good specification R1 and R2 must be matched.

PH4705/ET4305:Instrumentation

Amp

A circuit rearrangement enables gain to be set by a single resistor – a much more convenient arrangement. Gain is sifted to the input OP Amps and the ground connected R1s are replaced by a single resistor Rgain. The following Op Amp now has unity gain ( all resistor the same) and is responsible for the CMRR.

PH4705/ET4305:Instrumentation

Amp

The resultant circuit shown above is termed an Instrumentation Amplifier. Its gain is set by the single resistor Rgain, its CMRR is very good. Instrumentation Amps are so useful that semiconductor manufacturers make specific ICs, e.g. Analog Devices AD620. Single resistor sets gain between1 & 10k: wide power supply range ±2.3 to 18 v: low power 1.3mA max supply current: high bandwidth 120kHz, fast settling: low noise: input current 1nA max: CMRR 100dB: 8pin package.

PH4705/ET4305:Instrumentation

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Amp

For background on non-ideal behaviour of OP Amps and the development the instrumentation amp see: http://www.mines.edu/Academic/courses/physics/phgn217/lab8/lect13/

Data sheet for AD620 http://www.analog.com/static/imported-files/data_sheets/AD620.pdf