Differential amplifier

Simple differential amplifier circuit to measure a signal source with low impedance. Amplifies the difference between two inputs, thus it performs a differential gain. Hence, even though the input voltages may be of the same polarity and high, the output voltage can be small unless there is more gain.
Because the input impedances are balanced, any external noise, such as of electromagnetic induction as of radio signals, will be rejected by a differential amplifier because this external noise couples equally and with the same polarity, of course, on both + and - inputs so it cancels out.
This method allows the use of longer cables while reducing sensivity to external noise. If the source has too high impedance, the inputs have to be buffered. For this reason, a development is commonly called instrumentation amplifier.

Instrumentation amplifier

Is a differential circuit, usually a dedicated IC or three op amps, providing a high input impedance with ease of gain variation with a single resistor adjustment. Inputs are buffered to give higher input impedance. This may eliminate the need for external input impedance matching which makes the amplifier suitable for measurement and test equipment. Other characteristics for instrumentation amplifiers are very low noise, very low dc offset, very low drift, very high open loop gain, very high common-mode rejection ratio. However not all of these parameters can be present at the same time with best performance. Some are more intended for high frequency use and some may be intended for high gain etc.

Instrumentation Amplifier Test Equipment Portable Instrumentation Amplifier An instrumentation amplifier can also be a measuring unit, either for lab or portable use.

Links:

Instrumentation Amplifier - National Semiconductor
A Designer's Guide to Instrumentation Amplifiers - Analog Devices