Instrumentation amplifiers (in-amps), on the other hand, let you set a gain from one to several thousand with an external resistance you calculate based on a formula from in-amp data sheets.
In-amps have other advantages: a high common-mode rejection ratio that attenuates the effect of noise present on both inputs, and a low-impedance output that can connect directly to an ADC input. Manufacturers of data-acquisition equipment often put in-amps on ADC boards or in modular packages that directly drive an ADC input. (Some in-amps use an external reference voltage rather than a resistor to set a gain.)
An Analog Devices AD8426 in-amp, which provides gain between one and 1,000, uses the formula below to calculate the gain-control resistance:
Rgain = 49.4 x 103 ohms / (gain - 1)
Rgain = 49.4 x 103 ohms / (1.51 - 1) = 96.9 x 103 ohms
Like all amplifiers, in-amps have a gain-bandwidth product that describes how bandwidth decreases as gain increases, and vice versa. The AD8426 has a gain bandwidth of 1MHz, so if you set it for a gain of 50, for example, the bandwidth decreases proportionally to 20kHz. If you need a bandwidth of 40kHz, you can have a maximum gain of 25.
Be sure to check back in with Design News for Specifying & Creating Data-Acquisition Systems, Part 3.