Test & Measurement

Pre-amplifiers extend digitisers dynamic range & sensitivity

1st September 2015
Siobhan O'Gorman

For applications where low level electronic signals need to be acquired and analysed, Spectrum has added a range of pre-amplifiers that can be used to boost signal levels. When combined with a Spectrum digitiser, these low noise pre-amplifiers can greatly increase the measurement systems overall dynamic range and sensitivity. 

The pre-amplifiers are designed to complement the built-in amplifiers of the digitiser, effectively extending the available input ranges, and allowing the detection and measurement of much smaller signals. Seven different pre-amplifiers are available, making it easy to select a model that can best match the digitiser being used. They offer gain ranges from 20 to 60dB, bandwidths from 10MHz to 2GHz as well as 50Ω and 1MΩ terminations. Most of the pre-amplifiers also have true DC coupling and an adjustable output offset control to further extend their operational use.

To utilise the resolution and maximise the precision of a digitiser, it is important that the signal being measured covers as much of the ADC's dynamic range as possible. Spectrum digitisers already feature highly configurable input amplifiers that offer a variety of input ranges. However, the pre-amplifiers extend this even further.

For example, the 16-bit M2i.46xx series digitisers come with eight programmable input settings offering from 50mV to 10V full scale range. Using an SPA.1231 pre-amplifier can provide an additional gain of either 40dB or 60dB (x100 or x1000) effectively delivering a Full Scale Range (FSR) that can go as low as 50μV. The low FSR together with the 16-bit resolution of the digitiser makes it possible to detect signal variations that are way down into the nano-volt range.

The fastest pre-amplifier SPA.1841 with a bandwidth of 2GHz perfectly matches the Spectrum M4i.22xx series digitisers that offer up to 5GS/s sampling rate and a bandwidth up to 1.5GHz. Using the combination of a M4i.2230-x8 and a SPA.1841 amplifier, one can sample signals in the GHz range with a FSR of 2mV, resulting in a resolution of 15μV per step at the given digitiser resolution of 8-bit.

Such high gain often means high noise, especially when amplifying small signals, so careful design of both the digitiser and pre-amplifier is crucial. All SPA series pre-amplifiers are optimised for minimum input noise with models available that reach values as low as 0.9nV/√Hz. For applications that need to push the technology even further, signal processing functions, such as averaging and filtering, can be used on the digitised data. Such signal processing techniques can help to reduce the effects of random noise and greatly improve the overall signal-to-noise ratio of the measurements being made.

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