Pre-amplifiers extend digitisers’ dynamic range & sensitivity

September 01, 2015 // By Graham Prophet
For applications where low level electronic signals need to be acquired and analysed Spectrum (Grosshansdorf, Germany) has 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.

These 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 with gain ranges from 20 dB to 60 dB, bandwidths from 10 MHz to 2 GHz as well as 50Ω and 1 MΩ terminations. Most of the pre-amplifiers also have true DC coupling and an adjustable output offset.

To use 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 new pre-amplifiers extend this even further.

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

The fastest pre-amplifier SPA.1841 with a bandwidth of 2 GHz matches the Spectrum M4i.22xx series digitizers that offer up to 5 Gsample/sec sampling rate and a bandwidth up to 1.5 GHz. Using the combination of a M4i.2230-x8 and a SPA.1841 amplifier allows sampling of signals in the GHz range with a FSR of 2 mV resulting in a resolution of 15 μV 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 0.9 nV/√Hz.