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Extending the Dynamic Range and Sensitivity of Digitizers with a Pre-Amplifier

Spectrum SPA.1411 amplifier11 September 2015 - For applications where low level electronic signals need to be acquired and analyzed Spectrum has added a range of pre-amplifiers that can be used to boost signal levels. When combined with a Spectrum digitizer these low noise pre-amplifiers can greatly increase the measurement systems overall dynamic range and sensitivity. The new pre-amplifiers are designed to compliment the built-in amplifiers of the digitizer, 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 digitizer being used. They offer gain ranges from 20 dB to 60 dB, bandwidths from 10 MHz to 2 GHz as well as 50 Ω and 1 MΩ terminations. Most of the pre-amplifiers also have true DC coupling and an adjustable output offset control to further extend their operational use.

To utilize the resolution and maximize the precision of a digitizer it is important that the signal being measured covers as much of the ADC's dynamic range as possible. Spectrum digitizers 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 8 programmable input settings offering from 50 mV to 10 V 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 digitizer 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 2 GHz perfectly matches the Spectrum M4i.22xx series digitizers that offer up to 5 GS/s sampling rate and a bandwidth up to 1.5 GHz. 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 2 mV resulting in a resolution of 15 μV per step at the given digitizer resolution of 8 bit. The above shown picture shows the acquisition of a 2 mV 40 MHz sine signal with plain digitizer and with digitizer + pre-amplifer in comparison.

Such high gain often means high noise, especially when amplifying small signals, so careful design of both the digitizer and pre-amplifier is crucial. All SPA series pre-amplifiers are optimized for minimum input noise with models available that reach values as low as 0.9 nV/√Hz. For applications that need to push the technology even further signal processing functions, such as averaging and filtering, can be used on the digitized 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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