vb Instrument Anti-alias Filters

The information provided is for help with Commtest Instruments products:

• vbSeries® instruments

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QUESTION

What anti-alias filters does the vb instrument use on its analogue inputs, prior to digitization?

ANSWER

There is sophisticated anti-alias filtering on the analog inputs to the instrument. Most of it is provided by the advanced type of analog-to-digital converter which we use - it is a Sigma-Delta codec which internally oversamples the signal at 64 times the output sampling rate.

To quote the datasheet:

'The sigma-delta ADCs incorporate a proprietary fourth order modulator. A single pole of passive filtering is all that is required for antialiasing the analog input because of the ADC's high 64 times oversampling ratio. The ADCs include digital decimation filters that low-pass filter the input to 0.40 x Fs. (Fs is the output sampling frequency).'

For example, when the vb instrument records a 20 kHz spectrum it is receiving samples from the codec at 51.2 kHz, but the codec is actually sampling at 3.27 MHz. So very little external filtering is required, especially considering that the accelerometer's output drops rapidly after its resonance at around 30 kHz. But not only do we have a single pole filter on the inputs, also the op-amps themselves drop below unity gain above 1 Mhz.

For display of spectra with lower Fmax values, we use digital decimation filters in the DSP. These have been demonstrated to have aliasing noise rejection ratios in excess of 90 dB.

Our use of these over-sampling and digital signal processing techniques results in anti alias filtering which has better stability and phase response than the 'old fashioned' multi-pole analog filters which others use.

FAQ ID: 11649 Last Reviewed: 1 August 2001