Non-stop ADC for full quantization of arbitrary time period signal average. Do ADCs work like this

adcembeddedsensor

Lets say I have a light intensity sensor and I want to measure the exact average light intensity for one hour using an ADC sampling time that is much less than the input signal frequency. Lets also say I could setup as many ADC's as needed so that the moment one is converting, one is sampling… no overlaps or gaps between any sampling period.

Will this allow me to obtain the exact average light intensity for as long as this sampling happens?

From my understanding an ADC uses an RC circuit, charging up to the voltage of the source. Does it also deplete if the voltage drops during the sampling?

A simplified example:

The ADC has a sample time of 1ms. The sensor gives out a voltage equivalent to 20 units for 0.5ms. The next 0.5ms of sampling the sensor is at 0 units. Do I get an ADC value of 10 units for that sample period?

I am not after any particular signal frequency, just the average intensity. Is it possible to do this or do I have to focus on ensuring the sample rate is >= the Nyquist frequency to obtain this data.

Best Answer

Most types of analog to digital converters have intervals of time when they are sampling their input, and intervals of time when they are not. If one needs a time-averaged signal, there are two ways of accomplishing this:

  1. Filter the signal significantly before sampling; if the cutoff frequency for the filter is well below half the sampling rate, the output of the filter will not vary much in the interval between samples. Consequently, no matter when samples are taken they will be relatively close to the average signal value.

  2. A Delta-Sigma modulator, as MarkU noted, samples continuously. Not all delta-sigma ADCs do so, however. If a delta-sigma ADC counts off fixed intervals and generates a reading each time, then it may be able to guarantee that a long-term average value of the readings will equal the average value of the input signal. If the converter only generates readings when triggered, however, or if not all of its readings are captured, then such a guarantee would not hold.

Both filtering and the use of delta-sigma converters may offer useful averaging behaviors. Which is better in a particular application will depend upon one's exact requirements.

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