Using a changing signal on VREF

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Does anyone know if it is possible to use a varying signal for your reference voltage to your ADC? I have three signals, one of which is always bigger than the other two. If I used the biggest one as the reference, I could ensure that almost full resolution is always achieved. Does this affect readings? Does anyone know any analog ways to normalize signals? Perhaps an OpAmp circuit?

Thanks,
Andy

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I think the Vref voltage should be stable for the complete conversion. I that is the case it should
by no problem if it varies from conversion
to conversion.

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Right, it is that "no change during conversion" that is a problem. Here is one scenario.

The input signal is sampled and held at the start of the conversion. The conversion starts and after a few bit, the reference shifts slightly. The input may also shift but that is invisible to the ADC because of the sample and hold. Further, the reference shift may throw off the whole conversion, giving a result that bears little relationship to the actual input.

"Normalizing" usually means a scale factor change, not just offset. That is very difficult to do with any precision.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Its relatively easy to scale down an input signal with a resistive divider. You may need an opamp as a buffer to drive the divider, which should be about 10 - 50 KOhms. Resistors with 1% presision are easy and 0.1% precision not so difficult either.

A change of the ref voltage during a conversion will give some extra errors for the conversion. The lower bits will tend to be the same more often than normal, since the approximation method can not change the higher bits later on. If the change in voltage is slow enough (e.g. battery voltage, temperature sensor) its OK to use ref. as the input.