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A/D ranging and windowing

How do you do auto ranging and windowing with A/D converters? What I'm considering is using voltage divider resistor arrays and an analog multiplexer to change reference voltages in the A/D converter. Other options might be to us an analog multiplexer to select the A/D input from different taps from the resistor array. Thought there may be a better way that I'm overlooking, I've never used the analog multiplexers (or digital pots?) and not sure if that's the normal way to go.

The windowing I'm referring to is like when you get all your A/D resolution in a smaller range, like 12 bits from 0V to 5V window or 12 bits from a 5V to 10V window.

RogerN

Reply to
RogerN
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By switching voltage dividers on the input.

Any DVM would have a switch where you would change the dividers.

Works the same way.

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What I'm

Reply to
hamilton

Or, you could have an inverting preamp (the inverting op amp configuration allows diode-clamping to ground, to protect against overvoltages) with switch-selectable feedback resistors. The second stage could be a fully differential instrument amp with your chosen DC offset on its other input terminal. The instrument amp, too, can have a switch-selectable gain resistor.

It can all be done with amplifiers, because gain < 1 is an available operating condition.

Depending on your frequency range and sensitivity requirements, it can be an easy job, or a hard one. A 100 MHz o-scope input gain stage would be hard.

The closest to a single-chip solution is a gain-control amplifier like THAT 2181, but the gain-control signal is logarithmic and continuous in range (maybe hard to calibrate).

Reply to
whit3rd

If you have something like a PIC micro then you can set up whatever gains and offsets you need and feed the (multiple) signals into each A/D input. Selection is done in software and there is no switching to think about (need to think about over-voltage protection though.

Reply to
David Eather

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Not so. You would not want a PIC or any other uC to measure 10V directly. It's good to have at least two range selections: 0 to 5V and 0 to 10V.

Reply to
linnix

What part about variable gains and offsets didn't you understand? For your example - assuming it was in the range of interest - a gain of .5 would do. Also note the explicit mention of the need for over voltage protection

Reply to
David Eather

Thanks for the replies. This is for a microcontroller board, arduino, to measure battery voltage and current for charging experimentation. I'm considering digital pots to adjust gains and ranges and perhaps an op-amp to get offsets. I'm hoping to be able to write a program that will calibrate ranges by applying a voltage and entering what my meter reads.

RogerN

Reply to
RogerN

Myself, I like having something on the front end before the ADC input, like a unity gain or settable gain op-amp with optional offset using the other input of the OP-Amp. It's easier to replace a Op-amp than a uC if they soldered in!

jamie.

Reply to
Jamie

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