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Voltage Divider into ADC

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logictom:
I want to take a 0-10V signal and feed it into a 0-3.3V ADC, are there any general rules to using a voltage divider into an ADC.
I usually stick with values in the kohms range to limit current draw, should I go higher/lower - are there any specs to look up in the micro's datasheet to design against?
How do I go about designing the divider if I know it's xbits and y-zV range so I get sensible bits/voltage scaling in the software?

alm:
This recent thread is about almost the same topic:
https://www.eevblog.com/forum/projects-designs-and-technical-stuff/voltage-divider-on-the-output-of-a-voltage-reference-can-i/msg105854/

ADC datasheets will usually specify a maximum source impedance. You can scale the voltage however you like. You could divide it by ~3 to get the highest resolution. In the case of a 10-bit ADC, 1 bit would now correspond to 10 V / 1024 (3.3 V / 1024 * 3). Stability of the voltage divider will determine the effective accuracy of your ADC.

free_electron:
keep in mind that some a/d have a pretty low input impedance. also , this impedance may change depending on what you apply.
it's good to always put a buffer amplifier between your sensing node and the input of the a/d

Leo Bodnar:
ADC usually draws current in short bursts when their S&H cap is being charged.  If you are concerned about low ADC input impedance you can place a capacitor in parallel with ADC input.  In practical terms a divider and a good quality ceramic 0.1uF is all you need unless you have to maintain ADC's accuracy when measuring this 10V bus voltage.

Leo

logictom:
Thanks for the feedback - some food for thought.

--- Quote from: alm on April 20, 2012, 07:55:50 pm ---Stability of the voltage divider will determine the effective accuracy of your ADC.

--- End quote ---
Would this be down to voltage supply stability, resistor drift over age and due to temperature? Any other major contributors?


--- Quote from: Leo Bodnar on April 20, 2012, 11:26:04 pm ---In practical terms a divider and a good quality ceramic 0.1uF is all you need unless you have to maintain ADC's accuracy when measuring this 10V bus voltage.

--- End quote ---
Would you mind elaborating on this a little more? How does it effect the accuracy and if you wanted to maintain the accuracy what would you do differently?

Thanks.

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