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How to measure negative voltage on ADC?

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hal9001:
I have a charge pump that outputs -10 V. Theres no other negative power supply on the board. How can I monitor the -10 V supply with an ADC?

I can send the -10 V through an inverting opamp with voltage divider but the negative supply of the op amp will have to be powered by the same -10 V charge pump and put extra load on it. Is there a better way?



Cheers!

sicco:
Check out the latest ‘over the top’ from Analog Devices.
https://eu.mouser.com/datasheet/2/609/ada4099_1-2238218.pdf page 24.

voltsandjolts:
The op-amp can still be powered as normal from the positive rail, it doesn't need to be connected to the negative rail.
Bias the op-amp input appropriately for the value of resistor used to connect to the -10V.
It's similar to using a resistor divider to measure, say, +48V rail using an op-amp running from 3v3 rail, with some extra biasing from a volt reference or such.

Or use an ADC that has a negative input range like the ADS8665 (which uses this input biasing technique).

David Hess:

--- Quote from: hal9001 on October 28, 2021, 11:33:34 am ---I can send the -10 V through an inverting opamp with voltage divider but the negative supply of the op amp will have to be powered by the same -10 V charge pump and put extra load on it. Is there a better way?
--- End quote ---

The common mode input voltage of an inverting operational amplifier is the same as the voltage at the non-inverting input, which could be ground, so there is no need for a negative supply.  Inverting amplifiers are sometimes used to do exactly what you need without a negative supply.

mikerj:
You could simply use a potential divider with the top connected to your reference voltage, and bottom connected to your -10v rail and the ADC in the centre.  Scale the resistor values appropriately so that 0v on the ADC represents your minimum negative voltage (i.e. full scale).  You then just need to invert and scale the value in your software.

Watch out for the divider current pulling the -10v rail up past 0v if the rail could ever be in a high impedance state.

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