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[SOLVED] TC7107 liminting input current to Vin+ pin

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d123

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Hi!

I think this is a simple question for anybody who is familiar with the TC7107, or voltmeter applications.

I'm using the TC7107 ADC as a 0 to 20V voltmeter for a power supply, and the datasheet says in the electrical specifications on page 5:
"Note 1: Input voltages may exceed the supply voltages, provided the input current is limited to ±100 μA."

The datasheet is here:

https://ww1.microchip.com/downloads/en/DeviceDoc/21455d.pdf

How would I calculate/size the series input resistor (usually 1M in the schematics) to limit a possible maximum power supply output current of 0.75A to the stated ±100 μA value? Do I also need to take into account the maximum wattage that will pass though the output that the voltmeter will be measuring?

Thanks.
 

KlausST

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Hi,

I think you misunderstood something.
The limit to 100uA is for safety reasons only, not to destroy the chip. It is not meant for normal operation.

Tell us about what you want to measure. Is it the power supply voltage in the eangd of 0....20V?
Then just use a two resistor voltage divider (maybe with an additional C) to meet the ADC operating input voltage range.

Klaus
 
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    d123

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d123

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Hi. Ahhh, I misunderstood the datasheet specification and note, then. Thanks.

I only want to monitor the output on a home-made power supply, with voltages ranging from 5VDC to 15VDC. I'm using the 200mV scale; I know that when set up like this the 7107 reads from +19.99VDC down to -19.99VDC, but (never say never...) I don't expect to need a supply with a negative voltage reference for the time being.

Thanks for setting me straight on that matter, I'll just use the 1M and 10K resistors that nearly all the schematics show for the divider, and leave the 10nF capacitor as it is across the + and - input pins.

Pardon my ignorance, but can I use 1/4 Watt resistors for this if the output current may sometimes (but is unlikely to) reach 0.75A?
 

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