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ADC0820 and drum pad

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Danturner81

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Bit of a newb with electronics, so if this question has an obvious answer, please excuse me.

I am trying to attach an electronic drum pad to the Vin input of an ADC0820 flash ADC. I am powering it with 5v and using a 3.3v reference voltage (the voltage ref is a differential input on this IC, with a VRef- of 0V. When I attach a 10k pot to the ADC Vin input, everything works as expected.

Here's the thing that's confusing me: when I hook up a scope and read the drum pad when it's not attached to the ADC, it reads at 0 Volts when not being hit. However, when I attach it to the Vin input this level rises to 0.1 Volts and causes the ADC to read above 0. It seems the Vin pin is charged from the VRef+ pin and this is being sent back down the drum pad. I have confirmed this by connecting a pot to the VRef+ pin and turning it up and down - the charge leaving the Vin pin changes! This doesn't happen when I experiment with different ADC chips. Why is it happening here?

Thanks for the help

Dan
 
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