Questions on ADC0820 and piezo drum pad

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Questions on ADC0820 and piezo drum pad

Postby olaf123 » Wed Nov 30, 2016 7:14 am

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(see datasheet at flash ADC. I am powering it with 5 V and using a 3.3 V reference voltage (the voltage ref is a differential input on this IC, with a VREF- of 0 V.) When I attach a 10 k 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 V when not being hit. However, when I attach it to the VIN input this level rises to 0.1 V 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?
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