ADC0820 and drum pad

Discussion in 'General Electronics Chat' started by danturner81, Jul 26, 2016.

  1. danturner81

    Thread Starter New Member

    Aug 25, 2015
    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