Detecting an impact with a piezo sensor and ADC

Question

I am trying to use piezoelectric sensors to detect when an object strikes one of 9 plastic panels. I have 3 ADS1115 ADCs (they are the QIFEI ones that actually are ADS1015), so 12 channels at 12-bits over I2C. Using a Pi-4 1GB. Each piezo disc is attached to a little protection circuit with a 1 MΩ resistor and a diode. I'm supplying a 5V reference from the Pi to the ADC and the protection circuit, though I don't understand why the protection circuit needs it.

ADC: HiLetgo 3pcs ADS1115 16 Bit 16 Byte 4 Channel I2C IIC Analog-to-Digital ADC PGA Converter with Programmable Gain Amplifier

Piezo & Diode: GeeekPi 5Pack Piezoelectric Sensor Analog Ceramic Vibration Sensor Module Piezoelectricity 3.3V/5V for Arduino DIY KIT

When I test the ADCs with nothing attached, each channel reads a random voltage (no pull down resistor attached), but if I connect a voltage to any channel, every channel changes. i.e. attach 1.5V battery to to CH1 and it goes to ~1.5V but Ch2-4 go to 0.590V. On some of the ADCs, measuring a battery using CH1 causes CH2 to also read that value.

1. Are these ADCs broken?
2. How can I detect a transient signal from the piezo? The sampling rate is listed as over 3000 Hz, but I can't find any way to set it via the adafruit_ads1x15 library. I really just want to detect if an impact happens, and don't particularly care about capturing the waveform.

From my tests, there is a big difference between the signal you see when the panel is struck and the signal for when a neighboring panel is struck (with or without the diodes). I taped the piezos directly to the panels and then struck the panels and sampled the output with a scope so I know what the output should be.

Note: I haven't used a Pi or I2C before, so I just read the datasheet for the ADS1015 said 3300 samples/s, and assumed I could sample at that rate. In reality with my simple polling loop in my test code, I can read 4 channels every 8-12 ms. I don't think I need to sample fast enough to capture the actual waveform, I just want to know when it gets hit, and be able to filter out hits to a neighboring panel, which produce much lower amplitude oscillations

Answer 1

No, the ADC isn't broken. It can measure any value on unconnected channels, including a value which depends on the signal on neighboring connected channels.

If you want to create a slow "amplitude" signal from a faster signal, you need an envelope detector. That's how AM radios used to work.

Answer 2

This is not your problem but I would be cautious with QIFEI branded ADS1115. I bought two from different UK eBay resellers in 2020 and both had the same bizarre fault, they are 12-bit but with upper 4 bits fixed at 0 and the i2c communication at 100kHz is unreliable. More detail in Adafruit Forums: Strange Case of Adafruit ADS1115 and QIFEI ADS1115.