I am designing a weigh scale for a bee hive, using AD7780, using four load cells, 20kg each, 1mV/V, powered at 3.3V.
I ordered 2 samples and I etched a small evaluation circuit (schematics and layout attached), based on the CN0107 reference design. The part is powered with 3.3V, the AVDD and DVDD pins are decoupled using tantalum and ceramic capacitors, and the AVDD is powered through a low pass filter. The part is configured with 128 gain and 10Hz filter.
I tested with two individual load cells and the behavior is the same: above a specific load () the DOUT/RDY pin remains HIGH. If I reverse the sense wires and the force direction, the behavior is the same.
Vref is 3.3V. Gain is 128, so the maximum measurable voltage should be ~25mV. My multimeter, set on mV, indicated 0.09mV between AIN+ and AIN-. Referenced to GND, I measured 1.532 at AIN+ and AIN-. Excitation voltage is 3.281V and AVDD is 3.290 (the load cell is powered through the BPDSW pin). DVDD is 3.332V.
Filter an Gain pins are solidly connected to 3.3V and GND. All solder joints and wiring is intact. The load cell does not appear to be faulty (as measured with the multimeter set on Ohms)
As for the interface, the level is 3.3V, the clock frequency is 200KHz and the MCU waits for DOUT/RDY pin to go low before attempting anything. At no load, the status byte reads correctly: no conversion errors, gain and filter are as set by hardware, and the PAT bits are correct.
I also attached the interface code, in case there might be something wrong with it. It is written in Arduino IDE, targeted for the ESP8266. Also, at no load, the measured voltage was 12.84mV, so I think I am doing something wrong there.
What am I mistaking ?