I am using an AD7730 in AC excitation mode. For AC excitation of a Wheatstone bridge circuit, I connected the ACX ports of the AD to a MIC4427 as recommended in the datahseet. REF+ and REF- are connected to the IN+ and IN- of the bridge, respectively. The whole setup is exactly as depicted on page 41 of the datasheet.
For controlling the AD7730, it is connected to a Arduino Nano via SPI. Communication is running fine.
The AD7730 is performing the following operations:
1. Internal Full Scale Calibration (unipolar, 24bit, 5V ref, 0 to 80 mV)
2. Internal Zero Scale Calibration (unipolar, 24bit, 5V ref, 0 to 80 mV)
3. Setup of Filter Register (= turn on AC excitation)
4. Setup of Mode Register (continuous conversion, unipolar, 24 bit, 0 to 80 mV)
5. Continuous Reading of the Data Register on RDY signal
So far, everything seemed fine, if only I would get real conversion results other than only 'ones'.
I read the datasheet in detail and found that the Data register is filled with all 'ones', if the Reference Voltage (Vref+ - Vref-) is below 0.3V. Also, the NOREF bit in the Status register is set in this case. When reading the Status register, I found NOREF being set, so no wonder I am only getting only all 'one' results.
How do I fix this issue? Obviously, the reference voltage will be -5V at some point due to the AC excitation.