Hello,
In my project, I am using AD7177-2 32-Bit Sigma-Delta ADC. AVDD1 and AVDD2 are connected to +2.5V, and AVSS is connected to -2.5V. Desired to convert analog signal from -2.5V to 2.5V that is inverted and connected to AIN0(inverted signal),AIN1(positive signal) inputs.
I am also using an external reference that is connected to the ADC as +2.5V to REF+ and -2.5V as REF-. The VREF range is then 5V for two differential inputs(AIN0 and AIN1). I have connected the input signal to AIN1 and AIN0 (negative signal) to use the differential input of ADC. I have successfully set the registers of the ADC to use continuous conversion mode, 5SPS, external reference, swapped AIN0 and AIN1 inputs, 32bit output, bipolar coded output and so on.
When I leave the default settings of internal/external reference and use the internal +2.5V reference, the ADC is fully working in the range -2.5V to +2.5 in the range 0 to 2^32 bits as supposed to. But when I set the register 0x20 to use the external reference the ADC is working only from -1.25V to +1.25V in 0 to 2^32 bits. The full range from -2.5V to +2.5V is not possible to get. I also tried to change the gain of ADC using register 0x38 but this leads to resolution only to 2V, after this value the maximum (or some value between 2^31 and 2^32) value is set and the value is not changed with higher voltage.
What is the reason for the behavior of the ADC as described? I would assume that the ADC is working in full range with my external voltage reference.
Best regards, Martin Indrych