i'm having troubles in using the adau1446 ADC. I've read its datasheet and AN951 about GPIO, but i haven't found anything helpful.
I have ADC input 0 (MP0) pulled down through a 100K resistor, pin MP0 is set for aux adc input, other neighbour pins are set as digital inputs as well.
The DSP is internally working at 192KHz and if i set a readback cell on the ADC output i read 0.9942-something value (?).
I measured the voltage on the ADC input pin and this is about 2.2V with the pin pulled down (??), no other connections are made to
I then tried to change all GPIO pin working modes, the pin is always stuck at some positive voltage (2..3.3V) , except if i set it as an output. In this case the voltage read is zero.
So i set MP0 back as an analog input and tried to set the other three ADC GPIO pins (MP1 to MP3) as analog inputs: as i was swtiching the input configuration of the other pins, i had the voltage read on pin MP0 falling down to about 0.5V when i set all GPIO pins MP0 to MP3 as ADC inputs.
First question: is this normal ?
With 0.5V on MP0 i had a readback value of about 0.5 (?)
Second question: as the maximum voltage input is 3.3, which i suppose to be the maximum value corresponding to a readout of 1.00, shouldn't it be 0.5/3.3=0.15 ?
I then tried to ground the pin directly, no voltage reading on it, but on the dsp readout i was reading about 0.17..
Third question: why i'm not reading zero?
Note: in all cases, except the first when i was reading 0.999something, the values are quite unstable, no matter on how i set the ADC input filtering.
I carried out these tests on two proprietary PCBs and i got the very same results but i didn't tried on the evaluation board yet.