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ADAU1446 ADC

Question asked by drugoimir on Aug 22, 2014
Latest reply on Aug 26, 2014 by drugoimir

Hello,

 

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

it.

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.

 

Regards.

Tom

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