I'm debugging a board which uses the AD7998 12-bit multiplexed ADC converter and appears to have channel-channel interference. There appears to be an issue with around 40% of the boards to a lesser degree.
The ADC device is used in MODE2 where 6 channels are automatically sequentially converted once the conversion registered is written to. There is crosstalk in the form of channel conversion result can be affected by the value of the previous converted channel, of course we can only see this if the previous channel is at a vastly different ( >0.5V x Vref ) potential. The worst case we have seen is 72ADU difference between the expected and actual result (biased towards the prior channel polarity) and we are frequency seeing the previous channel in the conversion sequence being affected by around 20ADU.
In the circuit there is also a AD5254 Nonvolatile Memory Digital Potentiometers to vary the gain in the signal conditioning circuit however due to the sequencing of the conversion, and that this issue is also seen in the channels not connected with the digital POT this is not thought to be the cause of the ‘crosstalk’.
Can you tell me the Track and Hold sampling time for the AD7998 operating in mode 2. Could this be an issue?
Some further information:
I2C bus at 400KHz.
Sampling sequence is channel 1 to channel 6.
The ADC is set up as follows:
configuration register: 0x03FA
The above register is set-up once only, on processor power-up.
During the conversion MUX_A0 (WRT schematic) is static.
The ADC is triggered to convert a sequence of channels 4 times in succession every 160mS.