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AD779X Configuration Register Channel Select Bits

PhilOuellette
on Jun 3, 2020

I am looking at the data sheet for the AD7797 and there is something not obvious (at least to me) about the Channel Select Bits CON2, CON1 and CON0.

Legitimate choices include:

CH2  CH1  CH0    Channel

0       0        0         AIN(+) - AIN(-)

0       1        1         AIN(-) - AIN(-)

1       1        0         Temp Sensor

0       0        0         AVDD Monitor

I imagine I understand what happens when you select AIN(+) - AIN(-). This must mean the differential input channel. But what they heck does Analog Devices mean with AIN(-) - AIN(-)? Is this simply a way of shorting out the input channel under software control? If so why the hell don't they say so in the data sheet?

I see that the selection does make a difference, I have the AD7797 eval board and the waveform is different with the two selections (still using the default configuration with AVDD/2 applied to both AIN(+) and AIN(-). The AIN(+) - AIN(-) selection is offset a bit from zero and the AIN(-) - AIN(-) selection is balanced around zero.

There is absolutely nothing in the data sheet that mentions what this means that I can find. I looked in the AD7799 data sheet which has 3 input pairs and it also contains the same single selection for AIN(-) - AIN(-).

I hate mysteries, can anybody explain what this setting does and what I might want to use it for?