I need to test a differential ADC (with AIN and /AIN).
After some tests, I don't understand the behavior.
Let's use the AD6644 (taken from this Analog devices document):
As shown in the functional block diagram, the AD6644 has complementary analog input pins, AIN and /AIN. Each analog input is centered at 2.4 V and swings ±0.55 V around this reference (Figure 21). Because AIN and AIN are 180° out of phase, the differential analog input signal is 2.2 V peak-to-peak.
Does this mean I cannot feed AIN or /AIN with a voltage higher than 2.95v (2.4+0.55) or lower than 1.85v (2.4-0.55). Then the input range is 1.85-2.95v. What is the behavior if I put /AIN at ground (0V) and the AIN input signal at 2.4v +-0.55v ?
Maybe my misunderstanding comes from the board I am using. This board doesn't contain any differential output opamp and contains only two different signals for feeding AIN and /AIN. With the possibility to set one or both inputs at 2.5V DC. It uses the ADC with no AC-coupled inputs... just as a "differential" (AIN - /AIN) ADC...
Obviously, the board uses this diff ADC as a single-ended ADC.
Does the input offset of the signal matter or not ?
A) /AIN=0V AIN=SINE 0v offset, 2.2V p-p B) /AIN=2.4V AIN=SINE 2.4v offset, 2.2V p-p
A and B are supposed to give the same result. Aren't they ?
Or A doesn't work or is not efficient because doesn't respect the 1.85-2.95v input range.
And AIN and /AIN AC-coupling at 2.2v p-p with 0V offset AND 2.4V offset (or any other offset) are behaving same ? Only the 2.2v p-p does matter or the 1.85-2.95v input range too ?