AD7949 input configurations

Question 1. In the datasheet page 19 (input configurations) we see in figure 29-A the single ended inputs are reference to ground and COM pin is NC. In our board we connected the COM pin to ground but configured the DC to single ended. Is it allowed? Can it be A problem? 2. In the typical application diagram you use driver amplifier in the inputs. Do you have any recommendation when to use amplifier or direct connection to the source? 3. We connected 1.77V regulator output to IN1 input(IN2 is open) and read the measured output of OUT1 and OUT2. OUT1 shows 1.73V, and OUT2 shows 1.4V from some reason. Do you have any idea why OUT2 shows this readout? Answer 1. The AD7949 can be operated essentially in the following configurations : 1) For a unipolar differential input configuration, all the inputs IN [7:0] have the range of GND to VREF and the COM is always connected to absolute GND±0.1V as shown in figure 29A and it is set by CFG register bits INCC [12:10] = 11X. 2) For unipolar differential pairs configuration, the positive input channels have the range of GND to VREF and the negative input channel (in your case CH1) is always referenced to absolute GND±0.1V and it is set by CFG register bits INCC [12:10] = 10X. We call it "unipolar or bipolar differential" because the result will still be the difference between the positive input channel and the negative input channel or COM. 3) In bipolar differential input configuration, the positive input channels have the absolute range of -0.1V to VREF + 0.1V and the negative input or COM is always connected to the absolute VREF/2 ±0.1V and it is set by CFG register bits INCC [12:10] = 010. The figure 29B shows this case and the above case (1), where the COM- indicates it is always referenced as a negative terminal whether the COM is connected to GND or VREF/2. 4) In bipolar differential pairs input configuration, the positive input channels have the absolute range of -0.1V to VREF + 0.1V and the negative input (INx-) is always referenced to VREF/2 ± 0.1 V and it is set by CFG register bits INCC [12:10] = 00X. If the INx [9:7] is even, then IN0, IN2, IN4, and IN6 are used as the positive channels and if INx [9:7] is odd, then IN1, IN3, IN5, and IN7 are used as positive channels (as shown in Figure 29C - channels with parentheses). For example, for IN0/IN1 pairs with the positive channel on IN0, INx [9:7] = 000. For IN4/IN5 pairs with the positive channel on IN5, INx [9:7] = 101. In case of (2) and (4) above, the COM pin can be left floating as shown in figure 29C. 2. We recommend to use one of the datasheet recommended drivers (table 8). If you have fast switching signals and you are concerned about the AC performance, the driver is definitely recommended. You can directly connect the DC type input signals to the ADC. Have a look into this article about the front end design considerations for the SAR ADC : http://www.analog.com/library/analogdialogue/archives/46-12/sar_front_end.html 3. It seems like you are seeing some kind of crosstalk between IN1 and IN2. You have to probe each input/output in your circuit to make sure there isn't anything unusual.

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AD7949 Production
The AD7949 is an 8-channel, 14-bit, charge redistribution successive approximation register (SAR) analog-to-digital converter (ADC) that operates from...
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AD7949 on Analog.com