AD7949 input configurations

Document created by analog-archivist Employee on Feb 23, 2016
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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?

 

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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