Need suggestion on schematic of AD7682

Hi, Richie.

The AD7682 can be operated essentially either in a Unipolar or Pseudo-Differential mode in the following configurations, which means that the AD7682’s positive input swings between 0V and Vref and its negative input is always connected to either a ground OR a fixed DC value- Vref/2 – as shown below.

In this case, the result will still be the difference between the positive input channel and the negative input channel or COM pin for “unipolar or bipolar differential” configuration.

1)      For a unipolar differential input configuration, all the inputs IN [3:0] have the range of GND to VREF and the COM is always connected to absolute GND±0.1V as shown in figure 30A 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 is always referenced to absolute GND±0.1V and it is set by CFG register bits INCC [12:10] = 10X.

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 30B 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 and IN2 are used as the positive channels and if INx [9:7] is odd, then IN1 and IN3 are used as positive channels (as shown in Figure 30C - channels with parentheses). For example, for IN0/IN1 pairs with the positive channel on IN0, INx [9:7] = X00. For IN2/IN3 pairs with the positive channel on IN3, INx [9:7] = X11.

In case (2) and (4) above, the COM pin should be connected to the ground.

With the inputs configured for unipolar range (single-ended, COM with ground sense, or paired differentially with INx− as ground sense), the data output is straight binary. With the inputs configured for bipolar range (COM = VREF/2 or paired differentially with INx− = VREF/2), the data outputs are twos complement.

Hope that helps.

Regards,

Karen

Could anyone give me a little help on these two questions?

I really need your suggestions.

Thank you very much.

Best regards

Richie CHEN

1)Is figure 30A implying a unipolar differential mode? It seems that it is referenced to GND directly.

2)For a unipolar differential input configuration, should COM be connected with GND outside the circuit?

3)In bipolar differential mode, should COM be connected with Vref/2 by outside circuit? Or will Vref/2 is connected internally?

If the unipolar, referenced to GND mode is chosen, could you please kindly help check my schematic?

Really appreciate for your warm help.

Best regards

Richie CHEN

Hi, Richie.

We have explained this in detail in the previous post.

1. Yes, that's unipolar mode referenced to system ground.
2. Yes, you should connect COM to a ground sense.
3. Yes, you do have to connect COM to a VREF/2 externally. See Figure 27 on page 18 of the datasheet.

Note that the AD7682 is a single-ended device so it cannot accept input voltage below ground and the bipolar signals ranging from -2V to +2V will have to be level-shifted.

Regards,

Karen