AD7687 Operates Single-Ended, not differential.

maithil wrote:

Hello,

It would be helpful if you can share your schematic and scope screenshots showing input and output signals (SDO, SCK, CNV, etc.).

Are you using one of the driver amplifiers recommended in table 10 of the datasheet?

I don't have a good way to capture from the scope I am currently using, and do not have a sharable schematic.

The AD7687's are being driven by a pair of MAX7420 Switched capacitor filters, but I have also been testing by directly driving the AD7687 from a programmable function generator ( using due caution to keep the input within safe operating limits of the device).

Note that the AD7687 is a true differential 16-bit part and it does not have a single-ended or pseudo-diff mode, so when you try to use the part in this mode it is still 'looking' at the world in a differential way and therefore it can NOT be driven in a single-ended or pseudo-diff mode. The AD7687 samples the voltage difference between the IN+ and IN− pins. The AD7687 requires the inputs to be completely anti-phase of each other between 0 V and REF with their DC common-mode around VREF/2.

I am trying to use it in true differential mode. The connecting of IN- and IN+ to ground have been experiments to try to understand the parts after seeing the strange operation in differential mode.

It may not work at all if you ground either the IN+ or IN- pin.

The trouble is, with IN- grounded seems to be the only way the part wants to work.

Right now I have connected a programmable function generator directly to the IN+ and IN- inputs.

1Hz sine wave, 3Vp-p.

With it connected differentially, with 2.5V common-mode (VREF=5V), I get three steps in my data: 15B5H, 2B99H, 55B7H.

If I connect it with IN- grounded, and the signal on IN+, centered at 2.5V with a +/- 1.5V swing (3Vp-p), I get a beautiful sine wave on my data display, with the data values centered around 8000H. It is behaving exactly as I would expect a AD7685 Psuedo device to operate, including the output being straight binary (not two's complement) centered at 8000H.

The parts are marked C03 #535, which to my reading of the data sheet indicate they are indeed AD7687's. Is there any possibility that there was a batch of mismarked parts? These were purchased about 16 months ago, but not used until now.

Thanks for any possible help,

Arnie

maithil wrote:

Hello,

It would be helpful if you can share your schematic and scope screenshots showing input and output signals (SDO, SCK, CNV, etc.).

Are you using one of the driver amplifiers recommended in table 10 of the datasheet?

I don't have a good way to capture from the scope I am currently using, and do not have a sharable schematic.

The AD7687's are being driven by a pair of MAX7420 Switched capacitor filters, but I have also been testing by directly driving the AD7687 from a programmable function generator ( using due caution to keep the input within safe operating limits of the device).

Note that the AD7687 is a true differential 16-bit part and it does not have a single-ended or pseudo-diff mode, so when you try to use the part in this mode it is still 'looking' at the world in a differential way and therefore it can NOT be driven in a single-ended or pseudo-diff mode. The AD7687 samples the voltage difference between the IN+ and IN− pins. The AD7687 requires the inputs to be completely anti-phase of each other between 0 V and REF with their DC common-mode around VREF/2.

I am trying to use it in true differential mode. The connecting of IN- and IN+ to ground have been experiments to try to understand the parts after seeing the strange operation in differential mode.

It may not work at all if you ground either the IN+ or IN- pin.

The trouble is, with IN- grounded seems to be the only way the part wants to work.

Right now I have connected a programmable function generator directly to the IN+ and IN- inputs.

1Hz sine wave, 3Vp-p.

With it connected differentially, with 2.5V common-mode (VREF=5V), I get three steps in my data: 15B5H, 2B99H, 55B7H.

If I connect it with IN- grounded, and the signal on IN+, centered at 2.5V with a +/- 1.5V swing (3Vp-p), I get a beautiful sine wave on my data display, with the data values centered around 8000H. It is behaving exactly as I would expect a AD7685 Psuedo device to operate, including the output being straight binary (not two's complement) centered at 8000H.

The parts are marked C03 #535, which to my reading of the data sheet indicate they are indeed AD7687's. Is there any possibility that there was a batch of mismarked parts? These were purchased about 16 months ago, but not used until now.

Thanks for any possible help,

Arnie