[SOLVED] ADC Using Differential Inputs

[skywalker898]

Hi,

I am trying to sample the single ended output of an opamp (AD820) with an ADC that has differential inputs (AD7731). I'm wondering if I can simply ground one of the differential inputs on the ADC and attach the other input to the single ended output of the opamp. Does anybody know of any issues with doing this?

Thanks,
skywalker898

 

[barry]

No, you can't do that because the common-mode input range only goes down to 1.2V ABOVE ground. Why not use a different ADC if you've got single-ended signals?

 

[skywalker898]

Thanks, I'm new to working with ADC's so I'm trying to get up to speed with the typical specifications that are stated in the datasheet. I would use a different ADC, but I'm having a surprising amount of difficulty finding a singled-ended ADC that has between 16 and 24 bits of resolution. I'll probably start a new thread with my circuit design and see if anyone has any suggestions about how to properly sample the data.

 

[barry]

There are lots and lots of ADCs that meet your requirements (depending on speed) Look at Texas Instruments and Analog Devices for starters

 

[skywalker898]

I've tried looking again but I am not able to find a part that meets my requirements. My requirements right now are:

-18 to 24 bit
-single ended input
-SPI interface
-at least 1kSPS
-DIP package

Do you know of any parts that meet these requirements? Or, do you know how to use differential inputs on the AD7731 (or in general) to measure a single-ended output from a simple current to voltage converter (an opamp attached to a photodiode)? You made a good point about not grounding one of the differential inputs, but there must be a way to use them with a single ended output of an opamp.

Thanks again for all the help.

 

[barry]

Look at this LTC2411 - 24-Bit No Latency Delta Sigma ADC with Differential Input and Reference in MSOP - Linear Technology. Linear has many, many delta-sigma ADCs that will work for you. This one is also differential, but the common-mode range goes below ground so this should work for you.