A DAQ Card with DC Offset removal ?

[dalvares]

Hi all, 

I was wondering if there was any data acquistion card with multiple input channels (At least 20) that would allow me to remove the DC offset from my signal before A/D conversion. Therefore maximising my output resolution. I know there a e-daq single chanel system that does it so I am sure that NI has a system too. Just couldnt find it online. 

Cheers, 

Darren 

[Peter-E]

Hi Dalvares!

What I understand that you are looking for is a device that AC-couples the signal before getting to the ADC. Generally, our S-series DAQ cards and our Dynamic Signal Analyzer (DSA) modules are the only devices we offer that allow for this coupling configuration.

However, with these devices, you will not get the 20+ channels that you were hoping for. The S-series cards go up to 8 differential analog input channels while the DSA modules go as high as 16 single-ended channels. See below for a link to these different devices.

Keep in mind that for S-series and most DSA devices, you will need to select the input coupling in software based on this knowledgebase article.

The few exceptions to this are the NI 4496 and NI 4498, which are always AC-coupled, and the NI 4495, which is always DC-coupled (and therefore will not work with your application).

NI Products and Services:

S-series cards with 8 differential input channels

DSA devices with 16 input channels

I hope this helps!

[dalvares]

Hi Peter, 

Thanks for your email. I did think about the AC coupling option. The problem is I am interested in the steady state output from the device too. My understanding is the Dynamic signal analysis are suitable for capturing dynamic changes in data.

For instance I would like to know the initial value of the input voltage. Then remove the offset and then measure the output as if there were no intial offset. The output might increase to a value and remain at that value (which I should know) and then decrease again. So its about removing the DC offset without lossing it al together. Hope its not too confusing. 

Cheers, 

Darren 

[Peter-E]

Hi Darren,

Thank you for the clarification! I now understand what you are wanting. Unfortunately there is not a way to remove a known DC offset in hardware before the ADC. The only way we offer this is through the AC-coupling method which will not allow you to characterize the steady state output. It simply passes the signal through a high pass filter and any part of the signal below 20Hz will not be recognized. Although, the actual specifications of the filter is different for each DSA device.

Have a great day!

[AnalogKid2DigitalMan]

Maybe I misunderstand, but can't you simply read the initial offset voltage then subtract that value from your output readings?

-AK2DM

[Peter-E]

Hi AK2DM,

You are exactly right, you can do it this way in software to remove the DC offset. This will be a very effective way to do this.

However, we are looking for a way to remove this DC offset before reaching the A/D converter on the device, and therefore perform this removal in hardware.