DDC114, measurement of fA currents

Daniel,

Sorry this took so long, but we've talked to one of the system engineers with some experience on this part. In theory, the noise would stay he same as you increase the integration time. However, we've noticed that as you get longer, you get slightly more noise.

As for integrating the noise for longer periods of time, I don't think this would buy you much. In some of the tests we've tried, we have more change to introduce noise from random external sources (our lab environment isn't isolated enough for such small current measurements).

Regardless, he'll take one more pass at the measurement to see if he can get it to work. However, if you are trying to integrate for long periods, a discrete solution may serve you better. I think that you'd need to post more information and requirements about your application before we could make a good recommendation.

I'm posting his response below.

Joseph Wu

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I'm running an experiment but I can't even get the EVM to integrate much past 2s, let alone 100s.

I would be concerned with the input bias current (Ib) and voltage offset (Vos) at the inputs dominating the reading, as the Ib is typically around 1pA and integrating fAs with 1pA running through the input seems counterproductive.  The Ib changes with temperature too.

So far it appears that noise increases as integration time increases, but upon inspecting the data, it looks as if a handful of readings are extreme outliers and are dominating the noise number.  I'm seeing higher than normal noise on the readings I'm taking at longer integration times, but I fear the environment the setup is in may be somewhat noisy.  I will repeat it again shortly when there aren't as many people around.

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