LMP7715: Requirement to measure small currents

David Pfaltzgraff

Part Number: LMP7715
Other Parts Discussed in Thread: LMP7721, OPA397, TLV9061, OPA392

Tool/software:

I have an interesting requirement and thought I'd bring it up here for an open discussion. I need to measure small currents - less than 1nA. As a starting point, I am considering the LMP7715 and am looking at two alternative implementations.

The first is simply a transimpedance amplifier as described in the corresponding Ap Note (Section 4.1.4) I would need to be concerned with offsets, etc., but think I can handle those.

The second is to implement an integrator. This would work for this application as the real concern is not the instantaneous current, but the sum, or the total charge passed through. However, in this case, I would need to find a suitable hold-and-reset circuit that would work for these high impedances.

Any suggestions?

Thanks, Dave

2 months ago

0 David Pfaltzgraff 2 months ago

Expert 2445 points

Sorry for the confusion. The part number should be LMP7721 not LMP7715.

0 Marek Lis 2 months ago in reply to David Pfaltzgraff

TI__Guru 67601 points

First, the question is whether you need to measure 1nA at room or at elevated temperature? If the former, you may use any of the CMOS input op amp like OPA392 (max offset of +/-10uV), OPA397 (max offset of +/-100uV) or TLV9061 (mx offset of +/-1.6mV). If the part must operate at 125C, LMP7721 is a very good choice to measure currents below 1nA. Keep in mind you need to consider IG1 range and output swing to rail in your choice of Rgain.

As far as circuit implementation goes, I would recommend here to use TIP with inner-loop T-gain stage - see below.

3515.TIP OPA397.TSC

0 David Pfaltzgraff 2 months ago in reply to Marek Lis

Expert 2445 points

Sounds good. The OPA392 is lower cost and it seems better specs than the LMP7721. (Looks like the LMP7721 is about 10 years older.)

I'll probably go with this.

Thanks, Dave

0 Marek Lis 2 months ago in reply to David Pfaltzgraff

TI__Guru 67601 points

All of these is true for as long as you operate the part at low temperature (LMP7721 has lower IB at high temperature). If you're concerned about the cost, OPA397 uses the same die and is a low cost version of OPA392.

0 David Pfaltzgraff 2 months ago in reply to Marek Lis

Expert 2445 points

Hi Marek,

I was reviewing the circuit in terms of my application and came across a stumbling block (for me). I need to measure both positive and negative currents. To do this, is it as simple as using two supplies with the '+' input tied to the common, which is also common to the current source?

Sorry, analog is not my strength.

Thanks, Dave