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Can I use LMP7721 or similar 5V precision op-amps with REF200 for low current generation?

Nikhil Kulkarni45
Prodigy 20 points
Other Parts Discussed in Thread: REF200, LMP7721

Hello Folks,

My application is a precision low current source where I need to generate current as low as 1nA and maximum of 100uA. This would be used to test my analog input circuit which interfaces to a amperometric sensor.

I plan to use the circuit (b) shown in the datasheet of REF200 page 9 wherein I can generate current of 1nA. This circuit recommends the use of OPA128 opamp. I have done a POC (Proof of concept) using samples from TI and it works fine apart from the noise that is obviously present due to noisy power supplies and lack of shielding during POC.

The OPA128 uses a dual +/-15V supply and it very costly, somewhere around USD 40. I was thinking of using a cheaper precision opamp like LMP7721 instead of OPA128. The only problem is that LMP7721 and most other precision opamps use a supply of 5V.

My question is that if I replace OPA128 with LMP7721, will there be any issues with the circuit operation? As per the REF200 datasheet the compliance voltage range is 2.5V to 40V, so the 5V supply should work. Am I right?

Regards

Nikhil

  • 0
    TI__Guru* 93105 points

    Hi Nikhil,

    Indeed the REF200 can be used with a low voltage supply such as +5 V and ground. But be aware that the compliance range will be much reduced because it requires about 2.5 V stay operating. Figure 3 in the data sheet labeled, Current Source Output Current vs Voltage, shows how the constant current begins to deviate from constant when the voltage across the device drops below about 2.5 V. So if the minimum voltage is 2.5 V on the low end, then 5 V would be the high end with a +5 V supply.

    You mention considering the LMP7721 for the application. One issue I see is its common-mode voltage (CMV) range runs from V- to (V+) - 1.3 V, or 0 V to +3.7 V, with V+ set to +5 V and V- to 0 V. Taking the low-end voltage constraint of +2.5 V for the REF200 and the +3.7 V LMP7721 CMV input constraint, there isn't much range left to work with.

    The OPA128 is a high-end performance, ultra-low input current operational amplifier. There is a similarly low input bias current operational amplifier that costs less, the OPA129. You may want to take a look at what it has to offer in relation to your sensor application. Here's where you can find the OPA129 information:

    http://www.ti.com/product/opa129

    http://www.ti.com/lit/ds/symlink/opa129.pdf

    Do note that like the OPA128, the OPA129 is specified with +/-15 V supplies.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 20 points
    Hi Thomas,
    Thanks for your inputs. I understand the limitations with using a opamp with 5V supply. I am looking at OPA129 and will probably do a quick test regarding its stability. Looks like it would suit my requirement.

    Regards
    Nikhil