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August 22, 2019, 07:15:22 07:15


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Author Topic: Suggest a Pico-ampere measuring circuit  (Read 1129 times)
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itp
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« on: August 01, 2019, 06:35:15 06:35 »

Dear all,

Can anybody suggest  Pico ampere measuring circuit or IC for a measurement range of 1pA to 50nA.

Thanks & Regards.
Ittoop.
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Ahmad_k
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« Reply #1 on: August 01, 2019, 07:15:30 07:15 »

Directly you can't.

First, what is the required sampling rate ?

Supposing it is low, you have to use an amplifier + Low pass filter (x1000) (Multiple stage is better) then feed output signal to an analog to digital converter (Sigma Delta is great).
you can use MCP3421 (I2C 18 bits Sigma Delta converter with internal Programmable gain amplifier 1 - 2 - 4 - 8 )
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itp
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« Reply #2 on: August 01, 2019, 11:15:48 11:15 »

I am planning to use an analog front-end, followed by an A/D converter.
The analog front-end and its pcb designs are challenging to me.
Can anybody share any document/circuit of this analog section.

Thanks and regards
Itp
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PICker
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« Reply #3 on: August 01, 2019, 12:09:12 12:09 »

If you want very high precision in the lower pA range I suggest the LMP7721
http://www.ti.com/product/LMP7721
this is useful for making electrometers. But the choiche of the feedback resistor and the lowpass capacitor is crucial.
You can also use a switching integrator instead a classical transimpedance amplifier (I/V converter); plese have a look to this appnote:
http://www.ti.com/lit/an/sboa034/sboa034.pdf
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mexpcb
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« Reply #4 on: August 01, 2019, 12:37:37 12:37 »

Hi, for those Ranges it may be better if you use TIA (Transimpedance amplifier)

You can take a look at TI OPA859..

not sure if the full details but after that you may need to amplify it like an VCA824 also from TI

Regards
mexpcb

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BharatSujanani
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« Reply #5 on: August 05, 2019, 06:07:06 06:07 »

You can use any low noise operational amplifier in Trans impedance mode. And use very high resistance like 500Megaohm  to measure your current range. If you signal is noisy than use low pass filter after this opamp output.


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Bharat Sujanani
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« Reply #6 on: August 14, 2019, 01:53:04 13:53 »

As a general comment: Try to avoid super-high resistors (> 10 Mohm) in your circuit. Because leakages on the PCB become significant sources of error. Putting guard traces on your PCB (google it) can help reduce errors due to leakages.
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OscarH
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« Reply #7 on: August 15, 2019, 04:55:35 04:55 »

I also found these super-high resistor are sensitive to humidity and moisture.
It does mean, on top of Bigtoy comment, you can see value changing as a function % humidity in the air.
I don't remember the specification of the resistors I used, but I remember I didn't had too much choice in models, so took what I found...
OH
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PICker
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« Reply #8 on: August 15, 2019, 09:31:06 09:31 »

I have experience with transimpedance I/V converters with a feedback resistors in the range of 50-100 MOhm and I confirm that sometimes is the resistor the key component of the circuit. I suggest quality carbon or precision (0.1%) metal oxide thick film resistors  (i.e. https://www.ohmite.com/) with very low inductive effect.
I confirm also the sensitivity to temperature changes (sometimes counteracted by using a diode that responds in the opposite direction).
Another interesting approach is to use diodes  (LEDs) as feedback components for log I/V converters:
https://pdfs.semanticscholar.org/2ecd/fdcbedf2ea6d171cbc0b3f6336758457a58f.pdf
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