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DDC112: DDC112U Suitability to Small Signal Measurement

Gowtham R
Prodigy 120 points
Part Number: DDC112
Other Parts Discussed in Thread: ACF2101

Dear Sir,

I have doubt regards DDC112U can support to measure -100nA to 100nA with 1pA precision. This doubt i asked in Texas customer support and the case number is CS2211553.

Reply from Customer Support: The precision is too small for this IC. The device cannot support a -100nA reading with 1pA per LSB.  DDC112U can read 24.4pA per LSB for -100nA measurement.

As per my understanding of DDC112U, precision is 0.383pA/LSB. 

Explanation:

ADC Resolution: 20 bit -> 2^20 = 10,48,576 ADC Count.

Range 4: 200pC

Tint : 500us

Negative FSR is -0.4% = -0.8pC

Total Measuring Range = 200.8pC, In terms of Measuring Total current is 200.8pC/500us = 4,01,600pA or 401.600nA.

Precision pC/LSB = 200.8pC/1048576 = 0.0001915pC

Precision pA/LSB = 4,01,600pA/1048576 = 0.38299pA.

 

To measure Bipolar input, simply current source circuit added in input side. This recommended by Texas support, link Creating_a_Bipolar_Input_Range_for_the_DDC112.

In this Bipolar configuration, we are simply shifting the measuring scale.

Without Bipolar Config circuit:

-Full Scale(pC) is -0.8

+Full Scale(pC) is 200

In terms of current -1600pA to 400000pA or -1.6nA to 400nA

With Bipolar Config circuit:

-Full Scale(pC) is -103.2

+Full Scale(pC) is 97.6

In terms of current -206400pA to 195200pA or -206.400nA to 195.200nA

So, it can be able to measure -100nA to 100nA.

 

Please kindly check my above comments and refer the attached schematic and doc.

Range:200pC

Range:50pC

Schematic:

And eagerly waiting for your reply.

please check and reply, is this design possible to measure -100nA to 100nA with 1pA precision.

One more question, could you please suggest ACF2101 or DDC112U which one is best suitable for my application.

 

Thanks & Regards,

Gowtham R,

Panacea Medical Technologies

  • 0
    TI__Expert 4640 points

    Hi,

    Sorry for late reply. We will send you a response on Monday.

    Best regards,
    Eduardo

  • +1
    TI__Expert 4640 points

    Hi again, Gowtham,

    Your calculations are correct. I am glad you double checked with us as someone in TI had giving you the wrong answer.

    The only thing is that those calculations account only for the LSB size. You have to consider also all the other non-idealities of the device. From a DC perspective, you may be able to easily remove/calibrate some (like offset or gain) but some may be much harder (like linearity). In that sense, notice that linearity error increases with the reading, so, you may be able to meet 1pA at low input currents but not for the large ones. Finally you got to include noise if you have to base your result in a single measurement.

    Anyhow, I believe you are probably already very familiar with these but please let us know if you have any other questions.

    Best regards,

    Eduardo

  • 0 in reply to Eduardo Bartolome
    Prodigy 120 points

    Thanks for reply.