Low Burden Voltage Current Measurement Circuit

C

chemelectronics

DMMs typically have a burden voltage when measureing current. I want to create an active circuit using descrete components and ICs where necessary (such as for a transimpedence amplifier circuit) that can be used in conjunction with any multimeter that will drastically reduce the burden voltage on the unit under test while meauring extremely low currents at high resolution. Input will be the current to be measured and the output terminals will connect to a typical DMM with banana plugs. Please ask for any clarity you need for helping me get started toward a SMT component based product.

flux

I haven’t made schematic changes yet; the project is still empty with 0 components and 0 nets.
To start this SMT µCurrent-style adapter correctly, I need a few design requirements because they materially affect the topology, op-amp choice, protection, range switching, and output scaling:
  1. Current ranges: What current range(s) do you want to measure?
    Example: pA–nA, nA–µA, µA–mA, or multiple selectable ranges.
  2. Target burden voltage: What maximum voltage drop across the DUT is acceptable?
    Example: <1 mV, <10 mV, <100 µV.
  3. Output format to DMM: Should the banana output be a voltage proportional to current?
    Example: 1 mV/nA, 1 mV/µA, 1 V/µA, etc.
  4. DMM input mode: Will the DMM measure the adapter output in voltage mode, not current mode?
    That is the usual approach for reducing burden voltage.
  5. Power source: Preferred power supply?
    Options: coin cell, AAA/AA, 9 V battery, Li-ion/LiPo, USB-C, or external supply.
  6. Polarity: Do you need bidirectional current measurement, or only one current direction?
  7. Accuracy target: Desired accuracy/resolution?
    Example: 0.1%, 1%, 10 pA resolution, etc.
  8. Product constraints: Any size, cost, calibration, enclosure, or banana jack preferences?
Once these are defined, the next step is to design the schematic around a low-bias-current transimpedance amplifier with protected current input, precision feedback networks/ranges, low-noise power, output protection for the DMM terminals, and calibration/test points.
Set current ranges
Set burden target
Define DMM output
Set power and polarity
Start architecture
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