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Precision Current Sourcing Below 200uA

Hardware: Modular Instruments>>Power Supplies>>PXI-4110

Problem:
The minimum current that any of the channels on the PXI-4110 can source is spec'd at 1% of the range. This means that for the lowest current range of 20mA, the minimum current that the PXI-4110 can source (in constant current mode) is 200uA. In certain applications, however, it becomes important to set current limits below this 200uA limit. Can the PXI-4110 source below 200uA?

Solution:
Fortunately, there is a clever solution for achieving this functionality which has been successful in attaining current set points down to 1uA.

This solution employs both isolated channels working together: channel 1 delivers current to the load and channel 2 subtracts the 200uA offset from channel 1 through a shunt resistor. This way, if both channels are in constant-current mode, the difference between the current limit on both will be delivered to the load. For instance, if we set channel 2 to 200uA and channel 1 to 220uA, the load will get 20uA.

For this particular application, to be able to eliminate any offsets and achieve better precision, we chose the resistor value to be 50KOhm. Then, the settings for the PXI-4110 would be:

  • Channel 2: Current range: 20mA; Current limit: 400uA; Voltage setting: -20V

  • Channel 1: Current range: 20mA; Current limit: 400uA + I(DUT); Voltage setting: maximum voltage required/tolerated by the DUT

where I(DUT) is the current to be delivered to the DUT. This current value can be in a range from 0uA to 19.6mA with 0.4uA resolution. If better precision is required, the offset can be adjusted by changing the 400uA current limit in channel 1 to achieve a value below 0.4uA.

Figure 1: Channel Configuration for Low Current Sourcing


Related Links:

Attachments:


250px-Lowcurrent.bmp

Report Date: 04/02/2007
Last Updated: 04/17/2007
Document ID: 481BNQNP

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