Using the PXI-4022 for Low Level Current Measurements with a DMM

The PXI-4022 has different applications but one common use is a low-current preamplifier. You can use the PXI-4022 with a DMM to measure very small currents. The PXI-4022 works by amplifying and converting a current into a voltage that varies linearly from 0 to ±10 V depending on what current range the device is configured for. This voltage can be accurately measured with a DMM and scaled back to a current in software.

Since the PXI-4022 is amplifying and converting a current into a voltage any voltage measurement DMM example that is supported by your hardware can be used. Only a few small modifications will need to be made in order to use the DMM with the PXI-4022.

1. Configure the PXI-4022 by using the niDMM 4022 Control.vi
2. Connect the PXI-4022 to the DMM
3. Convert the DMM voltage measurement to display current

Configure the PXI-4022

To configure the PXI-4022 in software you only need to use the niDMM 4022 Control.vi. This VI requires two inputs:

1. Resource Descriptor - the resource name of your PXI-4022 (ex. pxi1slot5).
2. 4022 Configuration - allows five different configuration options, three of which allow you to set the current range of the device. These three options are: Current Amp 1mA, Current Amp 10uA, and Current Amp 100nA.

Depending on the option selected, the PXI-4022 will apply a different gain. The table below displays the different current ranges that can be selected and the gain that will be applied.

Current Range	Gain Setting (V/A)
1mA	10^4
10uA	10^6
100nA	10^8

Table 1: Current Amplifier Gain Settings

For example, if you want to measure a current of 4uA. You would use the niDMM 4022 Control.vi to configure the device as Current Amp 10uA . The PXI-4022 would amplify the 4uA current by applying a gain of 10^6 V/A which results in a voltage of 4V. Then you can use a DMM to accurately measure the 4V signal.

Connect the PXI-4022 to the DMM

The most common way of making the connections for a current measurement is shown in Figure 1. Other configurations are available and they are described in the PXI-4022 Specifications document which is linked below.



Figure 1: PXI-4022 and DMM connections for a Low Current Measurement

Convert Voltage to Current

Since the DMM is used to measure a voltage, you need to scale this voltage to a current. To do this, divide the voltage by the gain that was applied by the PXI-4022. The gains applied are shown in Table 1 above. One important thing to note is that the resulting current will be reversed in polarity. This can be fixed by just multiplying the result by -1.

For example, if 4V was measured by our DMM and the current range selected was 10uA (Gain Setting of 10^6 V/A), you can divide 4V by 10^6 V/A and you will get 4uA. An example program of how to scale the data is linked below.