Current Sourced By the PXI-54xx Arbitrary Waveform Generators

Updated May 24, 2018

Reported In

Hardware

  • PXI-5404
  • PXI-5401/5411
  • PXI-5421
  • PXI-5422
  • PXI-5431
  • PXI-5412
  • PXI-5441
  • PXIe-5413
  • PXIe-5423
  • PXIe-5433

Issue Details

The specifications for the PXI-54xx series of Arbitrary Waveform Generators show maximum output voltages for 50 ohm loads but does not specify how much current is sourced.

Solution

First, the open circuit voltage of the particular Arbitrary Waveform Generator (ARB) must be known.
 
±2 V±10 V±12 V
PXI-5404PXI-5401PXI-5412
 PXI-5411PXI-5421
 PXI-5431PXI-5422
  PXI-5441
  PXI-54x3

The analog output of the ARB can be viewed as a ±2 V,±10 V, or ±12 V source in series with a 50 ohm or 75 ohm resistance, depending on the software setting and the specific card. You can use the standard circuit equations to determine the peak voltage/current available for a given load as seen in Figure 1.
 

Figure 1 - ARB Output Diagram
 

Iout = Voc / ( RO + RL )
Iout = peak current output level
Voc = peak circuit voltage of the ARB
RL = load impedance in ohms
RO = output impedance on the ARB


For example, with the PXI-5411 driving a 800 ohm load, the maximum current sourced is approximately 10 / [800+50] = 8.31mA. In theory the PXI-5411 should deliver 200 mA when short circuited. In practice, the maximum current available is slightly lower than the calculated value for low impedance loads. Keep in mind, ARBs were not designed to drive loads less than 50 ohms.

Additional Information

If you are attempting to drive a device such as a sensor or an LVDT and want to find the voltage/current seen by the device, you will need to convert everything to RMS values. 

Maximum voltage swings can be calculated using Vout = ± [ RL / ( RL + RO ) ] * Voc

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