Damped Signals When Using Attenuating Probes with the NI 5911, 5922 and 5105

Updated Jun 15, 2018

Reported In

Hardware

  • PCI-5911
  • PCI-5922
  • PXI-5922
  • PCI-5105
  • PXI-5105
  • PXIe-5105

Driver

  • NI-SCOPE

Issue Details

When I use an oscilloscope attenuation probe with my NI 5105, NI 5911, or NI 5922 my signal does not look right. Why is that?

Solution

The NI 5911, NI 5922 and NI 5105 should not be used with most oscilloscope attenuation probes, including the SP200B available from National Instruments. When an attenuation probe is coupled with an instrument with a larger input capacitance than the capacitance for which the probe can compensate (typically 35 pF), the signal will be damped and the measured amplitude will be less than expected. 

The NI 5911, NI 5922 and NI 5105 have a higher input capacitance of 70 pF, 60 pF, and 50 pF respectively which causes this signal damping with most attenuation probes. When selecting a 3rd party probe, the digitizer's input capacitance should be considered to ensure quality measurements can be made. 

When using one of the mentioned cards, it is recommended that you connect the source directly to the input of the digitizer without a probe in order to avoid performance issues. The below images illustrate the difference between using a direct connection (Figure 1) and a probe (Figure 2) with a NI 5105. Figure 2 shows how probe compensation is not possible with the NI 5105.


Figure 1: Direct connection to NI 5102


Figure 2: Connection through probe to NI 5102

Below are common scenarios your application might fall under, with suggested work-arounds:
  • If the probe is being used for attenuation, a better solution would be to use an in-line attenuator.  You can find various kinds of attenuators over the frequency range of interest.  The SMB 120 is an example of a cable with built-in 10dB attenuation (attenuates to 32% of the original signal).
  • If the probe is needed to connect to the source (eg. alligator clips, etc.). Though a more stable connection is often preferred, in this case you should consider using an SMB to alligator clip cable. The SMB 300 is a good example of such a cable.
  • If your application requires a lower input capacitance, then you will need to consider a more comprehensive system design.  One solution may be to design in an active converter (eg. an electrical op-amp circuit) which converts from low impedance to high impedance.

Additional Information

For more information about attenuation probes please refer to the NI High-Speed Digitizers Help (Probe Compensation). Also, note that the process outlined in White Paper: Probe Compensation (Calibration) will not work for these digitizers. Users will need to provide their own square wave with which to compensate. 
 

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