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SCXI Overvoltage Issues

Hardware: SCXI>>Modules>>SCXI-1520, Multifunction DAQ (MIO)>>E Series, Multifunction DAQ (MIO)>>M Series

Problem: I get crosstalk or ghosting on the channels of my SCXI module when the inputs are railed at their maximum values. How could this be solved?

Solution:
Crosstalk and Ghosting are defined as signals which are replicated on other channels of a device's scanlist, which can cause distortion or offsets on those signals. Ghosting can be a problem when the inputs to the SCXI module are driven to the maximum limits of their range. This results in an output from the SCXI module which is higher than the maximum supported voltage of the connected MIO device (+-10V). For most MIO devices, the settling time from one channel to the next when a voltage outside the supported range is supplied at the inputs is significantly longer than the settling times that are listed in the device's specifications. The reason for this is because many amplifiers have longer settling times when their inputs are saturated, which is what happens when the DAQ boards read a voltage that is outside their nominal range.

When the inputs to a DAQ device device are saturated, the settling time is significant and ghosting will occur if the interchannel delay is too short. Interchannel delay is determined automatically by the driver based on your Sample Rate and the characteristics of the SCXI module. You can set the interchannel delay manually by setting the convert clock rate property of your task in DAQmx. So the main ways to resolve this ghosting issue are:
  1. Reducing the convert clock rate.
    The convert clock rate can be changed by setting the Convert Clock Rate property in DAQmx. The minimum convert clock rate can typically be computed by dividing your Sample Rate by the number of channels you are measuring.
  2. Upgrading to a high-speed M-Series device.
    The NI-625x M-series boards are designed to settle much faster than the maximum SCXI sample rate of 333k, so they can minimize the settling time issues associated with voltages that are above the +-10V limit.
  3. Remove the channel which is saturating the inputs from the scanlist.*

There are several common applications which can potentially be affected by this problem with SCXI:
  1. Open-Thermocouple Detection of SCXI-1102 or SCXI-1112 modules.
    When an open thermocouple is detected, the inputs to the SCXI module will be railed which can cause ghosted readings on adjacent channels in the scanlist. This problem can be easily resolved by reducing the sample rate for your acquisition in order to maximize the settling time for the DAQ board. Reducing the sample rate may not always work if you have other SCXI modules in your scanlist, so in those cases you may need to set the convert clock rate manually.
  2. SCXI-1520 with disconnected or broken strain gages.
    When a strain gage becomes disconnected or broken, the inputs will typically rail to their maximum values, which can cause ghosting onto other channels in the scan list. In DAQmx, the SCXI-1520 modules typically use the fastest possible convert clock rate by default regardless of the chosen sample rate, so in order to resolve this issue you will need to set the convert clock rate manually using the DAQmx Timing Property Node. The other solutions listed above will also solve this problem.
  3. Scanning of open channels on any SCXI module.
    Anytime a channel on an SCXI module is left unconnected, the readings will often float to the minimum or maximum range of the amplifiers, which will rail the inputs to the DAQ device and cause ghosting onto other channels. In this situation you should leave any unconnected channels out of the scan list, or simply tie the inputs of those channels to ground.*

* Note: Some SCXI modules, like the SCXI-1102, must scan the channels in sequential order, so removing a railed channel from the scanlist may not solve the problem with all SCXI modules. Consult the user manual for your SCXI module to determine if the channels can be scanned in an arbitrary order.

Related Links:
E Series Help
M Series Help
M Series Data Acquisition
KnowledgeBase[2W3HKQBS]: Is it Possible to Scan Open Channels on an SCXI Module?

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Report Date: 09/08/2005
Last Updated: 05/23/2006
Document ID: 3P7A0CBP

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