Load Switching

This section addresses analog signal-path related challenges associated with connecting a DUT, a switch, and a high-performance DMM.

Switching Voltages

When switching high-voltage signals, the parasitic capacitance of the cable connecting the DMM with the switches, as well as the input capacitance of the instrument, need to be charged to the source voltage. Depending on the voltage present at the DMM input during the measurement on the previous channel, a significant current can flow from the high–voltage source to charge this capacitance. This action is referred to as hot switching, which can reduce the life of the switches. Refer to your switch documentation for information about extending switch life.

When switching high-voltage signals into a low-voltage range of any DMM (such as applying 300 V on the 10 V range), the input impedance will be lower than it is when the proper range is selected. Repetitive operations like this may shorten switch life. To maximize switch life, always select a measurement range that can handle the maximum voltage expected.

Switching signals with large common-mode components (Input LO switched from +250 V to -250 V) may, over extended periods of time (tens of Hz for several weeks) begin to degrade the relay performance. The switching life of the relays can be extended in many cases by inserting some small common-mode resistance in series with the Input LO terminals. Even several hundred ohms (200 Ω to 500 Ω) helps significantly. Refer to the following figure, where VCM1 or VCM2 >50 V:

Figure 72. Adding Common-Mode Resistance in Series

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In these situations, break-before-make switching is essential to obtaining any kind of reliable relay life. Should S1 and S2 be turned on together, even momentarily, VCM1 + VSIG1, is shorted to VCM2 + VSIG2, which could cause high currents to permanently damage the contacts of S1 or S2.

Low Voltages

When setting up systems using relay switching, keep in mind that low–voltage performance is largely limited by the switches selected and the input terminals used on the switch.

Good low–thermal electromotive force (EMF) relay switching cards are available from NI.

Common, uncompensated reed relays can reach tens of µV of rather unstable offset, but high–performance reeds can reach below 1 µV.

Many armature–style latching reed relays can give very low drift (one such device is commonly used in NI-DMM's signal path). It is sometimes possible to compensate for this drift by using similar relays in both Input HI and Input LO connections or by placing adjacent poles of a 2–pole relay back–to–back.

Lastly, a common metrology technique involves making a measurement, reversing the leads, taking another measurement, and subtracting and averaging the two readings.

High Voltages

Caution All cables, connectors, and fixtures used in your system must have specific specifications to handle, with an adequate safety margin 1000 V signals.

The DMM inputs can measure sub-µV level signals with low drift, handle 1000 V DC or 1000 Vpk and peak V AC signals, and recover very quickly for sub-µV level measurements in a system with switching

However, when switching high-voltage signals, the parasitic capacitance of the cable connecting the PXIe-4081 with the switches, as well as the input capacitance of the instrument, needs to be charged to the source voltage. Depending on the voltage present at the DMM input during the measurement on the previous channel, a significant current can flow from the high–voltage source to charge this capacitance.

This current transient does not cause any degradation effect on the PXIe-4081, but this action, referred to as hot switching, can reduce the life of the switches. Refer to the switch documentation for information about extending switch life.

When switching high-voltage signals into a low-voltage range of any DMM (such as applying 300 V on the 10 V range), the input impedance is lower than it is when the proper range is selected. Repetitive operations such as these may shorten switch life. To maximize switch life, always select a measurement range that can handle the maximum voltage expected.

Remember that you might need to increase the settle time of your measurement. Consider the following figure showing a DMM connected to a 100 V DC source, a thermocouple, and device under test (DUT) through a multiplexing switch.

Figure 73. Connecting a DMM to a High Voltage

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For example, assume you are switching from the 100 V range to the 100 mV range and want to obtain the best sensitivity using the thermocouple. When the switch module advances from the 100 V channel to a thermocouple channel, the input signal conditioning of the DMM (along with the cabling, switching module, and so on) must discharge any stray capacitance from 100 V to 100 µV. If you need a sensitivity of 1 µV from the low–voltage channel, the system must settle from 100 V to 1 µV (10 parts per billion, or eight decades).

When you require resolutions better than 10 ppm, dielectric absorption is a major factor.

Depending upon the source resistance (R), system capacitance (C), and dielectric qualities of the system components, you must allow more settle time in this example. In practice, you may need to allow settle times of 10 ms or more in situations where you are switching high voltages (>30 V).

Voltages with Large Common-Mode Components

Switching signals with large common-mode components (Input LO switched from +250 V to -250 V) may, over extended periods of time (tens of Hz for several weeks), begin to degrade the relay performance. The switching life of the relays can be extended in many cases by inserting some small common-mode resistance in series with the Input LO terminal. Even several hundred ohms (200 Ω to 500 Ω) helps significantly. Refer to the following figure, where VCM1 or VCM2 are >50 V:

Figure 74. Connecting to a Large Common-Mode Voltage

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In these situations, break-before-make switching is essential to obtaining any kind of reliable relay life. If S1 and S2 are turned on together, even momentarily, VCM1 + VSIG1, is shorted to VCM2 + VSIG2, which could cause high currents to permanently damage the contacts of S1 or S2.

Ensure that you do not exceed the Volts-Hertz (V-Hz) limit of the device. Exceeding the V-Hz limit is more likely to occur when measuring high-voltage/high frequency signals. The transient signal generated when switching signals with large common mode voltages closely resemble the effect of applying signals that exceed the V-Hz limit. This could affect the internal transfer of data in the device and would generate an error prompting you to reset the board and reconfigure your measurement.

Tip   If the PXIe-4081 experiences the conditions where the common-mode transients cause the violation of the V-Hz limit of the instrument while the instrument is making measurements, NI-DMM driver returns error NIDMM_ERROR_SERIAL_PORT_ERROR (defined as 0xBFFA401F, or -1074118625). If your system can potentially face this situation, your application should look for this error code after a call to:
  • niDMM Read (niDMM_Read)
  • niDMM Read Multipoint (niDMM_ReadMultipoint)
  • niDMM Fetch (niDMM_Fetch)
  • niDMM Fetch Multipoint (niDMM_FetchMultipoint)

Your application can handle this error by calling the following VIs or functions:

  1. niDMM Reset (niDMM_reset)
  2. Any configuration VI or function to reconfigure the instrument to the required settings
  3. niDMM Initiate (niDMM_Initiate), niDMM Read (niDMM_Read), or niDMM Read Multipoint (niDMM_ReadMultipoint) to restart the measurement

Switching Current

The current measurement path within the PXIe-4081 utilizes an electromechanical relay that provides very low resistance to assure a low burden voltage. While this relay has an exceptionally long life, you can take special precautions to ensure reliable operation.

Whenever possible, switch the PXIe-4081 into the current measurement function before applying the current. Switching inductive current sources generally creates flyback voltages that stress the relay and, if done on a frequent basis, can shorten the reliability of the relay. Also, avoid interrupting the current by switching out of the current measurement function or resetting the DMM when currents are flowing through the circuit.

While the internal shunts are ideal for many applications, in applications where more than one current needs to be measured using the same DMM and the conditions require that the current cannot be interrupted across the load, you can use a break-before-make switch in multiplexer topology and place external shunts across the channels. Connect the shunts in series with the circuit under test and then measure the voltage drop across the shunts.

If the conditions allow interrupting the current flow through the circuit under test, it is still recommended to use external shunts. It is not common to find switches that can switch currents higher than 1 A, and switching currents can diminish the life of the switch.

The benefits of using the break-before-make switch in multiplexer topology with external shunts across the channels are as follows:

  • Allows uninterrupted switching of current sources because the multiplexer switches voltages developed across the shunts instead of routing currents; ensuring that the current loop is not interrupted is generally helpful to avoid disturbing the current sources
  • Extends the switch life, especially if the switch consists of electromechanical relays

The tradeoffs involved may be as follows:

  • Reduces accuracy, although using the DMM to measure the resistance of shunts periodically (in 4-wire) can address this issue
  • Introduces a leakage current from the switch that could significantly affect low-level current measurements; refer to your switch documentation for methods to reduce leakage currents
  • Introduces errors due to resistor self-heating in high-level current measurements if the shunt resistor characteristics are insufficient

You should choose a shunt resistor whose characteristics and value help you reduce noise and errors, as follows:

  • Consider the tradeoff between creating a large enough voltage across the shunt for resolution, while at the same time keeping the shunt voltage small enough to prevent high heat dissipation or burden voltage errors during the measurement.
  • Look for a shunt resistor with low temperature coefficient (preferably not more than 10 ppm/°C) and absolute tolerance matched to your application requirement.

Recall that voltage offsets introduced by the switch could affect your measurement, because they add an additional burden voltage to the one specified by the ammeter itself. For the burden voltage per range, refer to the specifications documents.

In cases where you must use the built-in current measurement capability of the PXIe-4081, use a make-before-break setup for the switches for ensuring uninterrupted current loops. You may also choose to use specially designed current routing switches for this application.

Refer to DC and AC Current for general information about current measurements with the PXIe-4081 and for recommendations when taking low and high level current measurements.