Differential Measurements Using High-Speed Digitizers


There are two ways to make differential measurements using NI oscilloscopes, which have single-ended input channels. The first method simply involves using a differential probe. This document details how to acquire the two components of the differential signal on two individual channels of an NI oscilloscope, and then use built-in driver functionality to perform waveform math (subtract channels) for an accurate, fast differential measurement. This document also discusses the calibration of your differential measurement system.


Measurement Calibration

Accurate differential measurements require a calibrated measurement system. Perform the following tasks to calibrate your NI oscilloscope before making differential measurements.

For differential measurements that involve coaxial cabling or test fixtures found in most automated test systems, it is important to calibrate gain and offset errors induced by the testing environment and test cabling. In these cases, calibrate your oscilloscope for differential measurements using the following method:

  1. Hardware self-calibration (internal calibration)
  2. Gain and offset calibration

For differential measurements that involve using attenuation probes, calibrate your oscilloscope using the following method:

  1. Hardware self-calibration (internal calibration)
  2. Probe compensation
  3. Gain and offset calibration

Hardware Self-Calibration

All precision instruments suffer measurement drift as accuracy fluctuates with time and temperature. Self-calibration allows your NI oscilloscope to characterize this drift as a set of calibration constants stored in the onboard EEPROM. The oscilloscope uses these calibration constants to automatically compensate for measurement drift and to return highly accurate calibrated data.

NI recommends frequently performing self-calibration to update the calibration constants and ensure the best possible measurement accuracy. You can perform a self-calibration by calling Cal Self Calibrate in NI-SCOPE, or by selecting Utility>>Self-Calibration in the NI-SCOPE Soft Front Panel (SFP). After being called, self-calibration does not require user intervention. Refer to your oscilloscope specifications document for recommended self-calibration intervals.

External calibration is performed less frequently than self-calibration and requires a precise external voltage source, such as a highly accurate oscilloscope calibrator. Because precision voltage sources and calibration equipment are costly, external calibration is usually performed by a metrology laboratory or by National Instruments. Refer to your oscilloscope specifications document for recommended external calibration intervals.

The following figure illustrates the self-calibration programming flow for NI oscilloscopes.

Figure 1: The coding flow for calibration includes Opening a session to the hardware, calibrating the oscilloscope, and closing the instrument session.

Gain and Offset Calibration

Differential measurements require matching gain and offset between the two test leads. You can automate the calibration of your test leads using the following method:

1. Connect both probes or BNC cables to the probe compensation signal on PFI 1, as shown in the following figure.

Figure 2: Compensating for a differential measurement on two input channels requires multiple BNC adapters to perform.

2. Run the probe calibration VI, which is downloadable at ni.com. The probe calibration VI returns the necessary calibration constants for gain and offset adjustments for channel 0. You can also manually adjust the array gain and array offset settings in NI-SCOPE to best match channel 0 and channel 1 as described below.

NOTE: The probe calibration VI returns gain and offset calibration data for channel 0 to best match the signal on channel 1. It is important to connect both channels to the same signal before you run the calibration routine.

You can manually calibrate gain and offset differences between two test leads using the following method:

  1. Acquire the same signal, such as the probe compensation signal from PFI 1, on both channels of the digitizer.
  2. Use the voltage histogram standard deviation (VHSD) measurement directly from NI-SCOPE on channel 0 and channel 1 to calculate the gain adjustment needed.
  3. Apply the gain adjustment to that array gain attribute/property of the necessary channel.
  4. Use the voltage average (VA) measurement directly from NI-SCOPE on channel 0 and channel 1 to calculate the offset adjustment needed.
  5. Apply the offset adjustment to the array offset attribute/property of the necessary channel.

For example, gain adjustment on channel 0 equals (VHSD channel 1)/(VHSD channel 0). Offset adjustment on channel 0 equals (VA channel 1) - (VA channel 0), after the proper gain adjustment has been applied.


Taking Differential Measurements

NI-SCOPE allows you take differential measurements with relative ease. NI-SCOPE includes measurement functions that quickly return the results of your differential measurement.

NOTE: Remember that you need to calibrate the high-speed digitizer hardware before taking differential measurements (refer to the Measurement Calibration section above).

To configure the high-speed digitizer for differential measurements, set the active and other channel attributes/properties. Then call the NI-SCOPE Fetch Array Measurement function with the array measurement set to subtract channels, and then retrieve your differential measurements directly.

Differential reading = active channel - other channel

Figure 3: This illustration shows the overall programming flow for making differential measurements with two signal probes on NI oscilloscopes.

Refer to the following LabVIEW example programs to acquire differential measurements. 


Figure 4: Driver functions can be used to perform channel math and show a differential product of the two channels.

NOTE: When using the NI 5112 to measure differential signals that have individual components well below 20 MHz, NI recommends activating the bandwidth-limiting filter to reject any high-frequency noise that may be present on your signal.



When making differential measurements using the method discussed above, it is important to remember the following limitations.

No Differential Triggering

The high-speed digitizer can only be triggered by one of the two channels in use, a separate single-ended analog signal, or a digital pulse. Therefore, the oscilloscope cannot trigger off the true differential level.

Vertical Range Setting

Improper vertical range settings can give rise to invalid differential measurements. Channel subtraction takes place on the acquired data after the analog-to-digital (A/D) conversion, so if the signal on either channel exceeds its vertical range, data is incomplete and processing operations give rise to invalid results. Ensure that the signal you are acquiring on each channel of your oscilloscope does not exceed the vertical range setting for that channel. Therefore, set your vertical range of your input channels based on the individual components of the differential signal, and not based on the differential result.

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