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SCOPE Soft Front Panel Offset With Same Signal Applied to Both Channels

Hardware: Modular Instruments>>High-Speed Digitizers (Scopes)

Problem: I connect the same signal to both channels of my National Instruments High Speed Digitizer. I open the SCOPE Soft Front Panel to verify the hardware is working properly and there's an vertical offset between the two channels. Do I need to return the High Speed Digitizer to calibrate the instrument?

Solution: Before returning the instrument, attempt self-calibration.

Using the SCOPE Soft Front Panel, you can perform self-calibration by selecting Utility»Self Calibration.

If you're using LabVIEW, you can navigate to Start»Program Files»National Instruments»NI-SCOPE»Examples and use the niScope EX Calibrate.vi.

Self-calibration has the capability to yield the following benefits (depending on device):

• Corrects for DC gain and offset errors within the digitizer. This is done for all ranges, both input impedance paths (50 Ω and 1 MΩ), and all filter paths (enabled/disabled).


• Calibrates trigger level offset and gain.


• Calibrates trigger timing, as well as the time-to-digital conversion (TDC) circuitry to ensure accurate trigger timing and time-stamping.


• Compensates 1 MΩ input frequency flatness, then compensates input capacitance so that it is equal regardless of selected attenuator range.

Self-calibration is supported by the following devices: NI 5102, NI 5112, NI 5122, NI 5124 and NI 5911. However, the self calibration only calibrates the Random Interleaved Sampling (RIS) timing on the NI 5102.

Using self-calibration, you can adjust the digitizer for use in environments other than those in which it was externally calibrated. Self-calibration does not rely on external standards. Instead, it adjusts the digitizer with respect to a precise, high-stability internal voltage reference. Use self-calibration whenever you use the digitizer in an environment where external variables such as temperature can affect measurements. Also, you want to use self-calibration only after the digitizer has been externally calibrated with respect to a known standard.

For optimum performance, use self-calibration when the digitizer is placed in a new system, any time the temperature changes more than 5 °C from the previous self-calibration, or 90 days after the previous self-calibration. The result is a product that yields full performance over its operating temperature range and two-year calibration cycle for DC accuracy, AC response, and trigger level/timing. When the two-year calibration interval expires, an external calibration is required. (Note: This paragraph includes specific information for the NI 5122 and NI 5124. Please check the specifications for your device to determine temperature range and calibration cycles.)

Internal circuitry is automatically isolated from the input during self-calibration. However, problems may occur if high-voltage, high-frequency signals (in excess of 500 V/µs slew rate) are present during self-calibration. When in doubt, disconnect the inputs as directed. If you are absolutely certain that the input signal's maximum slew rate is below 500 V/µs, then it is acceptable to leave the input signal connected during self-calibration.

Many times an offset such as the one below can be fixed using self-calibration.

Before Self-Calibration




After Self-Calibration



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Report Date: 03/22/2005
Last Updated: 03/23/2005
Document ID: 3JLGINLE

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