You can use the pin parametric measurement unit (PPMU) to make DC parametric measurements on DUT pins and force voltage or current.

The PPMU supports the following operations:

  • Force voltage, no measure
  • Force voltage, measure voltage
  • Force voltage, measure current
  • Force current, no measure
  • Force current, measure voltage
  • Force current, measure current
  • Measure voltage, no force

Force Voltage

You can perform the following operations while forcing voltage to the DUT with the PPMU:
  • Force voltage, measure current
  • Force voltage, measure voltage
  • Force voltage, no measure

When you program or interactively set the PPMU to force voltage to the DUT, you must specify the following values associated with the voltage level. Refer to Digital Pattern User Manual for more information about PPMU DC voltage API.

  • Voltage Level (VF)—Specifies the voltage level that the PPMU forces to the DUT pin.
  • Current Limit Range (I Limit Range)—Specifies the current range to use when forcing a voltage from the PPMU to a DUT.

    Selecting the smallest range that includes the maximum current expected to be required by the DUT provides a degree of current limiting for force voltage operations. This current limiting functionality uses a range setting rather than a current clamp, and therefore you should not rely solely on it to limit current for sensitive DUTs or test setups. If the selected current range is smaller than the actual current consumption of the DUT pin, the voltage output may not reach the specified voltage level due to the sourced current being limited.

    Note A voltage glitch occurs if the digital pattern instrument is already sourcing a voltage when you change the current range. For example, when switching from a smaller to a larger current range, the PPMU output impedance value decreases. If the digital pattern instrument is driving a resistor to ground, the instrument forms a voltage divider between the PPMU output impedance and the output load. As a result of the voltage divider effect, when the PPMU output impedance decreases, the output voltage instantly increases before the loop can compensate for the change. Likewise, when switching from a larger to a smaller current range, the PPMU output impedance value increases, which results in a decrease in the instantaneous voltage observed on the DIO channel.
    Note Lower current limit ranges requires a larger settling time for the sourced voltage to reach the programmed level.

    • Force Current

    You can perform the following operations while forcing current to the DUT with the PPMU:

    • Force current, measure voltage
    • Force current, measure current
    • Force current, no measure

    When you program or interactively set the PPMU to force current to the DUT, you must specify the following pin levels. Refer to Digital Pattern User Manual for more information about PPMU DC current API.

  • Current Level (IF)—Specifies the current level that the PPMU forces to the DUT.
  • Current Level Range (I Level Range)—Specifies the current range to use when forcing a current from the PPMU to a DUT. Selecting the smallest range adequate for the desired current level provides the best current accuracy for the force current and measure current operations.
  • Voltage Limit High (VCH)—Specifies the nominal voltage at the pin at which the high side voltage clamp activates when the PPMU forces current to the DUT.
  • Voltage Limit Low (VCL)—Specifies the nominal voltage at the pin at which the low side voltage clamp activates when the PPMU forces current to the DUT.
    • Note Refer to PXIe-6570 Specifications for more information about when the voltage clamps begin to conduct.

    Measure Voltage

    You can perform the following voltage measurement operations with the PPMU:
    • Force voltage, measure voltage
    • Force current, measure voltage
    • Measure voltage, no force

    You can perform measurement operations at any time, even if you are not forcing current or voltage with the PPMU.

    In the Digital Pattern Editor, navigate to the PPMU section of the Pin View pane to view voltage measurements. To programmatically make PPMU voltage measurements, use the following:

    • niDigital PPMU Measure VI
    • DigitalPpmu.Measure .NET method
    • niDigital_PPMU_Measure C function

    Measure Current

    You can perform the following current measurement operations with the PPMU:
    • Force voltage, measure current
    • Force current, measure current

    You can only measure current while forcing voltage or current with the PPMU.

    In the Digital Pattern Editor, navigate to the PPMU section of the Pin View pane to view current measurements. To programmatically make PPMU current measurements, use the following:

    • niDigital PPMU Measure VI
    • DigitalPpmu.Measure .NET method
    • niDigital_PPMU_Measure C function

    Aperture Time

    The PXIe-6570 supports discrete aperture times that are based on the sample rate of the ADC. You can configure aperture time on the PXIe-6570 to achieve a desired accuracy and/or speed.

    When you use the NI-Digital Pattern Driver API or the Digital Pattern Editor Pin View pane to measure voltages and currents, the PXIe-6570 begins acquiring measurements immediately. The PXIe-6570 uses sampling ADCs to sample voltage and current. These ADCs also allow for the hardware averaging of acquired measurements using the aperture time. The number of hardware averages is represented by this equation:

    Number of Averages = (Aperture Time/4 μs)

    Refer to PXIe-6570 Specifications for more information about the relationship between measurement noise and selected aperture times.

    To set the aperture time for your measurement, use the following:

    • niDigital PPMU Configure Aperture Time VI
    • DigitalPpmu.ConfigureApertureTime .NET method
    • niDigital_PPMU_ConfigureApertureTime C function

    Optimizing PPMU Measurement Time

    Each digital pattern instrument channel has dedicated per-pin PMU circuitry, but analog-to-digital conversion of the PPMU circuits is split into banks to increase operational efficiency: DIO<0..7>, DIO<8..15>, DIO<16..23>, and DIO<24..31>. An independent ADC is connected to each of these four banks of eight channels, meaning that PPMU current and voltage measurements can be run in parallel by taking measurements on channels in different banks.



    When taking a measurement on two or more channels within a given bank, the connected ADC must be multiplexed between the selected channels. This results in an increase in measurement execution time that is dependent on the following characteristics:

    • The number of PPMU measurement channels being multiplexed
    • Whether the measurement channel is configured to make a voltage or current measurement
    • Current measurement range
    Note Measurement execution time is only dependent on the current measurement range if the channel is configured for current measurement.

    The effect of measurement multiplexing on measurement execution time can be calculated using the following system of equations:

    Total Multiplexing Time Impact = ( Number of Multiplexed Channels * Aperture Time ) + T V + T HA + T LA

    where

    • T V = N VC × ST V

      where

    • TV—Voltage Channel Settling Time Impact
    • NVC—Number of Voltage Channels
    • STV—Voltage Channel Settling Time
    • T HA = N HCC × ST HC

      where

    • THA—High Current Channel Settling Time Impact
    • NHCC—Number of High Current Channels
    • STHC—High Current Channel Settling Time
    • T LA = N LCC × ST LC

      where

    • TLA—Low Current Channel Settling Time Impact
    • NLCC—Number of Low Current Channels
    • STLC—Low Current Channel Settling Time

    Refer to the following table for the settling time constants of different measurements.

    Table 10. PPMU Measurement Settling Time Constants
    Measurement Settling Time Constants Note
    Voltage Settling Time 40 μs —
    High Current Settling Time 130 μs Applies to measurements in the 32 μA, 128 μA, and 2 mA ranges.
    Low Current Settling Time 250 μs Applies to measurements in the 2 μA range

    The settling time for multiplexing between two channels is determined by the channel being switched to, not the channel being switched from. Because of this, you do not have to factor in the channel you take your initial measurement on in a bank when calculating the impact of settling time on measurement execution time for that bank.

    Measurement multiplexing has the highest impact on banks filled with low current measurement channels. For example, the highest possible multiplexing time impact for measurements taken with an aperture time of 1 μs is 1.76 ms:

    (8 ✕ 1 μs) + (7 ✕ 250 μs) = 1.76 ms

    The software overhead related to measurement multiplexing is the largest factor in measurement execution time for measurements with short aperture times. As aperture time increases, software overhead will have a progressively lower impact as measurement execution time comes to be dictated by the aperture time itself. Additionally, longer aperture times have a larger impact on measurement time if the measurements are taken on channels banked together and routed to the same ADC. Channels banked together share an ADC and need to be multiplexed, while channels routed to different banks are able to use unique ADCs and run in parallel. The following graphs demonstrate these relationships and show the improvement in execution time that running measurements in parallel on separate banks can provide.

    Figure 14. PPMU Current Measurement (2 μA), Long Aperture Time


    Figure 15. PPMU Current Measurement (2 μA), Short Aperture Time


    Figure 16. PPMU Voltage Measurement, Long Aperture Time


    Figure 17. PPMU Voltage Measurement, Short Aperture Time