Test Connections
 

Characterizing the DC parameters of digital and mixed signal devices has typically been a task best done on large, proprietary ATE. Geotest's GX5295 PXI digital instrument, with its per pin parametric measurement unit (PMU), now makes it possible to perform this task quickly and cost effectively using Commercial Off-The-Shelf (COTS) hardware. The GX5295 is a 3U PXI card which features 32 digital I/O channels with a PMU per channel – making it possible to perform common DC tests (force current / measure voltage & force voltage / measure current) without requiring external switching, a dedicated source measure unit (SMU) and associated cables and connectors. The result is a test configuration that offers superior signal integrity, reliability and test time over test setups that rely upon external switching and a dedicated SMU. The following sections detail how you can use the GX5295 to perform a variety tests which include tester / DUT (device under test) connectivity verification and input / output DC characterization.

Continuity Test: The continuity test checks that the DUT's pins are connected to the test resources. It does this by forcing a small current through all pins, and measuring for the presence or absence of a voltage at each pin. The ESD protection diodes on the input of an IC will develop a voltage across the diode when a small current flows through it. If the ESD diode is missing or open, or in the case of a continuity test, if the tester is not connected to the pin, current will not flow, and the measured voltage will be equal to the commutating voltage of the current source (higher than the junction voltage of the diode).

The ATEasy code below details how the GX5295 can be programmed to force a small current on 18 pins (excludes Vcc and ground) of a 20 pin device, measure the voltage drop across the ESD diode and subsequently display the test results.


Dio Setup Channels Mode PmuForceCurrentMode RangeOfChannels(nMaster, 0, 17)
Dio Setup Channels PMU ForceCurrent RangeOfChannels(nMaster, 0,17, -0.5,
   aPmuCurrentRange_n2ma_to_p2ma)

For nPin=0 to 17
   Dio Measure PmuVoltage(nMaster, nPin, dVolts, 100,,)
   dContinuity[nPin]=dVolts
Next



ContinuityTest


Shorts Test: The shorts test looks for pin to pin shorts, pin to VCC, and pin to ground shorts. By forcing a small sink current on all pins, then sequentially forcing a small source current on each DUT pin , it can be determined if any of the DUT pins are shorted. If the voltage on the DUT pin being tested is positive, the pin is not shorted. If the voltage is zero, it's shorted to ground.  If the voltage is a relatively large negative value, there is a short to another pin. And if the voltage is a relatively small negative value, there is a short to Vcc. The relative "large" and “small” voltage values are determined through empirical measurement under test conditions as part of the test development process. Checking for the presence of shorts is fast requiring only a single measurement on each DUT pin to confirm a positive voltage.  

Examples of various shorts are shown in the GtDio Measurement Panel for a GX5295. In this example, Channel "0" is forcing a 1mA source current and all other pins are forcing a 1mA sink current. The panels in figures 1a – 1d show the resulting voltage on channel "0" for different shorted conditions:

Positive Voltage on Channel "0"
0V on channel 0
Figure 1a: Positive voltage on channel "0"
indicates no short. All other channels show a
negative voltage.

Figure 1b: 0V on channel "0" indicates a
short to ground

"Large" negative voltage on channel "0"
"Small" negative voltage on channel "0"
Figure 1c: "Large" negative voltage on
channel "0" indicates a short to another IC pin.
Further testing of the other pins will indicate
which pin(s) are shorted.
Figure 1d: "Small" negative voltage on
channel "0" indicates a short to Vcc

I-V Curve Plot: It is not enough to know that the tester is connected to an IC by measuring the voltage developed across the ESD diodes when forcing a small current. Once the DUT / tester connection has been verified, it is also necessary to characterize the device's input diode current – voltage (I-V) characteristics. An  I-V curve shows the relationship between current and voltage across a diode and is used to determine the diode's P-N junction characteristics. The ATEasy code below shows how you can use the GX5295's PMUs to measure and plot an I-V curve for two pins of an IC (For simplicity, the code has been limited to measuring only two pins). The resulting I-V graph is shown in Figure 2 below.


For nPin=0 to 1
!  Set channel mode to Force Current

   Dio Setup Channels Mode PmuForceCurrentMode RangeOfChannels(nMaster, nPin,
      nPin)

   For nStep =-100 to 20 ! Force current from -100uA to 20uA
      dCurrent=nStep*.000001 ! Scale to microamps
      Dio Setup Channels PMU ForceCurrent RangeOfChannels (nMaster, nPin, nPin,
         dCurrent, aPmuCurrentRange_n2ua_to_p2ua)
      Dio Measure PmuCurrent(nMaster, nPin, dCurrent, 100,,)
      dIVCurve[nPin, 0, nStep+100]=dCurrent
      Dio Measure PmuVoltage(nMaster, nPin, dVolts, 100,,)
      dIVCurve[nPin, 1, nStep+100]=dVolts
   Next
   Print
! Return channel to Dynamic mode
   Dio Setup Channels Mode DynamicIo RangeOfChannels(nMaster, nPin, nPin)
Next

Load frmIVCurve,False ! Load the Chart Form

!  Plot I-V measurements
frmIVCurve.chtIVCurve.SetData(0, dIVCurve[0,0], dIVCurve[0,1],,,True)
frmIVCurve.chtIVCurve.SetData(1, dIVCurve[1,0], dIVCurve[0,1],,,True)



Normal I-V curve, two pins
Figure 2: Normal I-V curve for two pins of an IC


Vil and Vih Input Threshold Tests: Measuring the input thresholds of a device is one of the parameters measured as part of a device's characterization. To characterize input threshold, a ramping voltage is applied to the input of the device, while monitoring the output. At the point where the output changes state, you have crossed the high or low input threshold, depending on the direction of the ramp. The concept is shown in Figure 3 below.

Measuring input High/Low thresholds
Figure 3: Measuring Input High and input Low thresholds


Since each channel of the GX5295 can force a voltage simultaneously, it is possible to ramp all inputs on a device under test (DUT) at the same time and then measure the corresponding output(s). When an output transitions from one state to another, the voltage level is recorded on the respective input. This process is repeated until all outputs have transitioned from low-to-high (Vih measurement) and high-to-low (Vil measurement). Figure 4 shows the hysteresis of input thresholds measured on an octal latch IC.

Input Thresholds
Figure 4: Vih and Vil Input Thresholds

Characterization of a device's output pins includes measuring its logic low (Vol) and logic high (Voh) voltages as well as measuring the device's output vs. load characteristics.

Vol and Voh Output Level Test: To measure the output levels of a device, it is necessary to set the device's inputs to correct state in order to generate the desired output on the device. The process may require a single step, like asserting a "Clear" signal to force all outputs to a low state, or it may require several steps, for example setting an input state and then clocking the device in order to generate a high output state for a latch or flip-flop.  Once the outputs are in the desired state, the voltage on each output pin can be measured. Often this is done with a nominal load applied to the output signal.

The GX5295's PMU ability to force or sink current and measure voltage is ideal for measuring output levels. Figure 5 shows the measured output levels for an octal latch.

Output Thresholds
Figure 5: Voh and Vol Output Levels


Output Loading: The Output Loading test verifies that the output levels will meet the device's specification across a defined load range. This test is very similar to the Output Level test, except that the output loading is a sweeping current load which is applied to each output and the output voltage for each load is measured. Typically, the output levels will decrease with an increasing load, but the output voltage should always remain above the minimum specified level. Shown below is the ATEasy code used to apply a load to the output of the octal latch. In this example, the load is ramped from 0mA to 8mA in 100uA steps. The results are then plotted and displayed on a chart (Figure 6), showing output voltage levels vs. loading for all eight outputs.


For i=0 to 79  !  Load in mA
Dio Setup Channels PMU ForceCurrent ArrayOfChannels(nMaster, nCountInput, nInput,
      (i+1)*-0.1, aPmuCurrentRange_n8ma_to_p8ma)  !  Scale the load current to mA range
   Dio Measure PmuVoltage(nMaster, stDutIO[nQxPin[0]].DioChannel,
      dQxMeasure[0,0,i], nAperature)

   For nPin=0 to 7
      Dio Measure PmuVoltage(nMaster, stDutIO[nQxPin[nPin]].DioChannel,
         dQxMeasure[nPin,0,i], nAperature)
      Dio Measure PmuCurrent(nMaster, stDutIO[nQxPin[nPin]].DioChannel,
         dQxMeasure[nPin,1,i], nAperature)
      dQxMeasure[nPin, 1, i]=dQxMeasure[nPin, 1, i]*-10
   Next
Next

!  Plot the load voltage for each of eight output channels
For nPin=0 to 7
   objChart=Controls("chtLoading"+Str(nPin))
   objChart.SetData(0, dQxMeasure[nPin,0], dQxMeasure[nPin,1],,,True)
Next



Voltage vs Current for Octal Latch
Figure 6: Voltage vs Current Output Loading for an Octal Latch


The GX5295 and the TS-900 PXI Semiconductor Test System which includes the GX5295, offer cost effective solutions for digital and mixed signal test applications. Please contact us to learn more about how you can use Geotest products for addressing your component test needs.
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