Introduction: Improved Semiconductor Curve Tracer With the Analog Discovery 2

The principal of curve tracing with the AD2 is described in following links below:

If the measured current is quite high then is the accuracy acceptable. However, lower current measurement, lack on:

  • Offset error and common mode limitation of the scope channel amplifiers
  • Slope error due to parallel resistors

These errors can‘t be eliminated with AD2 device calibration.

Step 1: The Circuit Diagram With AD2 Internal Load Resistors

There is connected the waveform generator (W1), the scope channel 1 sense the voltage drop on current sense resistor (CSRes) and the channel 2 sense the voltage on the device under test (DUT).

Step 2: Equivalent Circuit Diagram

The AD2 scope input pins have 1MOhm pull-down resistors on every input pin which influence the current measurement. Two of these resistors are in parallel of the DUT.

Step 3: Impact of the Error

At the above graphs was the DUT disconnected. Current sense resistor is 330Ohm

  • Left: Vertical scale of +10mA/-10mA looks correct
  • Top right: Vertical scale shows an error with increased resolution of +100uA/-100uA (parallel resistor of 500kOhm to DUT and limited common mode rejection (CMRR) of scope channel 1 and the offset is almost zero)
  • Bottom Right: Vertical scale is equal to the picture on the top. But here was shorted the current sense resistor. the graph shows CMRR error only ( 5V/500kOhm=10uA, 26uA-17uA=9uA it is near to 10uA)

Step 4: Error Compensation Via Linear Equation

A short Script can do this automatically.

How does it work:

In order to calculate the equation are four parameters necessary:

Min/Max of ch1 (current) and also of ch2 (voltage)

Because the voltage at ch1 is very low, that is why Math2 filter ch1.

Finally, the calculated equation will be written to Math1.

The script on the right will be executed by pressing the run button of the script window, without a connected DUT. Displayed will be Ch1 not Math2, because filtering produces some delay and that generate double lines.

Step 5: The Script

This is the whole script which eliminates the errors. A description of the key commands is available in the help of Waveforms Application software.

Step 6: Math Setups

Math2 filter Ch1, this is necessary to calculate the Min/Max parameter accurate. Math1 shows the calculated equation.

Step 7: DUT Example: LED

The left graph shows the behavior with compensation and the right one as usual . There is a significant difference visible in higher current resolution .

Step 8: Conclusion

This example shows the powerful capability of the AD2 scripting
language. Easy to use, well documented AD2 commands and excellent to debug.

There is the AD2 workspace file for download available.

Caution change the extension of the file to .zip and unzip the file before using with AD2. Upload of .zip extension is not supported by instructables.

Another project is available at trenz electronic: LCR-Meter (Excel VBA)