How to configure the Model 2450/2460 for Delta Mode Measurements using Model 2182A nanovoltmeter?
A large source of error when making low resistance measurements (e.g. 1 ohm) is thermal EMFs in the circuit. A technique called Delta Mode reduces the thermal EMFs in the circuit as described in the Keithley Low Level Measurements Handbook. The delta mode method sources current in one polarity and takes a voltage measurement. Then the current polarity is reversed and another voltage measurement is taken. Then the two voltages are basically added together and then divided by two. The result is a normalized measurement without the thermal EMFs.
Instructions for Setting up the Model 2450/2460 SourceMeter with the Model 2182A Using Delta Mode
The Model 2450/2460 SourceMeter and the Model 2182A Nanovoltmeter can be triggered together to perform “delta” measurements. In this configuration, a two-point list sweep (one positive current and one negative current) is setup on the Model 2450/2460 and the Model 2182A uses math to perform a current-reversal technique to compensate for thermoelectric voltages. The Model 2182A will display the “delta” voltage by averaging two voltage readings resulting from the two-point current list sweep output by the Model 2450/2460. Here is the Delta Voltage equation:
Delta Voltage = (Vi+ - Vi-) / 2
The following example will be measuring a 10 mΩ resistor using 20 µA test current. These instructions will work for any Graphical 2400 Series SMU: Models 2450, 2460, 2461, and 2470.
Setting up the Model 2182A:
- Reset the Model 2182A to the factory default setting. Press RESTR and select FACT.
- Enabled desired number of PLCs with RATE button. Use 1 (medium) or 5 (slow) PLC for best results.
- Select desired digital filter size using SHIFT key then FILTER. Default is 10.
- Select DELTA mode by pressing SHIFT V1/V2 button. NOTE: In delta mode, the units are displayed as “Vd”.
- Enable External Triggers by pressing the EXT TRIG button. Dashes should show up on the display indicating that it is waiting for an external trigger.
Setting up the Model 2450/2460:
Set up Measure and Source:
- Reset unit back to factory default conditions: Navigate MENU ->Manage -> System Reset. Press the HOME key.
- Press FUNCTION key and select Source Current and Measure Voltage
- Navigate to MENU -> Measure Settings: Change NPLCs to 0.01
- From HOME screen, change current source value to +20 µA and press OK.
- Save this value in the Source List. Navigate to MENU -> Source: Config List.
- Push the “New List” button to save the first source configuration point (20 µA).
- CLEAR the default name and use the name of the Source Config List to “delta” and press ENTER.
- Push the “System to List” button to save the first source configuration point (20 µA) to the list.
- From HOME screen, change current source value to -20 µA and press OK.
- Navigate to MENU -> Source: Config List.
- Push the “System to List” button to save the second source configuration point (-20 µA) to the list.
- Return to HOME screen.
Create Trigger Flow Model:
- Navigate to MENU -> Trigger Templates
- Push the Templates button and select “External Trigger”. The Input Line should be 1 and the Output Line should be 2.
- Navigate to MENU -> Trigger Configure. The External Trigger model will already be loaded into the TriggerFlow diagram. You will need to add a few blocks to load the source configuration list and to measure continuously.
- Press Block 1, Buffer Clear, and press Insert After. Select Action block and Config List Recall. Select Source List. Chose the Source List created earlier called “delta”. This will load the Source List into the trigger model.
- Press Block 2, Config Recall, and press Insert After. Add an Action block and select Source Output. This will turn on the output of the Model 2450/2460.
- Press Block 6, Measure, and press Insert After. Add an Action block to Config List Next. This will output the second value in the Source List (-20 µA). Press Source List and then “delta”.
- To make it repeat infinitely; delete Block 9, Loop Counter. From Block 8, press Insert After and press Branch. Click on Always, and branch back to Block 4 (Wait Block).
Save Configuration to a Script:
Before the start of test, save the configuration as a script so it can later be recalled and executed.
- Navigate to MENU -> Scripts: Create Config
- Press Create and name the new script, such as “deltascript”. Press ENTER
Begin Delta Measurements:
- Press TRIGGER key on Model 2450. (It is important to trigger the Model 2450/2460 first.)
- Press TRIG button on Model 2182A.
- The Model 2182A will display the average voltage.
- To check the stability of the voltage measurements on the 2182A, take the standard deviation of a 100 readings as follows: Press STORE and select 100 RDGS. Press ENTER. When done storing (asterisk disappears), press RECALL. Select STD DEB. Use right arrow key to view the standard deviation of the readings.
To Stop Measurements:
Press the RUN button at the top and select Abort Trigger Model and turn off the output.
To recall saved script, just go to No Script button at top of page and recall “deltascript”. Press the TRIGGER button on the Model 2450/2460 and the TRIG key on the Model 2182A.
Here is a photo of the set-up measuring the 10 mΩ resistor. Notice the Model 2450 will display the sourced current. Because it is setup for 0.01 NPLC the voltage reading will not be accurate. The Model 2182A will display the “delta” voltage. In this case, 199 nV (10 mΩ * 20 µA = 200 nV).
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