RMX-4122 With Inconsistent Rising Times

Issue Details

I have multiple RMX-4122 modules and would like to have the same time response to use the product for parametric current and voltage measurement for my device under test. But I noticed that the output voltages are inconsistent regarding ramp-up times, settle time, and voltage values.

RMX-4122 Number 1:
 

Time, Voltage

0.01,0.02
0.21,7.75
0.41,15.74
0.61,23.74
0.81,31.73
1.01,39.7
1.21,47.7
1.41,55.69
1.61,63.69
1.81,71.61
2.01,79.59
2.21,87.59
2.41,95.6
2.61,109.35
2.81,123.35
3.01,137.34
3.21,151.35
3.41,169.01
3.61,194.08
3.81,219.67
4.863,219.678

RMX-4122 Number 2:

Time, Voltage

0.01,0
0.21,7.81
0.41,15.81
0.61,23.81
0.81,31.77
1.01,39.75
1.21,47.7
1.41,55.7
1.61,63.65
1.81,71.65
2.01,79.66
2.21,87.64
2.41,95.63
2.61,102.72
2.81,109.01
3.01,114.24
3.21,119.08
3.41,147.25

Why is this?

Solution

This is known behavior and is due to the software time timing added by LabVIEW, rising time, crossover point, and how the calls at a low level of the driver add delays, this cannot be corrected. 

But you can increase the level of determinism by understanding deeper the causes of this behavior and how the rise time specification works in the settling time of the system. With this be able to tune the system requirements and have predictive behavior.
 

There are multiple causes for this behavior:

 

1. Rise Time Specifications.
2. Code Scheme Timing and Synchronization.
3. Hardware Output Load limits.

 

Rise Time Specifications:

The rise time is defined as the time period measured between 10% to 90% of the rated output.

For example, the Rise Time for the RMX-4122 is 100 ms, thus this is the minimum time to get the voltage level to the desired state.

 

Code Scheme Timing and Synchronization:

The RMX family of products (RMX-412X) does have the feature that all the communication between the hardware and the application level is serial communication, to be specific uses NI-VISA.

This adds delay to the acquisition of the voltage and the setting of the expected value. Depending on the code development we can add more or less determinism.

As a general recommendation in LabVIEW, the level of determinism is limited but if you considered that the configuration and setting of the voltage are higher than the Rise Time plus the delay in the serial communication we can achieve the desired voltage level in the time loop configuration.
 

Hardware Output Load Limits:

The RMX-412X modules have a load curve that states the minimum current requirements to get a specific voltage. This is known as the crossover point, the supplies would potentially swap from CV to CC if the voltage or current is not sufficient. As an example we can explore the manual for the RMX-4122:


In detail, we have a relationship between the voltage levels and the minimum current required to satisfy the voltage. In case the current is insufficient we will have an inconsistent value of voltage.

Additional Information

For applications in which the precision is on the lower end of voltages values (maximum 10 V), an SMU is recommended, similar in case the voltage values are lower and the determinism is required.