"“current limit” " means "compliance" or "range"?

[4135SMU]

Hi, All,

  I am using the example vi from NI DCPower . it is called “ Hardware-timed voltage sweep”.  Basically it used one SMU to sweep a voltage range and monitor the current at the same time. On the front panel, there is an input parameter called “current limit” which confused me. In my past experience using Keithely systems to do a voltage sweep and measure the current, there are two parameters: current “Compliance” and current “Range”.  Compliance means the maximum current you would allow to flow through your device during the test in order to protect it.  Also your device may only see pA during the test, but you can set the compliance as 10A, which should not affect the measurement at all.  The other parameter “Range” is to tell the instrument what maximum current the system would expect to see. “Range” and “Reading resolution” are close related:  a bigger “range” value corresponds to a larger reading resolution. For example, if the current range is set at 1mA, then the reading resolution can be 1uA; if the range set as1uA, the reading resolution can be 1pA.  Now back to this specific vi, which parameter does the “current limit” correspond to, “Compliance” or “Range”? I also opened up the block diagram, there is a property node with “current limitrange auto” as input .  So the “current limit” at the front panel should mean “Compliance”?

Regards,

ted

[santo_13]

Hi,

In NI SMUs, the current limit is the compliance limit as you've mentioned for Keithley systems. One condition is that current limit <= current range.

Example, current limit (compliance limits) = 1mA and current range = 10mA is accepted.

In the block diagram that you have attached, Current Limit AutoRange was configured, in this case, NI DC Power driver would take care of assigning closest larger or equal current range based on current limit configured.

If Current limit Autorange is disabled, you can configure the Current Range to any higher value possible. Example, Curren Limit = 10uA, Curren Range =10uA or 100uA or 1mA or 10mA or higher.

Even if you set any value of Current Range, the driver will auto coerce to the nearest Current range supported by the instrument. Example, setting current range to 2mA, the driver will choose 10mA since 2mA is not a supported range by the instrument.

[4135SMU]

Hi,

  thanks for the reply. the motivation behind my question is how to measure a device current-voltage  over a big range using this specific vi. For example, I have a diode whose IV response is not linear. at 1V, its current is 1pA; at 10V, the current is 1mA. then to do a IV scan from 0-10V, how should I set up the "current limit"? I can not set it as 1mA or bigger, because at 1mA range, the NI SMU's resolution is about 1nA or worse, it could not read 1pA at 1V accurately.  I wonder if NI has thought about it and developed a better vi for this kind of application.

regards,

ted

[santo_13]

Hi Ted,

I am afraid that may not be possible due to the architecture of the NI SMUs. The current limit range and current limit are both linked. Sourcing range and measuring range are always the same since both use the same Current ADC and when you send a measure, it will return the continuously available data from the ADCs.

An alternative is to use a different current range for different voltage ranges, example, 0-1V at 100uA range, 1-10V at 10mA range.

You can modify the program to achieve that. If you would be okay I can also help you with that customization.

What is the exact module you are working with?

Regards,

Santhosh

[4135SMU]

Santhosh,

 thanks for the reply and offering to help. I have a few things need to clear before I will seek your help.

1st,

"The current limit range and current limit are both linked", did  " the current limit range" means "current (measurement) range"? and the "current limit" means "compliance"?

 2nd,

you said "Sourcing range and measuring range are always the same since both use the same Current ADC", I am sourcing voltage and measuring currents here, I wonder if these two values share the same Current ADC because one is voltage , the other is current. I did not know the physical of the SMUs, and I may be wrong. 

regards,

ted  

[4135SMU]

we have three NI PIXe 4135 SMUs at hand, we are trying to do diode and transistor I-V measurements. 

[santo_13]

Ted,

1. Yes, your understanding is correct. I believe the current clamp (current limit or compliance) is implemented by continuously monitoring the ADC data from the current ADC which uses the current limit range (in other terms - measurement range). Hence, current limit <= current limit range.

2. Sorry for the confusion, what I meant was that NI SMUs have dedicated ADCs for Voltage and Current measurement. In source voltage mode, the current ADC is used to implement the current limit and current measurements. In source current mode, the voltage ADC is used to implement the voltage clamp and the voltage measurements. In any mode, you can get measurements from both Current ADC and Voltage ADC since they are dedicated.

regards,

Santhosh

[4135SMU]

Santhosh,

  thanks for the explanation. I think NI did a bad job to correlate the "current limit (measurement) range" to the "compliance". the "current measurement range" should only be related to the real current value, the machine should automatically set the best range for the real current value because the user could never predict at which voltage the current is going to be. if we can predict, we don't need to do the measurement. the only thing user would/should know is at what current value the device would burn, in other words, the "compliance". 

  I may need your help to build a practical vi for real word IV measurements. actually I suspect other users may already realize NI's problem and had developed some Vis.   

regards,

ted

[santo_13]

Ted,

I have modified Advanced Sequence mode example to support your requirement. Please test it and let me know if it works out.

Regards,

Santhosh

[4135SMU]

Santhosh,

  thanks for the program. It should work for this specific device . but one more important question: for general case, if I don't know how my device behave in advance, and want to find out,what program should I use?

  the purpose of electrical measurement is  to obtain the device performance. in many times, we saw device behaves abnormally. for example there may be a current peak(current increase then drops). in your program (thanks again), we are assuming the device current is monotonically increasing and  pre-set the measurement bias into several section, and assign different appropriate current measurement range into each section. however, in 99% of time, we don't know how the device is going to behave in advance (otherwise, we don't need electrical measurement), so we should not be able to know how to seg the bias range and which current measurement range  to assign, I wonder if NI has thought about it.

regards,

Ted