Measuring sub 125 microamps using PXI 4110 - Is it possible?

[TesterABC]

I have a question concerning the PXI 4110.  We are trying to use this card to both provide voltage to a device while at the same time using it to monitor the real time current being supplied.  What we have found that it is capable of providing voltage from 0 to 6 volts, but have not been able to read below ~500 micro-amps.  The data sheet says it has a resolution of 0.01 ma or 10 micro amps, but we have not been able to get the card to read consistently under the 500 micro-amps.

Is there a minimum current draw required before the 10 micro-amp range becomes true? 

Here is what we are trying to monitor using this card:

1. Voltage supplied between 3.5 to 6 volts.
2. When device is asleep, current draw is between 50 to 100 micro amps.
3. When device is awake, current draw can be as high as 100 mili-amps.

Will this card work?  Is there a better solution to this? 

Thanks

[Steve_B]

Hi TesterABC,

Thanks for posting. The PXI-4110 is a power supply module that is able to generate a voltage and measure both voltage and current with software timing for each. The Current Measure Resolution spec is the smallest change in the current measurement that can be detected by hardware. Resolution is typically limited by the ADC used for the measurement, but the measurement may also be limited by other factors, such as noise. It sounds like you are experiencing an offset and/or gain error in the readings which are specified in the accuracy columns to the right in our Specifications. If you were to read a 910uA signal we would be able to measure if the signal changed to 920uA. Are you using the 1A Range and if you are using channel 1 can you change to the 20mA Range for the sleep state of your DUT? 

Regarding your application, it would be interesting to know how fast you need to measure the signal and how many channels do you need at a time? Are you able to change ranges on the power supply when your DUT changes modes?

Based on the information you provided, if the PXI-4110 is not sufficient, I would recommend that you look at our newest SMUs with Source Adapt, the PXIe-4140 and PXIe-4141 or our Battery Simulator, the PXIe-4154 which is designed for mobile device test. 

The PXIe-4140/4141 would provide 4 channels and the PXIe-4154 would allow the test current to expand up to 3A. These modules have hardware timed generation and measurements. The measurement accuracy will be better than you see on the PXI-4110 for a comparable range. Let us know some more of your requirements and we can help you fit the PXI-4110 or another solution that is a better fit.

-I meant to include links for you:

PXIe-4140

PXIe-4141

PXIe-4154

[TesterABC]

Hi Steve,

Thanks for the response.  I am using the 1A range, but have tried the 20 ma range.  That works better while the product is in sleep mode.  The problem is that I cannot switch between these states due to my application that I am trying to test and monitor.  I have created a seperate thread that its only job will be to sample current at a defined rate (say every 5 sec.).  I am testing Firmware within the product, and want to have a large sample of the current draw by the product over a several day test sequence.  This baseline will be used to compare against new Firmware in the future to determine if the new Firmware is at least as good if not better with current draw by the product.  The product is running off of batteries, so current consumption is a critical parameter that we have to monitor while testing the product.  I find that the teststand allows for threading really easy, and makes this type of measurement running continuously in the background work well.

Due to my range of current when awake vs. sleep, I need some device that allows for an auto range in order to be able to handle both modes of operation.  I know that I could use an external DMM in auto range mode to do this, but wanted to see if NI had some type of card, particularly capable of providing power as well as monitoring the current before using an external DMM.  We are in the trial stage of the NI hardware, and want to make sure that I pick the correct card to be able to do what we are trying to do before purchasing our complete hardware system.

Thanks

John

[TesterABC]

Steve,

Wouldn't the 4130 Power Supply be capable at the 200 mA range for what I am trying to source vs. measure? 

Thanks

[Steve_B]

Hi John,

Thanks for the additional information. Based on your sampling rate you should be fine with a software or hardware timed device. The answer that you will need to determine is how accurately you need to measure your sleep current. The issue is that you do not know the mode that your DUT will be in to change your current range. At this time our power supplies have output and measure ranges that are coupled together. This means that when you change to a lower range your output current will be limited to the max of that range, which can be an issue when the DUT 'wakes up'.

I would recommend is to select a device that can remain in the higher current range (100mA or greater) and have the measurement capability that meets your needs. You would need to know that you would need to measure the sleep current within +/- X uA. When selecting the device you will then want to look at its measure accuracy on the range you will use. The calculation would be +/- (Measurement*Gain Error + Offset Error). You will find that the largest portion of the error will be offset due to the range you will be using. You also need to take into account the resolution of the instrument because that is the smallest possible change you can measure

One comment on offset error is that for a given test setup/temperature/humidity/etc and test voltage it will stay fairly constant. This means that you can characterize the offset of your system if all of those factors remain constant. I would recommend that you would set up your test, including all fixturing/cabling excluding the DUT. You can set the supply to your test voltage and measure the current. In this setup the ideal current would be 0uA because it is an 'open', but due to leakages in the system there will be an offset. You can take this reading as a baseline 'zero' and subtract it from future readings to improve your measurements. You will want to be careful of Dielectric Absorption (DA) because it can mislead you when making measurements like this, but it is less of an issue when talking about uA and more of an issue when measuring pA. It would be a good idea to repeat this characterization periodically to ensure that your measurements are accurate, ideally once per DUT, but you can scale that back as necessary.

I hope this is helpful. It is a good idea to evaluate the hardware in your test setup to ensure that the measurements meet your needs. I would also add the PXI-4132 to your list of options to consider for its 100mA range. I think that these other devices would be better than the PXI-4110 in your application because of the low current measurements you need. If you can use the additional channels the PXIe-4140/4141 are good options, if not the PXI-4132 would be a good option. You should also consider the different connectors for PXI vs PXIe and what will work for your chassis.