Measuring 'small' currents - Page 1
Forum section: electronics / testgear

author: AndyP
author_rank: Contributor
timestamp: July 07, 2017, 09:48:58 pm
content: All,I wanted some advice on measuring current, either using a 'smart' power supply or a current probe (ideally with an RTB2000 'scope, this would make synchronsing current with the voltage behaviour easier).I'm working on a BLE bike wheel rotation meter using a low power BLE device, my own software and a magentometer chip. In order to estimate the battery life, I'd like to measure the dynamic power consumption of the device, and see what impact my software is having on power, since it's BLE it can drop down to a few micro amp and upto severals tens of milliamps when all the components are active.I was thinking of getting access to a Keysight N6705, but I wanted to know if there are other alternatives to monitoring the current in a similar price range or lower. I'm assuming current measured at 200kHz or faster would be useful, possibly as high as 1M.Do R&S / Tektronix or other manufacturers do suitable power supplies or current probes?Any other ideas I've missed?

author: trevwhite
author_rank: Frequent Contributor
timestamp: July 07, 2017, 10:08:03 pm
content: Some DMMs have a current integration measurement. Basically you set the measurement time and the meter will measure average current over that time duration.I think a few meters do it but I only have experience with the Keithley 2110 and that has been brilliant for me with measuring average currents for low power devices with dynamic current loads.

author: AndyP
author_rank: Contributor
timestamp: July 07, 2017, 10:40:09 pm
content: I was after understanding the dynamic behaviour in order to better identify which areas to focus on power saving.Hence the interest in dynamic current behaviour.The compare section here:http://uk.tek.com/keithley-source-measure-units/keithley-smu-2400-series-sourcemeterThe faster ones seem to go to 50us (20000 samples per second)

author: alm
author_rank: Super Contributor
timestamp: July 07, 2017, 10:40:35 pm
content: Define low current? Current probes with a Hall sensor are generally not particularly good below 1 mA or so.The Agilent/Keithley mobile communication sources / measurement power supplies (e.g. Keithley 2302/3/4/6, HP/Agilent 66309/11/19/21) can to fairly high speed current measurements. They can not sample quick enough to capture 200 kHz signals, but they can integrate the current over a longer time span. The HP/Agilent supplies have a 20 kHz bandwidth and can sample faster than the Keithley units (something like 50 kS/s?). Dave did a recent teardown of one of the Keithley units:

author: AndyP
author_rank: Contributor
timestamp: July 07, 2017, 10:49:15 pm
content: Quote from: AndyP on July 07, 2017, 09:48:58 pm50mA down to 5uAA high resolution current probe e.g. Keysight N2821a may do the trick, but I don't have long term access to one of these.

author: alm
author_rank: Super Contributor
timestamp: July 07, 2017, 10:57:23 pm
content: The HP/Agilent communication power supplies will read down to 0.6 µA resolution in 20 mA range.

author: Someone
author_rank: Super Contributor
timestamp: July 08, 2017, 01:00:44 am
content: Quote from: AndyP on July 07, 2017, 10:40:09 pmKeithley (Tek) and Keysight are both blurring the lines of power supply and SMU, hunting through their dozens of models you'll find more flexibility costs more money and the "locked down" designs are cheaper. Is there some particular detail you need to resolve at 200kHz? otherwise integrating/filtering the result will get the total consumption figures accurately and cheaply.

author: AndyP
author_rank: Contributor
timestamp: July 08, 2017, 06:23:10 am
content: I'm trying to determine on a live system which hardware and software interactions are preventing the ble chip entering power save mode.Understanding how using cpu time and accessing the external peripherals impacts power would help me reschedule work from highly loaded sections of time to less loaded areas. Possibly changing the trigger source for some activities.

author: alm
author_rank: Super Contributor
timestamp: July 08, 2017, 02:41:18 pm
content: Not sure what BLE part you are using, but you may be able to get information about time spent in power states through the live trace features (e.g. SWV) of the part.

author: testpoint
author_rank: Regular Contributor
timestamp: July 09, 2017, 01:49:25 pm
content: you can use the Motech PPS1201GSM-A to test the consumption, the minimum current is 1uA, maximum is 3A, voltage is 0-8V, the used one in China price is 70 USD, but the shipping cost may high since its weight about 7kg.

author: Rich@RohdeScopesUSA
author_rank: Frequent Contributor
timestamp: July 09, 2017, 05:43:30 pm
content: Quote from: AndyP on July 07, 2017, 09:48:58 pmHi AndyP - we have a couple options:A high-sensitivity (1A/V) current probe that will work with the RTB2000 (uses a separate power supply). It is called the RT-ZC30. This is hall effect sensor based.https://www.rohde-schwarz.com/us/product/rtzc30-productstartpage_63493-158913.htmlWe also have a new probe with an integrated 18-bit ADC designed exactly for low current and voltage measurements down to potentially even nAs (uses a shunt resistor). However, it only works with our RTE and RTO oscilloscopes (uses the MSO slot for the interface to the probe). You can get up to 16ch with it (ideal for sequencing).https://cdn.rohde-schwarz.com/pws/dl_downloads/dl_common_library/dl_brochures_and_datasheets/pdf_1/RT-ZVCxx_dat-sw_en_5214-7058-22_v0300.pdfHope this helps and good luck with your project.-Rich