uCurrent Gold ( & Tiny Current clone) powering and accuracy issues  - Page 1
Forum section: electronics / beginners

author: slow_rider
author_rank: Regular Contributor
timestamp: September 12, 2021, 11:45:19 am
content: I have an original uCurrent Gold and two Tiny Current (which are a clone).When powering them from the CR2030 3V lithium cell, the accuracy I get is very good and all seems to be working as should.When I try to power them from a 3.3V lifepo4 battery I get errors, around 25%.When I power the units using a circuit built around the LT3045https://www.analog.com/en/products/lt3045.htmlI get even worse error of 50% or more. The voltage out is about 5V.As far as the documentation goes for the Tiny Current, it can be driven by up to 5.5V.The units were tested individually with different power sources and different current fed into them (using a Keithley Source-Meter).What could cause this strange behaviour?

author: SilverSolder
author_rank: Super Contributor
timestamp: September 12, 2021, 10:37:54 pm
content: Quote from: slow_rider on September 12, 2021, 11:45:19 amAll the units give exactly the same error on DC supply, but not when powered by batteries?

author: slow_rider
author_rank: Regular Contributor
timestamp: September 13, 2021, 08:26:27 am
content: I've hooked up a Keithley 2401 as a current source directly to the uCurrent Gold.I've powered it with a 3V lithium battery (CR2032) and later with a Siglent SPD1168X lab. power supply.Shown below are 6 points that I've measured.Input 1mAOutput with Battery: 0.96mV (4% err)Output with PSU: -2.37mV (~330% err)Input 5.004mAOutput with Battery: 4.93mV (1.4% err)Output with PSU: 1.55mV (~69% err)Input 9.9994mAOutput with Battery: 9.93mV (~0.7% err)Output with PSU: 6.57mV (~34% err)Input 0.999uAOutput with Battery: 0.93mV (7% err)Output with PSU: -2.52mV (~350% err)Input 5.003uAOutput with Battery: 4.93mV (~1.46% err)Output with PSU: 1.51mV (~70% err)Input 10.002uAOutput with Battery: 9.92mV (~0.8% err)Output with PSU: 6.53mV (~34% err)

author: Kleinstein
author_rank: Super Contributor
timestamp: September 13, 2021, 08:39:47 am
content: The error seems to be an offset of some -3.4 mV for the ouput, which is a lot.A known issue with some versions was the tendency of the virtual ground circuit to oscillate with too much capacitive load. Using an external supply can add a little bit extra parasitic capacitance. The meter used to read the result can make a big difference in this case.

author: slow_rider
author_rank: Regular Contributor
timestamp: September 13, 2021, 08:47:34 am
content: Quote from: Kleinstein on September 13, 2021, 08:39:47 amI am using a BM235 (Dave's own) DMM set on the mV range to read the output on all measurements. I would think the load is the same? My unit is # 1969 which is an early production and has the choppers with the ABAA markings. I know there was an issue with later models that had a different part. I am wondering how come the power supply I am using comes into play like this? I also tried a 3.3V battery and I've seen large errors.I need to use these with a PSU somehow...

author: Kleinstein
author_rank: Super Contributor
timestamp: September 13, 2021, 09:02:45 am
content: I faintly remember that some of the EEV branded meter have quite high input capacitance and cause the ocillation problem for Dave.The circuit kind of misses a simple series resistor for the meter output. There is a series resisor at the OP , but his was in the loop, so not helping, but making it even more suceptible to capacitive load.The other point was the OP used for the virtual ground - there are minute differences between brands / versions of 5 V RR CMOS OPs.AFAIR Dave changed the OP to an supposed to be equivalent type / other manufacturer and some version was than more prone to oscillation.An external supply (even a battery, but even more a lab supply) will add some extra parasitic capacitance to the meter. Capacitance to the inputs (e.g. SMU as a source) also adds capacitance seen by the virtual ground OP. The case with SMU for the input signal and lab supply for the supply could be a problem with many versions.There is an older video / thread from Dave on the issue.For a first test I would try it with some 100 Ohms in series to the meter and battery supply.

author: slow_rider
author_rank: Regular Contributor
timestamp: September 13, 2021, 09:55:43 am
content: Looking at the virtual ground portion of the schematichttps://www.eevblog.com/files/uCurrentRev5schematic.pdfU2 is used for the virtual ground and it has a resistor, R10 of 270 Ohms at the output. In theory this should increase the ability to drive a capacitive load. I must be missing out on something, but this looks fine to me?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 13, 2021, 10:31:56 am
content: The Rev. 5 has the 270 ohms from the OP to the virtual ground - that is the version after the fix.It would still help to have a capacitor from virtual ground to the supply ground, and R3 is still inside the loop, where it does not help.

author: slow_rider
author_rank: Regular Contributor
timestamp: September 13, 2021, 10:54:54 am
content: This worked... by adding more capacitanceMy uCurrent is Rev. 5 !!! with a low serial number?! I can also see R10 with the marking '271' on it and pin 4 from U2 (output) goes to one side, the other side goes to the negative voltage output plug." alt="" class="bbc_img" />