DIY uCurrent Design - Page 1
Forum section: electronics / projects

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: December 20, 2013, 12:47:17 am
content: Hi group,I am the process of building my own ucurrent. I have deviated a little from Dave's original design. Some of the design decisions were made because of the parts I had to hand.I certainly don't want to detract from Dave's uCurrent project.Here is my version:Schematic:I have decided to use separate jacks for the three current ranges. This simplifies the switching. I have also used a SPDT switch for the power and a tact switch for the battery test. I used a Linear Tech LTC2050CS8 for the main amplifierComponent side of the board:(Not quite finished)Top side:I have made the board a little bigger, to fit the Hammond 1591 BBK box, I have not been able to find the UB5 Jiffy box outside of Australia.Jay_Diddy_B

author: dannyf
author_rank: Super Contributor
timestamp: December 20, 2013, 01:09:25 am
content: Nulling would be very difficult in your design.NI (? and now TI) has an old application note on some micro-current opamps (LM-something) in which they talked about quite a few ua/pa-level ampmeters.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: December 20, 2013, 01:20:21 am
content: Hi Group,Quote from: wilfred on December 19, 2013, 11:11:16 pmThe key parameters in choosing and op-amp for this application are:1) VosThe offset voltage is multiplied by the gain of the amplifier. The idea is to get the burden voltage as small as possible, so that insertion of the uCurrent device has minimal impact on the circuit being tested. With a 10m Ohm shunt the voltage is 10uV / per mA of current. So a 10uV offset in the op-amp would be seen as 1mA of current flow. It is desirable to have as low a Vos as practical.There is a group of op-amps that fall in the class of 'Zero-Drift' (Linear Technology), 'Ultra-Low Offset' (Maxim), 'Zero Drift' (Intersil). The key feature of all these op-amps is that they are chopper-stabilized.A secondary feature is the temperature Coefficient of Vos.2) Input Bias CurrentThe next consideration is input Bias current. This is current that flows in or out of the input pins of the opamp. Lower is better. It will add to or subtract from the current being measured.Input Bias Current can vary depending on the common mode voltage of the inputs relative to the supply rails.3) Gain Bandwidth Product, GBW.The bandwidth of the amplifier is limited to the GBW of the opamp divided by the gain of the circuit.4) Slew RateSlew rate can limit the large signal bandwidth of the opamp.5) Rail-to-Rail IOThis parameter is not really important, because bad things happen if you operate the op-amp outputs or inputs near the rail. The gain is limited. The output voltage swing will determine the maximum current that can be measured on each range.6) Minimum Supply VoltageThis is fairly obvious. 2.7V is desirable if you want to power the circuit from a single Lithium coin cell.7) Supply CurrentThis will determine how long the battery will last. Very low power op-amps will tend to have low SR and low GBWI hope that this answers the question.Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: December 20, 2013, 01:32:57 am
content: Quote from: dannyf on December 20, 2013, 01:09:25 amThe LTC2050 op-amp chosen has a typical Vos of 0.5uV and guaranteed offset of 3uV. When multiplied by the gain of 100. This gives offsets equivalent to:50uA typical, 300uA max on the 1000mA range50nA typical, 300nA max on the 1mA range50pA typical,l 300pA max on the 1uA rangehttp://www.linear.com/product/LTC2050I don't see a need to adjust the offset.Jay_Diddy_B

author: george graves
author_rank: Super Contributor
timestamp: December 20, 2013, 02:17:47 am
content: Subscribed. I might learn something by building one myself!

author: Skimask
author_rank: Super Contributor
timestamp: December 21, 2013, 10:41:41 am
content: I think I know the answer to this already...but...What happens if you're drawing say 10mA but plugged into the nA jack?Conversely, maybe draw 1nA and plugged into the mA jack?I'm guessing one will max out the reading (but obviously not be correct) and the other will likely show something close to zero...but you won't smoke anything, right?

author: nctnico
author_rank: Super Contributor
timestamp: December 21, 2013, 11:28:57 am
content: If you use 2 opamps from the same chip with the same amplification the offset between them is zero (give or take the tolerance of the resistors). You can use any cheap jelly-bean opamp. I use this trick for interfacing thermocouples to a differential ADC or 2 seperate ADC channels and then subtract.edit: I have to take this back (and reclaim tuition fees). It seems that even though the transistors for both opamps are on the same die the differences between them can be quite large.

author: dannyf
author_rank: Super Contributor
timestamp: December 21, 2013, 12:12:41 pm
content: QuoteFat chance.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: December 21, 2013, 01:27:19 pm
content: Hi Group,I have been busy and built and tested my version of the uCurrent.Blank PCBHere is the component side of the PCB. The board contains an error. The pin connections for the TPS3809 are wrong.Here is the other side of the board which is the front panel:Assembled BoardComponent side:Front Panel Side:TestingThis is the most challenging test. I took the difference between the 1.000V and the 1.018V outputs of a Fluke 732A DC voltage standard. Feed this through a 1M Ohm resistor to uCurrent nA Input. I got a reading of 17.8mV which corresponds to 17.8nA. The reading does bounce around a bit. This is because of the noise from the op-amp. This is the noise from the datasheet. It is amplified by the gain of 100 at the output of the uCurrent:I had to turn off the fluorescent lights above my bench for this test. The noise from these lamps interferes with small signal measurements.I tried to measure the 18nA directly with my Fluke 289. It read 0.1uA or 100nA.Looks like it is working good.Jay_Diddy_B

author: SeanB
author_rank: Super Contributor
timestamp: December 21, 2013, 01:33:50 pm
content: Very nice job there.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: December 21, 2013, 01:39:47 pm
content: Quote from: Skimask on December 21, 2013, 10:41:41 amDont think so , 10 mA through the 10k nA range resistor would be 100V on the opamp input . So that opamp would go POOF . He needs to consider input overvoltage protection for the opamp (upping r12 considerably but then more offsets from input bias/offset currents to consider).

author: KerryW
author_rank: Regular Contributor
timestamp: December 21, 2013, 03:57:16 pm
content: You could try a circuit like this one. It won't go as high in current, but how high of a current should a MICRO current adapter handle?Down side - Current limited by power supply and op-amp+booster. Needs low input current op-ampUp side - Reads + or - current. No matched resistors, accuracy determined by range resistors. Burden voltage - Vos of U1 +/- (output voltage / U1 open loop gain), or basically 0.

author: nctnico
author_rank: Super Contributor
timestamp: December 21, 2013, 04:10:01 pm
content: Quote from: dannyf on December 21, 2013, 12:12:41 pmI guess you never heard of matched pairs. Two opamps on the same die (chip) have exactly the same errors.

author: dannyf
author_rank: Super Contributor
timestamp: December 21, 2013, 04:25:07 pm
content: QuoteFat chance.

author: KerryW
author_rank: Regular Contributor
timestamp: December 21, 2013, 04:30:26 pm
content: Quote from: nctnico on December 21, 2013, 04:10:01 pmA piece of string walks in to a bar and orders a beer. The bartender looks him over and says "Are you a piece of string?". The piece of string says "Why, yes, I am." The bartender says "We don't serve pieces of string here" and throws him out. The piece of string twists himself up and lays down in the street and lets cars run over him for a while. Then he gets up and goes back in the bar and orders a beer. The bar tender looks at him suspiciously and says "Wait a minute, aren't you a piece of string?". The piece of string says "No, I'm a frayed knot."If that were true, all op-amps would have 0 Vos. They would just put 2 on each die (or 4 for duals8 for quads) and have them cancel.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 21, 2013, 04:47:57 pm
content: The active IV converter is a great idea for small currents. Very low input impedance (basically zero) and the offset of the opamp is not as critical.

author: hans
author_rank: Super Contributor
timestamp: December 21, 2013, 05:25:05 pm
content: Quote from: nctnico on December 21, 2013, 04:10:01 pmA matched pair also has a tolerance, so not each one can be made perfect. For example, the MAT04 was specificied for a couple percent current gain matching. What makes you think it doesn't have that?See datasheet AD708 "Matching characteristics"Or OP227 from LinearOr LT1024The difference seems to be about half of the individual typical specification, so at best you're halving the offset voltage, but at 30uV it's not particularly exciting.http://e2e.ti.com/blogs_/archives/b/thesignal/archive/2012/10/30/matchy-matchy-how-alike-are-dual-op-amps.aspx

author: dannyf
author_rank: Super Contributor
timestamp: December 21, 2013, 05:34:42 pm
content: -The active IV converter is a great idea for small currents.-agreed. Plus, it has the added benefit of being easily converted to a u-volt instrument.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: December 21, 2013, 07:01:38 pm
content: Hi,I have been following the great suggestions that have been made on the thread today. I have a couple of concepts to share.Modification for IV converter nA rangeThis concept shows how to implement a current to voltage converter for the nA range while preserving the original circuit for uA and mA. The only downside to this scheme is that the nA range is inverted. Only one op-amp is used. I showed three circuits to illustrate the configurations.Addition of protection componentsThis concept shows how protection can be added to the design withou interfering with the measurements. The diode connected transistors were used by Fluke in a lot of their DMMs. The circuit is protected to 1A and 40V. The levels found in low voltage circuits.Jay_Diddy_B

author: SeanB
author_rank: Super Contributor
timestamp: December 21, 2013, 07:09:27 pm
content: Good and usable, but is getting away from the original idea of simple, robust and cheap. Probably best would be a 2A picofuse on the common lead for the load, and just buy a strip of 20 or so and store in the uCurrent box bottom. Problem with the transistors is that they only zener above the opamp supply voltage, you might be better off there making a bridge out of 4 with another 2 to shunt the bridge, giving about 2v of safe input before the diodes conduct

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: December 21, 2013, 07:20:47 pm
content: Hi,I was drawing faster than I was thinkingThis is corrected:Jay_Diddy_B

author: nctnico
author_rank: Super Contributor
timestamp: December 21, 2013, 07:22:16 pm
content: Quote from: hans on December 21, 2013, 05:25:05 pmI definitely have to get my money back for 4 years of theory on analog IC design... Using dual opamps seems to work to cancel offset voltages to a certain extend but not as good as I expected.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 21, 2013, 08:21:56 pm
content: I think active IV conversion would also be useful on the uA range. Also, it would be nice to maintain a low voltage drop during "overload" conditions, as might happen when measuring quiescent current of a microcontroller that draws a few mA pulses from time to time. A pair of Schottky diodes can do that, or even better, a pair of MOSFETs switched in with a comparator.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: December 21, 2013, 08:49:39 pm
content: Hi,My final version of the uCurrent will use Tellurium Copper connectors. I have named the device after these connectors, it will be called:CuTe CurrentJay_Diddy_B

author: dannyf
author_rank: Super Contributor
timestamp: December 21, 2013, 09:50:28 pm
content: QuoteSlight different implementation.

author: dannyf
author_rank: Super Contributor
timestamp: December 21, 2013, 09:50:59 pm
content: You can work out the math between full scale and R11/R15.

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: December 22, 2013, 04:16:22 am
content: Hi,Please correct me if I’m wrong, but the right hand side bottom two terminals shown on the back of the board.Is that the 4 terminal resistor there between them. Are you feeding all the other test currents through that voltage measurement terminal

author: EEVblog
author_rank: Unknown
timestamp: December 22, 2013, 05:19:13 am
content: Quote from: Jay_Diddy_B on December 21, 2013, 07:01:38 pmHow is the mA range resistor protected?

author: SeanB
author_rank: Super Contributor
timestamp: December 22, 2013, 06:51:39 am
content: Soldering iron and spare resistor method. Just keep a piece of P&P tape with resistors in case. Visual and aural warning of overload is a glowing resistor and that smell of well toasted electronics.

author: BravoV
author_rank: Super Contributor
timestamp: December 22, 2013, 09:42:09 am
content: Subscribed & thanks JDB.

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 22, 2013, 07:21:50 pm
content: The LTC2050 is not suitable for low current measurements, Ibm=75pA @ 25¤C, 4nA over temp. range.You can measure that by selecting 10k shunt and measure zero reading with jacks shorted (equals offset) minus jacks open (offset plus Ib* 1MOhm).I prefer the ICL 7650 for low bias.The offset voltage could be trimmed to zero by adding a pot over +/- supply, providing an offset current into the feedback loop,over 10MOhm into an additionsl 10Ohm resistor in series with the 1kOhm to GND.Frank

author: Kohanbash
author_rank: Regular Contributor
timestamp: December 23, 2013, 03:08:57 am
content: Looks nice. The one downside to not having a switch and using separate terminals is that you need to break the connection to switch ranges.

author: Legit-Design
author_rank: Frequent Contributor
timestamp: December 23, 2013, 03:10:34 am
content: Quote from: sdk32285 on December 23, 2013, 03:08:57 amNot if you use a jumper, one extra lead with banana jacks each end.

author: Kohanbash
author_rank: Regular Contributor
timestamp: December 23, 2013, 03:20:31 am
content: Quote from: Legit-Design on December 23, 2013, 03:10:34 amThats true.I guess we can debate what is easier to remember a jumper or a switch...

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: December 23, 2013, 06:24:42 pm
content: Hi,Access to the Forum is somewhat limited. I am living in area effected by an ice storm. I am running the computer from a Honda generator and accessing the internet through a hotspot on my iPhone.http://www.theglobeandmail.com/news/national/ice-storm-where-the-power-is-still-out-in-ontario/article16088754/Here is the new schematic:Here is the latest board design:To do:1) Add a SMD pico fuse in series with the common input.2) Change the 10m Ohm to a heavy-duty resistor. I have some 3 or 5W wire-wound SMD resistors. They are not four-terminal, but I will implement a four-terminal connection on the board.I will not be able to build or test until the power is restored.The weather is expected to be -10C tonight, -12C tomorrow.Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: December 27, 2013, 02:55:32 pm
content: Hi,I have had an opportunity to build my version of the uCurrent with protection circuit added.Here is the latest schematic:I have disabled the nA range and returned the feedback components back to the original values.Here is a picture of the new board:[The board is being held in a vice. I made the vice to hold boards while working with a stereo microscope. It is made from 1/2 inch square 6061 and 1/4-20 screws]I have tested the protection with a power supply set to 40V and current limited to 1A. No damage was sustained.I have a few questions for somebody with the original version of the uCurrent:1) How much low frequency noise do you see on the output?2) With zero current applied what is the average (filtered) output on the nA range?Jay_Diddy_B

author: hugo
author_rank: Regular Contributor
timestamp: April 03, 2015, 05:34:18 pm
content: Hi JDB,Have you used .1% resistors or "regular" 1% ?Thanks

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: April 03, 2015, 09:04:51 pm
content: Hi,I used 1%, because that accuracy was good enough for me. I can always change the resistors if I need better accuracy.Regards,Jay_Diddy_B

author: Yansi
author_rank: Super Contributor
timestamp: April 03, 2015, 09:22:05 pm
content: HiI am glad to see this thread here. As a coincidence, I've talked about uA range metering with my friend, but we haven't concluded or agreed anyhow on these thoughts:1) Why Dave did choose to sense the current on a shunt, instead of using direct I/V conversion? (only pure curiosity of a EE beginner)2) As the uCurrent is without overcurrent protection (correct me please, if I remember wrong), the IV convertor with OPamp offers nice solution for current protection, see folowing picture please. As the voltage between inputs of I/V converter is "almost zero", so two (or maybe a few in series) antiparallel diodes pops up as a beautiful solution. But what kinds of leakage current we are talking about, when using this diode protection at voltage levels of "between the opamp inputs", which is almost zero? Really no idea, if this simple diode protection would be usable (leakage low enough).thanx

author: Neganur
author_rank: Unknown
timestamp: April 03, 2015, 10:26:23 pm
content: Quote from: Dr. Frank on December 22, 2013, 07:21:50 pmold post, hoping you're still reading thisthe LT2050(HV) is in my spreadsheet of potential circuits for a design I'm developing. I found that the datasheet of the 2050HV claims Ibias ±7 pA typ and ±50 pA max at 25 C (5V supply). Also the bias currents look sufficiently flat up to the 50-75 C region. How important is input bias? I've until now not spent much thought and focused more on offset drift and GBW (I'd like to stay above 3MHz)

author: Marco
author_rank: Super Contributor
timestamp: April 03, 2015, 10:29:31 pm
content: Quote from: Yansi on April 03, 2015, 09:22:05 pmThe shunt has a constant impedance over a very wide bandwidth whereas the impedance to virtual ground for a transimpedance amplifier starts increasing relatively quickly with frequency. Also with transimpedance amplifiers if you have a low impedance source the offset voltage gets translated into current.There's pro's and con's to both depending on circumstances ... but the shunt method is more fool proof.

author: Yansi
author_rank: Super Contributor
timestamp: April 03, 2015, 11:17:37 pm
content: Thank you for explanation. The inconstant impedance makes sense. But it would depend on the application of the I/V (what stuff will be measured with it). The only question which pops up then is: For almost dc measuremets, like semiconductor leakage current measurement, maybe some lowpower circuits current draw measurement - is the direct I/V method usable, or are there some other hidden drawbacks, that I do not know about? I might like to test it, some small uCurrent probe for poking around in lowpwer micros would be handy for me at home. uA ranges on my MMeters at home sux a lot...

author: dom0
author_rank: Super Contributor
timestamp: April 04, 2015, 08:38:56 am
content: Quote from: Neganur on April 03, 2015, 10:26:23 pmCurrent flowing into the op amp inputs is current that isn't converted, i.e. input bias current creates an output offset.

author: Neganur
author_rank: Unknown
timestamp: April 04, 2015, 02:04:11 pm
content: hmm I see, because it flows in external impedances and can create additional offset voltage this way.It also depends on if there is an internal bias current compensation, in which case external compensation might not be meaningful and only add more thermal noise.an interesting read regarding input bias:http://www.analog.com/media/en/training-seminars/tutorials/MT-038.pdf

author: Kleinstein
author_rank: Super Contributor
timestamp: April 04, 2015, 07:54:28 pm
content: The OP-amp bias appears as an offset to the measured current. So with the LTC2050HC you may be off by something like 10 pA due to bias current which is not that much. Leakage on the board can be larger. Though a zero offset type, the maybe 0.1 µV of offset voltage usually gives a larger error.The bias of auto zero OPs is also relatively constant (that is not so temperature dependent), so one may manually compensate for it. Different from normal OPs the bias of both inputs is usually with opposite sign. So the simple compensation through equal impedance at both inputs does not work with auto zero OPs. If you want to compensate for the bias, just add a constant part (e.g. +-10 µV) to the output.The transimpedance circuit for very small currents may not need a auto zero OP, as the offset voltage is not amplified. So a good low offset / drift CMOS or JFET OP is likely OK. Noise is often dominated by the feedback resistor anyway. However this would be more like having a pA range in addition, though the nA and µA range could use the transimpedance circuit as well.If you are interested in DC current only, an extra capacitor in feedback of the transimpedance circuit helps to keep input impedance low and allows for large peak currents - this may be very helpful measuring something like the µC consumption with mA peak but less than µA average. A cap in parallel to the shunt would need to be much larger.

author: timofonic
author_rank: Frequent Contributor
timestamp: August 24, 2015, 02:41:55 am
content: Quote from: Jay_Diddy_B on December 27, 2013, 02:55:32 pmSorry for the necroposting.Why did you remove it? What was the reason behind it?