uCurrent Gold design question - Page 1
Forum section: electronics / beginners

author: Montzterrr
author_rank: Newbie
timestamp: October 09, 2014, 07:12:02 pm
content: Hello,I have been having a discussion on reddit/r/askengineers about some design choices on the uCurrent gold and the only thing that still doesn't make sense to me is the use of the 270 ohm resistor R8. It appears in the feedback network and gives the op-amp a gain of 10.27 instead of 10 and the cascade a gain of 102.7 instead of 100. I just watched the video explaining the use of cascading op amps to increase bandwidth, but there was a lot of emphasis on keeping the gains the same. I'm just really confused about why the R8 resistor was included. A user on reddit said it might be used for stability or minimizing ringing/overshoot but wasn't sure, so I thought I would come to the home of the uCurrent to find the reasoning behind using it.Here is a link to the reddit discussion.http://www.reddit.com/r/AskEngineers/comments/2iryfm/eeopamp_circuit_so_i_just_purchased_the_ucurrent/This is my first post on the forum so please let me know if I broke any forum rules by posting a link to reddit. It has the full discussion that I've had so far.Thanks

author: Montzterrr
author_rank: Newbie
timestamp: October 09, 2014, 07:13:16 pm
content: For easier access, here is the schematic with the resistor labeling I was referring to.http://www.eevblog.com/files/uCurrentRev5schematic.pdf

author: Dave
author_rank: Super Contributor
timestamp: October 09, 2014, 07:29:36 pm
content: The 270R resistor is there to limit the output current, in case you make a mistake and short the output, or even worse, connect some voltage to it.It does not affect the gain whatsoever, as the output is connected after that resistor and the feedback is taken directly from the output.You might see a gain of 10.27 directly on the output pin of the opamp, but that's not where the output of the circuit is.

author: Montzterrr
author_rank: Newbie
timestamp: October 09, 2014, 07:38:40 pm
content: Thank you! I can't believe I missed that about the gain, it seems so obvious now!

author: dinoboy
author_rank: Contributor
timestamp: March 08, 2015, 05:07:31 pm
content: ( let's post all uCurrent Gold design questions in the same thread?)R8is placed left to the node (U4 -R8- node - J3). The corresponding resistor in theold design(called "R9" there, with 100?) was placed right to the node ("U1 - node -R9- J3"). Does the placement, right vs. left to the node, affect inanyway any functionality? And why was the value increased from 100? to 270? or how was the 270 number calculated?In both the old and new designs the resistor directly after the range switch is calledR12("R12"). What's its function and is it absolutely necessary? And again, why was the value increased from 100? to 270? or how was the 270 number calculated?Is my assumption correct thatC3andC4are in place to dampen fast fluctuations of the output voltage (=to create a 'smoother graph') in a DC circuit? Is this what is commonly called 'noise reduction'? Is their value (10pF) a more experimental/simulational value and rather arbitrary in nature/a matter of taste or liking, or is it the result of stringent design calculations and optimization?The heart of uCurrent is thechopper amplifierbyMaxim, a venerable part which has been on the market for 10+ yrs afaik. Since the price is equally low at ~1.00$/part, I am wondering if it isn't "better" to just install a modern so-called 'current sense amplifier' instead?Texas InstrumentsandMaximhave quite a few of them. Are 'instrumentation amplifiers' the same as those?Analog Deviceshas them. Also seeon the topic.Thanks for some input!

author: dannyf
author_rank: Super Contributor
timestamp: March 08, 2015, 05:18:06 pm
content: QuoteR8 is inside the feedback loop so it doesn't matter. all it does is to protect the opamp in case of a short.