Proper usage of Earth-ref'd signal gen + isolated PSU + split rail op amp ? - Page 1
Forum section: electronics / beginners

author: smoothVTer
author_rank: Regular Contributor
timestamp: October 07, 2016, 01:06:52 am
content: I'm in the planning & experimentation phase of building an active-load circuit using the standard topology of an op amp & closed-loop feedback around an NMOS transistor. Using a 1-ohm sense resistor and a 1V input signal would give 1A current draw. I want the op amp to be powered by a split-rail power supply in order to guarantee the gate voltage can be pulled to zero. By the way: for purposes of this discussion the actual voltage levels & component values are arbitrary here and can/will be changed in the final implementation.The signal generator is earth-referenced. Can't change that fact. The PSU is floating but has a earthing post. Channel B & Channel C have a common negative that is internally connected inside the PSU. Channel A is isolated from B and C completely. For initial testing I have voltages set to what is show in the diagram.I've put ChanA & ChanB in series as in the diagram below; then, the point "Fx" is the actual floating ground of the bipolar power supply. Point "Gx" is essentially my power ground. At this stage, the PSU is floating and the waveform generator is earth-referenced. There is no common between them, and as such, voltage levels are undefined between the two instruments. Thus the need to find shared common between all circuit elements.Here is where I am stuck: My first thought was to attach point "Fx" to the earthing lug on the PSU, because this virtual floating ground could be attached to earth, so then the AWG, op amp and the PSU would be referenced to the same ground and everything would be OK. However, ChanB on the PSU immediately hit my preset 100mA compliance limit. Not sure why this occurred. Secondly, point "Gx" is my power ground. So this point should connect to the system ground now, which I've established to be point "Fx". If then I were to connect the negative output (Gx) of ChanC to Fx (now earth ground), ChanB's negative output would short to ChanA's negative output and negate the series-connected ChanA-ChanB.How do I establish a common reference between the AWG and the PSU in this setup?Does the internally-connected negative outputs of Chan B & Chan C preclude this particular setup?Why did I hit compliance when I simply connected point Fx to the earthing lug?Which of my assumptions in the text above is incorrect? There must be something!As always, much thanks to everyone taking the time to read/respond.

author: Ian.M
author_rank: Super Contributor
timestamp: October 07, 2016, 01:24:03 am
content: It would appear that either channels B and C are Earth referenced or you have a sneak ground path somewhere in your circuit to 'Gx' - e.g. a scope probe ground clip.Swap channels A and B so A provides the -ve rail to the OPAMP, and 'Gx' and 'Fx' will then be safely linked inside the PSU and should be safe to ground.

author: smoothVTer
author_rank: Regular Contributor
timestamp: October 09, 2016, 02:00:18 am
content: Ian.M:Your suggestion worked. Thank you kindly!For posterity, here is the working setup which Ian.M suggested and which I implemented with success. The power supply is a Rigol DP832A:

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2016, 09:59:10 pm
content: That is interesting. The feature I like most about my old Tektronix PS503 power supplies is that the dual tracking output is completely floating compared to the high current +5 volt output. Positive, common, or negative can be connected to ground or the +5 volt output.