How does SMU change its output impedance when it switches from CC to CV mode ?  - Page 1
Forum section: electronics / projects

author: emm386
author_rank: Newbie
timestamp: August 25, 2023, 08:06:20 pm
content: Hi all,Recently I had a chance to go through Keithley's SMU 237 service manual to understand its theory of operation better.Attachments show the simplified block diagram of the Keithley SMU 237.The way I understand its output impedance b/w CC and CV mode is as follows. Since it uses a famous two OPAMP-based precision current pump topology for its output, its output impedance with CC and CV mode is determined by series-series feedback (I-feedback path) and series-shunt feedback (V-feedback path), respectively.But would like to hear other's opinions regarding this topic. Thanks!

author: TimFox
author_rank: Super Contributor
timestamp: August 25, 2023, 08:13:29 pm
content: Simple answer:In CV mode, the feedback keeps the output voltage constant over a wide range of current, therefore very low output impedance.In CC mode, the feedback changes the output voltage to keep the output current constant over a wide range of voltage, therefore very high output impedance.The details are in the switching: both controls' measurements exist, but only one can control the output voltage at a time.Whichever condition is more limiting controls the result: the unit is programmed for a voltage and a current, but only one of those two variables is trying to exceed the setpoint, depending on the load.Diodes CR9 to CR12 determine which of the amplifier channels controls the output (four needed for both polarities of both variables).

author: emm386
author_rank: Newbie
timestamp: August 25, 2023, 08:47:15 pm
content: Hm... my question was how such impedance alternation is possible using a shared analog circuit. When you look at the block diagram, the error amplifier and output stage are shared b/w CC and CV mode.Plus, what you wrote is exactly what I mentioned in the original post.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 25, 2023, 09:04:45 pm
content: FYI, some comments here:https://electronics.stackexchange.com/questions/678852/how-does-an-smu-change-its-output-impedance-from-current-source-mode-to-voltage?noredirect=1#comment1807578_678852Tim

author: TimFox
author_rank: Super Contributor
timestamp: August 25, 2023, 09:08:57 pm
content: Quote from: emm386 on August 25, 2023, 08:47:15 pmExcept that I added the note about the analog logic performed by CR9 to CR12 to determine if the output voltage is regulated according to the current sense or the voltage sense as the load changes.(In careful usage, "regulation" means changing the output, "stabilization" is a servo that controls the regulator from feedback.)If you take feedback from the outputcurrent, that increases the output impedance.If you take feedback from the outputvoltage, that decreases the output impedance.

author: David Hess
author_rank: Super Contributor
timestamp: August 26, 2023, 12:39:18 am
content: Quote from: TimFox on August 25, 2023, 09:08:57 pmExactly, the output impedance, within the bandwidth of the feedback loop, depends on whether the feedback comes from the output voltage or the output current.I will add however that the switched current shunt is in series with the output and the current shunt resistance directly adds to the open loop output resistance. So the open loop output resistance depends on the current measurement range.The actual output stage is a class-ab emitter follower so had inherently low output resistance, but a high output impedance current output, like the output from a transconductance amplifier, could also have been used and I have seen it done both ways.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 26, 2023, 03:56:17 am
content: Quote from: David Hess on August 26, 2023, 12:39:18 amCareful -- that's floating ground. It's actually a pair of current sources (presumably, biased a little class A), so is indeed a gm amp first and foremost, and a follower (CV output) only through servo action.At least, I think... these three images aren't great context.Tim

author: emm386
author_rank: Newbie
timestamp: August 26, 2023, 04:11:36 am
content: Thanks a lot to TimFox, David Hess, and T3sl4co1l !All your responses greatly helped my understanding of the theory of operation of this instrument.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 26, 2023, 06:10:12 am
content: Quote from: emm386 on August 25, 2023, 08:47:15 pmDid you miss the diodes ?The diodes decide which control loop is the winner.They are both active, but only one has ultimate control in CC / CV modes.I guess you could argue exactly on the knee point, when CV and CC intersect, that if diodes have equal current, then they share the win