Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
You didn't analyze the simulation results thoroughly! Q11 is in saturation, because it's fully turned on by U5 although the loop
current is about 54 mA, which should never happen.
Apparently the OP is in a latch up or gain reversal state - or simply wired incorrectly. Unfortunately, neither the OP
type nor input polarity can be clearly recognized on the schematic.
Regarding the original problem, I stated a phase reversal of the OP due to negative input voltage. If it's correct, that
the problem doesn't occur with the real OP, it would be a pure SPICE model problem. The AD8627 datasheet doesn't discuss
phase reversal with negative overload, so I don't know about the real OP behaviour. For simulation purposes, you may
want to clip negative input voltages e.g. by a schottky diode.
I don't exactly understand the purpose of operating the current loop for both directions. I don't know of automation
equipment supporting this feature.
Regarding the "ground loop" problem, there are many ways to operate a 4-20 mA transmitter, but you don't clearly tell about
the intended grounding scheme. To assure operation within a limited common mode range (it's actually limited for both
driver and receiver), there must be a defined grounding. A current feedback through a low pass filter can only be
stable, if the feedback loop gets a dominant pole in conjunction with the driver amplifier. It doesn't look promising
in my opinion.
This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
By continuing to use this site, you are consenting to our use of cookies.
