EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: grouchobyte on August 14, 2016, 05:00:20 am
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One of my consulting projects, a high current DC to 1 MHz transconductance amplifier with rather stringent functional and performance requirements has me tasked with some redesign of a current product (excuse the double entendre). Multiple current cells (example of basic positive current cell attached below) provide scale-able bipolar current and there are numerous challenges in meeting thermal, compliance, accuracy, linearity and stability needs for a lab metrology-grade instrument.
That said, there are some legacy tweaks in the old design that need to be explained ( the documentation and the original designer are not available). My client wants to know if we still need some the questionable circuits that were implemented in the past.
T1 is implemented as a board structure of two parallel traces about 6 inches long. I only have a picture so I can only guesstimate the trace width length, spaces and the board material, etc.
T1 might have been implemented as an empirical tweak to compensate for some distortion at higher frequencies or to null out or rolloff some instability based on a specific opamp or existing board parasitic. I can do some sims on it but my fear is that won't really explain why its there and whether it is warranted in the newer design/layout
I scratch my head :palm: when I see T1 virtually "un-terminated" at the opamp inverting node. Does anyone have a better feel for what and why this was done?
You can be vague or detailed.....all feedback welcome.
Thanks muchos
Gracia
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@grouchobyte
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Kelvin connection for R1?
They're only parallel and straight, not coiled to specifically meander them to a certain "inconvenient" coupling length?
Without actually seeing if any of this makes sense, it structurally in the schematic almost looks like a "single point ground" or "kelvin connection" or common mode choke thing.
Of course if the fuse blows then it'll provide some input rail reference to the amp anyway.
Good one EVB149!....and thanks.
Yes, R1 is a 100 milliohm current sense resistor with a kelvin connection. Obviously at DC (in steady state mode) there are no issues or concerns and T1's contribution is nil. Its clearly a second board spin tweak to fix something that showed up unexpectedly
@grouchobyte
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Surely it couldn't have anything to do with matching the fuse inductance...
EDIT: Then again paralleling the tracks will couple them like a transmission line transformer / directional coupler, but it will also minimize the inductive loop area (like a non-twisted pair or microstrip) formed between their common connection point and the OAs node, so if there was magnetic flux it would induce less EMF around the loop due to the close coupling, assuming the final diversion to the opamp input is short.
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fuse inductance? maybe, but its likely not the lowest hanging fruit
Here as a shot of the old board structure( circa 1990):
@grouchobyte
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That looks like a directional coupler in series to sensing resistor... To have different feedback at dc and ac...???
Probably was tuned to have a specific step response of output current..