Let's play... Name the opamp circuit - Page 1
Forum section: electronics / projects

author: MattHollands
author_rank: Frequent Contributor
timestamp: September 12, 2016, 02:00:15 am
content: Hey, I am working on reverse engineering my LCR meter to understand exactly how it works - I know generally how they work but I thought it'd be cool to see a real world example. I've come across 4 op amp circuits that I don't recognise. I am pretty certain that they are all filters of some kind (from context in the overall system) but I don't recognise the topology. I haven't figured out all the passive values because they are all 0603 and I would have to desolder the components to get an accurate measurement. There is a chance that I may have one or two connections wrong as I have been trying to copy these circuits from a small pcb.Can anyone help me get a name for these circuits?The third one is clearly an RC followed by an LC filter, but I don't recognise the use of the other components.

author: JS
author_rank: Frequent Contributor
timestamp: September 12, 2016, 02:23:29 am
content: I think the biggest problem are your drawings, I don't know if there's something that strange on those circuits. Sometimes redrawing (moving components around in the screen) helps so much! That makes a good schematic to be easily readable. Also unnecessary traces should be left out and tagged, ref voltages distribution for example). If you put your components all facing a different way and have traces all wiggling over the place it's much harder to see what's going on.JS

author: MattHollands
author_rank: Frequent Contributor
timestamp: September 12, 2016, 02:26:45 am
content: I know exactly what you mean, but because I don't understand the signal path I'm finding it difficult to tell what components should go where

author: Berni
author_rank: Super Contributor
timestamp: September 12, 2016, 06:06:56 am
content: It looks like a high pass filter to me with one of the inputs simply used as biasing.Placement of components is quite important to make the circuit recognizable. I had a case where a friend was fixing a old all transistor PA amplifier, it was quite stubborn to fix so he eventualy got to tracing out the PCB in to a schematic and it resulted in a barf of transistors and passives over a sheet. He didn't really have any understanding how these things work so i went at the schematic and redrawn parts of it until they resembled a sensible configuration and then in the end redrawn the entire thing again to stick all the blocks together to finally get a schematic that made sense. I could then use the schematic to probe around to find a misbehaving point and within a few minutes found the open circuit resistor that caused the issue.A tip for rearranging your schematic is that the passives you have around your opamps should mostly be to the left of the opamp apart from what connects directly to the output pin. Usualy you want the components that take the signal from the cirucits input to the opamps input to be in a straight line, anythyng that branches away from that line towards ground should go down while things that branch off towards the output should go up. A lot of opamp circuits in literature follow such a arrangement.

author: bobaruni
author_rank: Regular Contributor
timestamp: September 12, 2016, 07:52:48 am
content: The way it's drawn is fine, it is indeed a very strange looking circuit possibly using an uncommon generalised immittance converter (GIC) topology to create a frequency dependant negative resistor (FDNR).seehttp://www.ti.com/lit/an/sbaa001/sbaa001.pdfThe circuit with the coil is called a Pi filter due to the arrangement of capaitors and coil looking like the symbol Pi.

author: Andy Watson
author_rank: Super Contributor
timestamp: September 13, 2016, 10:42:10 pm
content: The op-amps appear to be configured as integrators - assuming that "Amp Ref" is essentially a DC level. This and the odd arrangement of feedback loops makes me suspect that these filters have been designed using leap-frog ladder techniques.