Instrumentation Amplifer from LM123/LM224/LM324 - Page 1
Forum section: electronics / beginners

author: lk
author_rank: Regular Contributor
timestamp: February 16, 2014, 07:18:49 pm
content: Greetings,For the fun of it i wanted to simulate an instrumentation amplifier to measure the voltage drop over a resistor, but it is not working as i would expect it to work.This is set up so i have a voltage drop of 1 volt over my sense resistor R5, and as i have set the gain resistor r8, so that the gain of the circuit is approaching 1. So with this setup i would expect that i get some where around 1 volt on my output. But the problem is that the output ends up at ~6.7 mV.the big question is what i do wrong here? I have tried in several simulators, so i'm sure that its something that i have screwed up-lk

author: fcb
author_rank: Super Contributor
timestamp: February 16, 2014, 07:23:26 pm
content: 100meg (100 meg)!!!!!try 100 ohms

author: qno
author_rank: Frequent Contributor
timestamp: February 16, 2014, 07:30:31 pm
content: Yes you beat me with the post.Check your manual. 100 meg without a capital could be read by the simulation package as 100 mili ohms.Some sim packages cold be difficult about this.You could try a standard setup with all 10k resistors.

author: lk
author_rank: Regular Contributor
timestamp: February 16, 2014, 07:44:15 pm
content: Thanks for the replies,this is ltspcie so MEG = meg = 10^6, so i would imagine that is not the problem.The reason for R8 being 100meg is the gain approaches 1 as the gain resistor approaches infinity.fcb: if i set R8 at 100 ohm the gain of the circuit will be 201, so my output should be approaching the positive rail, it is not however.-lk

author: Bored@Work
author_rank: Super Contributor
timestamp: February 16, 2014, 07:51:59 pm
content: You have connected one positive input to the positive rail. But that opamp is not a rail-to-rail opamp.

author: lk
author_rank: Regular Contributor
timestamp: February 16, 2014, 07:52:53 pm
content: Thank you Bored@work,i selected 100meg, to approach and open r8, and the calculated gain for the was 1.0002, which was close enough for me. But again i must have made a more fundamental mistake in my circuit.-lasse

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 16, 2014, 07:56:15 pm
content: If you put a 5 ohm resistor between the top of R5 and the +12 you no longer have +12 on one of the inputs. (but it still gives the same result)If you cut R1 and R2 the output of both opamps differ exactly 1 volt but there must be more... time for me to look up instrumental opamps I guess.

author: lk
author_rank: Regular Contributor
timestamp: February 16, 2014, 07:58:16 pm
content: Good spot Bored@work,I changed the one positive input to 9 volt, but i still don't get the expected 1 volt on my output. now it just hovers at -78 mV.-lk

author: mrflibble
author_rank: Super Contributor
timestamp: February 16, 2014, 08:32:02 pm
content: Try adding a 100k resistor from output to ground. Also, try something lower for that 100 Meg. Maybe 1 Meg.

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 16, 2014, 08:32:21 pm
content: It works if you use dual supply voltages (6 and -6), it doesnt work with single supply voltage.

author: Bored@Work
author_rank: Super Contributor
timestamp: February 16, 2014, 08:59:08 pm
content: It works with a single supply. Only that he has the polarity of the input wrong. Upper opamp should have a lower input voltage than the lower opamp. It is the opposite in the schematic.

author: lk
author_rank: Regular Contributor
timestamp: February 16, 2014, 09:07:32 pm
content: Thanks guys,i swapped the input on the upper/lower opamp and now i get my ~1 volt out-lk