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From dual to single supply op amp circuit

by _Bd_
Tags: circuit, dual, single, supply
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analogdesign
#19
Dec17-13, 10:59 AM
P: 474
If the biomedical engineer thinks the circuit is good enough, it probably is. Remember fitness to purpose. Sophiecentaur makes a good point though. Obviously the output is saturating (you have a limiting amplifier) so the value of the filtering is not clear. If this is a done deal and it meets specs it doesn't matter though, but something to think about.
_Bd_
#20
Dec17-13, 12:10 PM
P: 89
I would like it not to be squared, TBH I would love to get a perfect sine wave as my output but thats as good as I could get.

My original purpose was to get the same circuit but with single supply as opposed to dual, but after everything was done according to all those articles I've read about single supply thats the signal I got, which is still a million times better looking that the first attempts .


The purpose of each stage:

First: (transimpedance amplifier)
This is just a current to voltage converter, AFAIK is to make it "easier" to read the outputs from the photodiode.

Second: 39 Hz Butterworth filter:

A low pass filter with a cutoff frequency of 39, TBH I dont know how to set the cutoff frequency but I assume is set by the values of the resistors/cap in that op amp, which is why I did not change the values on those resistors/caps.


So first you amplifiy the signal, then you filter it, then you re-amplify it to make it easier to read? (that would be my assumption, again Im an ME not an EE so I am not too sure about this)

But thanks for all the help you guys have been extremely helpful!
sophiecentaur
#21
Dec17-13, 01:04 PM
Sci Advisor
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P: 11,922
Your first stage isn't doing what it should, clearly. The red line on your graphs shows a massive 500mA swing(?) (If the right hand scale is really the input current). I see the input source is written 5m, on the diagram. Is that 5mA? Have you actually set it up as a current source? I can't be sure what the legend on the graph means: I[I1] ??? I am not familiar with that way of simulating a photodiode. A Google search throws up more complicated models- but you are only working at low frequencies so it may be OK. It will require some playing about with actual values in that part of the model, I think.
Reasons for my comments:
Vout of the first stage should be 4700*Iin so, for less than 5Vpk-pk out (to allow some headroom, you should be hitting it with much less than 1mA pk-pk (by my reckoning. If the source really is 5mA, then you are driving the circuit five times too hard, and that could explain the limiting behaviour. The DC Voltage gain of the filters is in the order of 3X, by my eyeballing so you want even less current input swing. That would be easy to try and it may give your filter a chance to do something useful (poor little thing, with all that signal belting through it!)


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