Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
So, I figured that since I actually was able to get the circuit working on the breadboard, that something was up with the PSpice modelling, so I downloaded the trial version of Multi, which is A BAJILLION times better!
Ah, I see. So what you're saying is that my work above to calculate the threshold peak voltages of the capacitor triangular wave aren't really accurate?
I was talking about what you were saying about the difference between the amplifier hook up and the Schmitt Trigger hook up.
To further and more simply illustrate the difference you can get with different initial conditions, for a second (or even less) imagine that you connect the positive supply to the negative power pin of the op amp and the negative supply rail to the positive power pin of the op amp. You'd get a very different solution than either one we talked about so far
Yes, you are seeing sample errors. Its caused by not having enough resolultion in your simulation. Try to turn up the number of points in the simulation setup applet. I don't know what it's called in your simulator; look at the documentation for simulation points or time division or similar.
Hey, I tried that but the peaks are still clipped. When I zoomed in I noticed that it is because the astable multivibrator doesn't go immediately rail-to-rail, it has some delay slope and it causes the triangular wave to be clipped.
Is there a certain parameter that classifies this rail-to-rail time? I thought since the signal is so slow that it would be a fast rise/fall. Any ideas?
EDIT***
Never mind, I modified the time step and it made things look better, the peaks aren't consistent, not clipped though.
That's strange that the peaks are not consistent. I tried a quick sim and they looked ok once i used a simple limiter circuit on the output of the first op amp. The limiter makes the positive and negative peaks equal. Maybe try LT Spice.
Awesome! I just realized that the R-R op-amp that I was using (LT1801CMS8) is a dual op-amp, so I used the 2/2 amp for the integrator, and it worked perfectly. I figured out that the peak voltages were under this relation, with the help of this tutorial (last bunch of slides): https://www.electro-tech-online.com/custompdfs/2011/08/Chapter_4_Op_Amp_Oscillators.pdf
I tweaked the resistors to allow for the peak voltage that I wanted, and voila! I have to do some more testing, but this is a very good result!
**broken link removed**
**broken link removed**
I am now wondering what kind of buffering, if I need it all, I will need to use this to drive electrode biasing.
In answer to your original question the offset of the output triangle wave is IMHO due to the inherent input offset voltage of one or other op-amp (see the op-amp datasheet for a likely value) being subjected to amplifier gain. Try connecting a high value (probably megohms) trimmer resistor between one of the op-amp inputs and one or other supply rail to see if you can compensate for this.
At a maximum, this thing should be able to deliver between 100 uA to 30 mA. The datasheet for the chip says it has an output current of 50 mA. I am just wondering if this triangular signal will be distorted at all, or if there are any precautions that I should take.
Well with an output current capability of 50ma i would think it could handle 30ma, but if you can test it to make sure that's always a good idea.
Maybe calculate or simulate to find the current through the capacitor while it is ramping and add that to the output just to make sure.
Both the op amps are part of a closed feedback circuit.
Provided the op amp can source/sink the required peak load current without going into current limit, it will drive the load with negligible distortion.
It will either work perfectly, or clip horribly if you hit the brick wall of internal current limit.
Make absolutely sure your op amp can comfortably reach 30mA over your intended supply voltage and temperature range.
Well with an output current capability of 50ma i would think it could handle 30ma, but if you can test it to make sure that's always a good idea.
Maybe calculate or simulate to find the current through the capacitor while it is ramping and add that to the output just to make sure.
Both the op amps are part of a closed feedback circuit.
Provided the op amp can source/sink the required peak load current without going into current limit, it will drive the load with negligible distortion.
It will either work perfectly, or clip horribly if you hit the brick wall of internal current limit.
Make absolutely sure your op amp can comfortably reach 30mA over your intended supply voltage and temperature range.
Thank you both for you suggestions, I will try to simulate these with my entire system.
Another thing that I am worried about is powering my entire system. I will be using this chip, along with three other op-amps. Is a +/- 3 V dual coin cell battery setup sufficient?
Hey all, I really appreciate the help that everyone has given me. My project is moving forward well, but I have hit another stumbling block, and I am hoping someone can help me diagnose the problem.
I have managed to get the triangular wave working, and I connected my entire circuit in Multisim and simulated it. Here is the basic circuit:
**broken link removed**
Between the WE and RE, the circuit can be modelled as: R + R||C. The same is between CE and RE. I have measured these values with an impedance analyzer and have come up with a range of 1 uF to 10 uF for the parallel capacitors, and roughly 1k to 10k for the parallel resistors. The resistors in series have <100 ohm.
The circuit is works by passing the Vbias voltage across the WE and RE electrodes. My problem is when I simulate it, I can't get the inverting input of the transimpedance amplifier to stay at ground. Here are my circuit and simulations:
**broken link removed**
**broken link removed**
----- Vbias from triangular oscillator ----- -Vbias set to reference electrode ----- Inverting input of transimpedance amplifier ----- Output of transimpedance amplifier
I am essentially trying to model my chemical solution based on the data that I measured. For this system to work, I really need the yellow signal (inverting input of the transimpedance amplifier) to be zero constantly.
Anyone have any ideas, this one's a toughy. Thanks!
JP
The output on U5A is the voltage proportional to the desired current flowing through the system. It's basically a transimpedance amplifier, and I need it there.
The inverting input of U5a should be a virtual ground, just as you say.
If there is significant voltage at that node, it can only be because U5a is not working properly.
Check the resistor values are what you believe they are, and that U5a is not dead, and that it has both Vdd and Vss supplies present. Also look very carefully at all the connections to U5a. It should work, but something there is faulty.
The output on U5A is the voltage proportional to the desired current flowing through the system. It's basically a transimpedance amplifier, and I need it there.
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.
