Sawtooth shape in CVs
I recently came across the rodeostat and was quite intrigued as it looks a lot like the design I've independently been kicking around in my head over the last few months - e.g. use the built in ADC and DAC in the teensy and use the arduino development environment for code (or in my case, coded poorly). I also sponsored a group of students to build a simpler version of a potentiostat using an Arduino Uno, but the R-C filtered PWM signal is not ideal.
I'm an interested buyer - in our lab we could use a second potentiostat that we could fit inside our anoxic gloveboxes and not take up too much space.
However, the CV's on the website look kind of "hairy". One example is this: https://cdn.shopify.com/s/files/1/1091/6464/products/ascorbate_cv_1024x1024.png?v=1501974528
I'm wondering if that is an issue from a previous iteration or is it still a drawback of the Rodeostat?
Also, looking at the design schematic, there is a capacitor in the feedback loop of the control amplifier connected to the counter electrode. In other words, the control amplifier is an op-amp integrator. Is it the discharging/charging behavior of this capacitor on each voltage step that causes the CVs to look like that? (I don't know, but it is a hypothesis.) I suppose alternatively it could be also due to charging of the capacitors in the transimpedance amplifier feedback loop. Clearly the problem doesn't manifest itself in the data I see for the chronoamperometric measurements you've posted on the website. That sort of makes sense because the potential is fixed in those measurements.
Do you have thoughts on this, and would there be a simple design change to avoid it?
I'm not sure the exact cause of the "hairyness" the CV's. I do know that it only seems to occur when using certain electrochemical setups. We don't see it with purely resistive loads such as dummy cells or even simple dummy cells with combined resistive and capacitive loads. You can do CV sweeps etc. and no hairiness . So it isn't really a matter of whether or not the potential is fixed - maybe it is some characteristic of the load in the cases where we see the hairiness. It would definitely be interesting to explore the issue. I've been considering designing some more general dummy cell PCBs for testing purposes and I would love some input on designs. It would be great to have a very general dummy cell design which could mimic all sorts of loading situations. In particular, could we find a dummy cell with the correct load characteristics to reproduce the issue? or is this really something to do with the electrochemical cell?
The capacitor in the feedback loop of capacitor of the control amplifier is a compensation capacitor used to improve stability. It is basically providing a high-frequency bypass. The load which the potentiostat is going to be hooked up to is essentially unknown (lots of different variations) so we wanted the control loop to stable under a broad range of conditions. Adding this compensation capacitor was found to help extend in stability over a broader range of conditions without causing a detrimental effect. I don't believe this capacitor is the cause of the hairiness you are seeing.
The compensation capacitors on the transimpedance amplifiers are also for stability. The values have been selected fairly conservatively. So we are probably getting a bit more lowpass filtering then absolutely necessary. For the most part this is more of an issue for the lower current ranges where the resistor in the feedback loop is larger. We've made some nA versions of the Rodeostat where we had to go to larger capacitor values in order to ensure stability. For example, for a +/- 60nA range with a 10MOhm resistor we needed a 100nF compensation capacitor for stability. Again I don't think these capacitors are the issue.