CheapStat Voltage modification
I am looking for the way to modify the cheapstat voltage to negative 2-2.5 V for my application. Is it possible to get negative 2 to 2.5 V. If so how can I get it?
It might be possible. It would require some minor changes to the firmware. Also, it won't change the overall voltage i.e. (Vmax - Vmin) . You can just shift the range in the towards either the positive or negative directions. Finally, I haven't tried it - so I can't guarantee that it will work. However, it is just a firmware change and I don't think trying it will cause any damage to the instrument.
The cheapstat is a 0-3.3V device and the mid-point 1.65V is used as the ground for the electrochemical cell. This how the device achieves an effective bipolar voltage range. The value used as the ground for the electrochemical cell is set by the value DAC1 on the microcontroller and is labeled REF on the schematic. DAC1 is a 12-bit digital to analog converter so there are 4096 possible values and it has a 0-3.3V range. It is currently set to 2048 (the mid-point value) which gives an output voltage of 1.65V. This puts the ground for the electrochemical cell midway between 0 and 3.3V. Increasing the value of DAC1 will shift the output voltage range in the negative direction - increasing the range of negative values and decreasing the range of positive values. . Similarly decreasing the value of DAC1 will shift the output voltage range in the positive direction.
So in the basic configuration, it is possible to use a potential from -1.65 to +1.65 ? I tough it was limited to -1 to +1V. Is it just a question of display on the screen that goes back to 0 after 990 mV ?
Ideally you would be able to run from -1.65V to 1.65V. However, in practice with the output cheapstat becomes nonlinear below about -0.9V.
I'll play with the potential of my reference electrode to shift my potential range then.