I am using the EVAL-AD5940ELCZ with the sensorPal application. I am confused about the functionallity of the Calibration resistor in Cyclic Voltammetry.
I have noted that on the board the RCAL resistor is value 200 ohms this is also the default value on the sensorPal GUI.
however if I leave it at the default setting I get a much higher current output than I would expect from my sensor 400 uA as opposed to in the range of 0 - 21uA
initial potential 0mv
Final potential 1000mv
Step potential 2.148 uV (default)
Scan rate 100 mv/s
current range to 0uA - 21uA (85K RTIA) this is the range i expect.
Calibration resistor 200 ohm
Are the sensorPal examples only applicable for the on board dummy sensor or can custom sensors be used.
Do need to change the RCAL resistor if using a custom sensor.
I have noticed if I increase the calibration resistor value drastically (on the GUI only) the current output will drop to range that I would expect.
Also should step potential units on the GUI be in mV instead of uV according to AN-1563 "The minimum step size is 537 µV on the AD5940"
The calibration resistor is used to calibrate the internal Rtia on the LPTIA. SensorPal can be used for your own sensor also. Ensure jumpers JP9, JP10 and JP11 have disconnected the dummy network from the CE0, RE0 and SE0 pins. Are you connecting your sensor via the cables or wired directly to P7?
RCAL should be chosen to be closer to the value of RTIA you plan on using to maximize accuracy of the calibration routine. I would suggest replacing RCAL with a 10k resistor on the evaluation board.
Yes, the units should be in mV. We will update.
Thanks for the reply
yes I am using the cables,
We are trying to replicate a lab based potentiostsat experiment on an embedded device.
the sensor used is a prototype with two electrodes for water sample analysis
A cyclic voltammetry scan is used to observe oxidization peaks in the samples current response
I am currently connecting the red crocodile clip (CE0) and blue(RE0) to the counter electrode of the sensor and the green(SE0) to the working electrode as is done using the lab potentiostat.
I am unsure if this is the correct method using the AD5940 should i use the usb cables or the BNC connector considering it is a water sample i am dealing with.
From reading the data sheet i think it is possible to route the Cyclic voltammetry waveform signal through the BNC connector by setting the appropriate switches in the programmable matrix but this is probably not possible using sensorPal.
I am also not sure if i should use the LTC6078 op amp in the circuit in this application or bypass it.
sensor impedence is 300-400k
Using the cables is certainly the correct approach. For the cyclic voltammetry scan, you don't need to use the op-amp. That opamp is for a pH sensor and also for measuring very high impedance sensors where the magnitude is several Mohms.
Have you been able to get some reasonable data yet?