EVAL-AD5940ELCZ sensorPal Calibration Resistor

Hi Ian,

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

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?

Hi

I can set and read registers, read temperature sensor etc and I can set the Low power DAC to output 12 bit and 6 bit voltage levels. 

I am having issues trying to generate a very slow Cyclic voltammetry waveform of scan rate 100mv/s, following the data sheet I need to use the low power DAC. 

I have set the WAVEGEN bit to 1 in the AFECON register

and Bit 6 in the LPDACCON register to 1 

and set up the various registers for the trapazoid generator to achieve the appropriate waveform.

Where I run into trouble is this statement p92 data sheet :

 " The 32 kHz oscillator must be selected as the system clock when using the waveform generator with the low power DAC"

if I do this by setting bits the SYSCLKSEL bits to 1,0 in the CLKSEL register, I kill my SPI communication to the AD5940 i assume because SPI 1MHZ is too fast now.

is there a method of using the 32 khz clock for the low power DAC without affecting the SPI interface 

Regards

Ian

Hi,

We have an example application in our SDK which carries out a cyclic voltammetry measurement without using the waveform generator. Refer to AD5940_Ramp example for details. The LPDAC is updated through software instead of the waveform generator. It is not possible to communicate with the part when the 32kHz is selected as system clock. 

Yes i've seen that example and was wondering why the generator was not used

Thanks

Hi can you confirm that this formula in the data sheet for ADC input voltage is correct 

in my application i am substituting VBIAS_CAP with  the value of LPTIA_N node as I am conducting a DC current measurement as below

I have noticed in the sample firmware that the ADC calculation used  is  the following

float kFactor = 1.835/1.82;

tmp = ADCcode - 32768;

fVolt = ((float)(tmp)/32768)*(AppAMPCfg.ADCRefVolt/1)*kFactor;

here there seems to be no reference to VBIAS_CAP or an alternative negative ADC input maybe this is added else where in the code ? And kFactor what is this 

Please advise

Regards 

Ian

Adding VBIAS_CAP in the equation is only required when the negative input to the ADC is the 1.11V reference, or ADCMUXN_VSET1P1 in the SDK

The kFactor is a constant that is required to calculate the correct voltage.