CN0429 evaluation board behavior

Hi

We have some question about CN0429 operation.

〇Accuracy 

〇Rcal measurement

External dummy Circuit is below. Dummy 1Mohm is connected to CE0,RE0,SE0.

1Mohm is discrete resistance and red line is discrete lead and jumper.(Leads insert CN0429 each terminals.) 

Configuration is below.

 We change Vbias from 0mV to 200mV.

they have some offset but increase current vs increase Vbias have accuracy less than 1ppm. 

Vbias Current Increase Current
0mV -21.9nA
50mV -76.9nA -55nA
100mV -134.5nA -57.6nA
200mV -230.7nA -96.2nA

But  read Rcal is always 122ohm.

Rcal is 200ohm in schematic and actual result is 199.1ohm with digital multimeter.

RTIA=40kohm is current range is 0.9V/40kohm=±22.5uA

RCAL is 200Ω.

22.5uA×200Ω=4.5mV 

Over4.5mV is saturated condition.

but their readrcal result don't depend on changing RTIA at 200ohm,1kohm. 

How voltage is supplied wtih "readcal"?

We consider the gas sensor diagnostic with EIS.

In user manual wiki, result are Frequency, Magnitude, Phase.

 The result is below.

Addtional items is Real Mag and Imag Mag.

But I think measurement result are only Frequency, Magnitude, Phase.

Frequency Magnitude Phase
1000Hz 961687 5.56
5000Hz 1071751 1.22
10000Hz 949025 2.44
20000Hz 1105400 7.12

Over 20kHz result cause parasitic capacitors.

1k-20kHz result are unstable.

Is Rcal=200ohm good for any MΩ measurement?

EVAL-AD5940_BIO is RACL=10kohm.

How impedance is targeted with Rcal=200ohm? 

Can we know measurement condition?

(each steps TIA, Rload,CE,RE,SE,supply voltage (AC and DC) and frequency)

UG-126 from p.159 to ・・.

Step 2: Measure RLOAD02 and External Sensor (RSENSOR)

STEP3: Measure RLOAD02

STEP4: Measure RCAL via the Impedance Measurement Engine

https://www.analog.com/media/en/technical-documentation/user-guides/ADuCM355-Hardware-Reference-Manual-UG-1262.pdf

 

Best regards

Thank you very much.

  • Hi Jma,

    I have forwarded your question, someone should get back to you shortly.

    Brandon

  • Hi Jma,

    I don't think the test case with 1MΩ makes sense for this. When you measure EIS on amperometric gas sensors, the impedances are normally in the range of single ohms. This is the reason the 200 ohm Rcal makes sense for the EIS measurement. This is the default Rcal on the main ADuCM355 eval board for the same reason. I didn't design this circuit note, but just looking through the code, it looks like Rcal is only used for EIS measurement and nominal values are used for TIA resistances. I would guess this design decision was because the sensitivity variation from sensor-to-sensor is larger than the internal TIA resistor variation in ADuCM355. There is already some code in M355_calibration.c that you could leverage if you wanted to calibrate individual R_tia gain values in your system though.

    Best regards,
    Scott