Hello,
I would like to design a conductivity (EC) measurement circuit with AD5941. I want to measure conductivity from 0.1 uS/cm to 100,000 uS/cm with an EC electrode which has a cell constant of C = 1 1/cm. (So, I would like to measure impedance from 10 Ohms to 10 MOhms.)
I've started designing a circuit based on AD5941 datasheet, the CN0428 and EVAL-AD5941ELCZ circuit schematics, and I have a few questions. (If I know right, the AD594x is essentially the analog front-end circuit of the ADuCM355.)
1. I connected the EC electrode to the CE0 and SE0 pins through a BNC connector. According to CN0428 (Figure 1), when I switch S2 to the INT position, I can measure impedance from 100 Ohms to 10 MOhms (Page 5). I don't understand the reason for this. How can I go below 100 Ohms? According to the AD5941 datasheet, when using the SE0 pin, I connect a 100 Ohm resistor (R_LOAD) in series with the EC electrode (Page 49). How should this be taken into account?
2. In the schematic of EVAL-AD5941ELCZ, the signal from the EC electrode cannot be connected to SE0, only to IMEASHZ. How can water conductivity be measured at all? According to CN0428, this mode (IMEASHZ) is designed for measuring impedances above 200 kOhms.
3. Do I need anything in addition to the EC electrode between the CE0 and SE0 pins? According to the CN0428 and EVAL-AD5941ELCZ schematics, it's not necessary, but for example, AN-1557 uses R_LIMIT resistors and C_ISO capacitors.
4. Among the software libraries (https://github.com/analogdevicesinc/ad5940-examples), which one is suitable for this application? I'm using an STM32 host MCU.
Thank you in advance for the responses!
I added page and figure numbers.
[edited by: BalintV at 10:14 PM (GMT -4) on 2 Nov 2023]
