Hello all,

I am working in a conductivity sensor and have some doubts. The base for my design is CN0349 but I have read AD5934 datasheet, AN1252, CN0217 and still not have clear how to calculate the Rfb and Rcal, so I would appreciate you give a hand.

(1) I need to measure conductivities from 2uS to 2mS, that is, impedances from 500KOhms to 400 Ohms, approx.

(2) On AN-1252, pag. 5, table 3, says that Zmax/Zmin for rebiasing should be 45 maximum, so cleary,

I need to divide the range of the impedances to measure, and I am thinking on something like this:

* Low Conductivity range (high Impedances)= 500K-16K, (500K/16K=31.25) and

* High Conductivity range (low Impedances)= 16K-400 (16K/400=40)

(4) As only need the conductivity, I am using Vout frequency= 2KhZ (CN0349,pag3), Not sweep, not increment. Just one measure in that frequency and maybe take several samples to average the measure with the repeat command.

(3) VDD= 3.3V

(4) PGA= 1

(5) Vpk=2Vpp (AN-1252,pag. 3 table 1)

(6) To calculate Rcalibration (An1252, pag 5) Rcal=(Zmin+Zmax)*1/3

so Low Range, Rcal= 516K/3=172K

High Range, Rcal= 16400/3=5K466

(7) For calcultating Rfb,(An1252, pag 5)

Rfb=((VDD/2-0.2)*Zmin/(Vpk+Vdd/2-Vdcoffset))*1/Gain

being:

Vdd/2=1.65

Vpk=2

Vdcoffset=Vdd/2 (rebiasing signal)

Gain=1

so Low range, Rfb=1,45*16K/2=11k600 Ohms

High Range=290 Ohms

(8) To calibrate I would use:

Low Range: Rfb=11k600 and measure for Rcal=16K and RCal=172K

With this two points I can calculate GF (Gain factor) and Nos(Offset System) (CN0349, pag. 4, eq (1) and (2))

Hig range: Rfb=290 Ohms, and measure for Rcal= 5k466 Ohms and Rcal=16K.

With this two points I can calculate GF(Gain factor) and Nos (CN0349, pag. 4, eq (1) and (2))

In total, I need three resistor to calibrate my system, 5k466, 16K and 172K and two Rfb 290 and 11k600, am I correct?

If not, how should I do it?

(9) To measure the unknow signal Yx, use the eq. (3) (CN0349, pag. 4), and finally, Ycell= Yx/(1-R2*Yx), being R2=100 Ohms.

(10) I dont undestand the gain of the system for fig 1, pag 1, of CN0349 so I dont know when signals are out of range for the ADC. Let me explain:

On Fig 1 of CN039, R10=R11=20K, so internal OA of AD5934 has Gain of -1. As internal PGA of AD5934 is 1, the Gain of the system is provided by U2B and the Rcal/Rfb ratio:

So in my above explanation:

low range, Rfb= 11K600, case Zmin=16K, Gain= 0.725 (116600/16000)

Case Zmax= 500K, gain= 11600/500K=0.0232, Can be this so small???

Hig range, Rfb= 290, Zmin= 400, Gain=290/400=0,725

Zmax= 16K, Gain=290/16000= 0.00125, Can be this so small????

What is wrong?

(11) Another questions, how calculate the influence of the temperature on the YCell, is there any equation/logic ? At the moment I am testing the Evaluation Board CN0349 with fixed resistors but when I use a real sensors how I can use the temperature to correct the conductivity.

Sorry for my large text but I hope people can understand my doubts,

Thanks in advance.

Joaquin.

hi lopezcontreras: the CN0349 measured range divided into two ranges: Rcell ≤ 900Ω or Rcell > 900Ω, when Rcell ≤ 900Ω the Rfb = R9 = 100Ω should be satisfied, when Rcell > 900Ω the Rfb = R8 = 1kΩ should be satisfied. please refer CN0349 Software User Guide [Analog Devices Wiki] for more detail.