# CN0349, Calculating Rfb and Rcal

Question asked by lopezcontreras on Dec 12, 2017
Latest reply on Dec 19, 2017 by Snorlax

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,