Optimizing RCAL,RTIA,CTIA and DAC voltage of ADuCM350 for 4-Wire Measurement.

Introduction

To achieve higher accuracy and repeatability of measured sensor impedance using ADuCM350, the RCAL, RTIA , CTIA and the DAC output voltage during the measurement needs to be optimized.

The accurate results can be produced when the ADuCM350 SNR is higher. To achieve higher SNR, the DAC output needs to be around 600 mVp and the ADC input should be  around 750 mVp.

600 mVp – Maximum allowable swing from DAC output.

750 mVp – Maximum allowable swing at ADC input.

To calculate the RCAL , RTIA and DAC Output code, the below items needs to be identified.

1. The minimum sensor impedance.
2. Access resistance and Capacitance.
3. Mode of operation such as 2-wire,4-wire and 4-wire Bio-Isolated.
4. Frequency of interest.

Based on the above parameters, the RCAL , RTIA , CTIA and DAC Output code values can be calculated.

Please refer to the application note “AN-1271-Optimizing the ADuCM350 for Impedance Conversion” for detailed explanation of this calculations.

4-wire Measurement Set-up with RACCESS:

The diagram shown below depicts the 4-wire measurement to be performed using ADuCM350 with RACCESS to identify the unknown Z value connected between the AFE channels 6 and 3.

RCAL,RTIA,CTIA and DAC Output Selection Procedure:

For the 4-wire measurement diagram shown above,consider the below parameters.

1. Minimum Zsensor = 2000 Ω ;
2. RACCESS1-4 = 10 KΩ;
3. Frequency of measurement = 50 KHz.
4. Mode of measurement = 4-Wire with only access resistances.

With the above mentioned inputs,the RCAL,RTIA,CTIA and DAC Output values can be selected.

RCAL Selection  Procedure:

The RCAL resistor value can be selected as same as the minimum Zsensor value. This allows larger signal during calibration and hence the improved accuracy.

RCAL  = Minimum Zsensor = 2000 Ω.

RTIA Selection  Procedure:

The RTIA value can be calculated by following the below steps:

1.Calculate the total impedance across the channels. In our case, between AFE6-3 :

      = RACCESS1 + RACCESS4+ Zsensor

      =10 KΩ + 2000 Ω + 10 KΩ = 22 KΩ.

2.Calculate the current through the channels(AFE6-3) :

  = (Maximum DAC output voltage * Attenuation Factor) / Total impedance across the channels.

• The Maximum DAC output voltage allowable is 600 mVp.
• The attenuation factor is the attenuation of RCF incorporated at the DAC. The RCF has a cutoff frequency of 50 KHz and based of the frequency of operation , the signal attenuation varies.

The attenuation factor can be calculated by using the below equation :

1/(SQRT((1+(F/Fc)^2))).

Where as

• Fc – cutoff frequency of RCF which is 50 KHz.
• F – DAC output frequency.

Considering our frequency of interest which is 50 KHz,The attenuation factor is 

= 1/(SQRT((1+(50000/50000)^2))) = 0.707

3.The current through channels can be calculated by substituting the above values.

= 600 mVp * 0.707  / 22 KΩ

= 19.28  µA

4.Now, Calculate the optimal RTIA by dividing the maximum allowable ADC input(750 mVp) with the calculated current through the channel.

  = 750 mVp / 19.28 µA = 38.9 KΩ.

5. Atlast,select the closest available standard resistor value

i.e. RTIA  = 39.2 KΩ 1%.

CTIA Selection  Procedure:

The CTIA value can be selected by identifying the cutoff frequency of the measurement signal, RTIA and attenuation factor.

CTIA = 1/(2*∏*Fc* RTIA).

Fc – Cutoff frequency of the RC filter.

The Fc needs to be carefully selected here. If Fc is too close to desired frequency of interest , the ADC input will be attenuated due to the RC filter rolloff.

Considering ,the cutoff frequency ten times the measurement frequency gives a attenuation factor of 0.995.The cutoff frequency selected is 500 KHz.

Now, the CTIA can be calculated by

= 1/(2*∏*500E3*39.2E3)

= ~ 8 pF.

Select the closest available standard capacitor value

i.e. CTIA  = 10 pF 5%.

DAC Output Voltage Selection Procedure:

To calculate the DAC output voltage that needs to be programmed in code, multiply the current through the channel with the sensor impedance.

  = 19.28 µA * 2000 Ω.

  =~39 mV

RCAL,RTIA,CTIA and DAC Output Calculator:

The excel sheet"RTIA_RCAL_CTIA_CALCULATOR.xlsx" attached with this FAQ incorporates the above mentioned procedure and calculates the RCAL,RTIA,CTIA and DAC Output as per measurement inputs.

Signal Swings Across AFE Channels

By using the calculated the RCAL , RTIA , CTIA and DAC Output code values, the DAC output swing captured across the AFE channels 3-6 are shown below :

Please note that the maximum swing across the AFE channels are only 848 mVp-p instead of 1.2 Vp-p. This is due to the 0.707 attenuation factor introduced by the RCF in TX path of ADuCM350.

Also  the DAC output code, we have programmed is ~39 mV only. This larger swing is introduced by the Raccess resistance of 10 K. The 39 mV swing will be present across the sensor impedance of 2K.

Programming a wrong DAC output in code could cause DAC output saturation or reduced SNR thereby affecting the measurement results.

Signal Swings Across ADC (RTIA) Input:

By using the calculated the RCAL , RTIA , CTIA and DAC Output code values, the signal swing captured across the ADC input is shown below :

Please note that the maximum swing across the ADC input is ~1.5 Vp-p. This helps in achieving maximum SNR and thereby producing accurate measurements.

Conclusion :

Thus optimum values for RCAL, RTIA,CTIA and DAC output needs to be selected for obtaining accurate results using ADuCM350.Use the excel sheet calculator attached with this FAQ for selecting them.