# Analog Devices Wiki

This version (01 Mar 2019 17:13) was approved by Micheál Lambe.

# High Speed TIA Calibration

## Overview

The high speed TIA gain resistor is configurable from 50, 100, 200, 1k, 5k, 10k, 20k, 40k, 80k and 160kΩ. To calibrate the Rtia a precision RCAL resistor needs to be connected to RCAL0 and RCAL1 pins. Ideally the RCAL should be close in value to the Rtia to be calibrated and should have a tolerance ⇐ 0.1%. The calibration process implements an impedance measurement where the magnitude of the impedance is the value of Rtia. A sine wave is applied through RCAL and the Rtia. The voltage drop across each is measured with a DFT calculated on each. Using ratiometric analysis the actual impedance of the Rtia is calculated by the following equation: |Rtia | = |Vrcal|/|Vrtia|.

It is important to calibrate at the correct frequency. For example, if the measurement of the sensor requires an excitation signal of 50 kHz, the calibration routine should be carried out at this frequency. This insures CTIA is factored into the calculation. If carrying out a frequency sweep, it is recommend to do a calibration for each frequency point in the sweep.

## Calibration Steps

The following are the steps involved for calibrating the HSTIA Gain resistor:

• Configure switch matrix to connect RCAL between the Excitation buffer and the HSTIA as per below figure: • Generate a sine wave with required frequency using the waveform generator, HSDAC and Excitation amplifier.
• The signal path is highlighted in above image.
• The P_Node and N_Node are connectted to the ADC mux and the DFT of the voltage is measured.
• Then, maintaining the same excitation voltage, the HSTIA_P and HSTIA_N nodes are connected to the ADC mux. A DFT of the voltage is calculated again.
• The real and imaginary parts of the DFT results are used to calculate the value of the Rtia with the following equations: • This value of Rtia is then saved by the host controller and used for calculating impedance's.

## Using the HSTIA Rtia Calibration Function in the SDK

The following function is an example on how to calibrate the HSTIA gain resistor:

```static AD5940Err AppRtiaCal(void)
{
HSRTIACal_Type hsrtia_cal;
hsrtia_cal.bPolarResult = bTRUE; /* We need magnitude and phase here */
hsrtia_cal.DftCfg.DftNum = AppBIACfg.DftNum;
hsrtia_cal.DftCfg.DftSrc = AppBIACfg.DftSrc;
hsrtia_cal.DftCfg.HanWinEn = AppBIACfg.HanWinEn;
hsrtia_cal.fRcal= AppBIACfg.RcalVal;
hsrtia_cal.HsTiaCfg.DiodeClose = bFALSE;
hsrtia_cal.HsTiaCfg.HstiaBias = HSTIABIAS_1P1;
hsrtia_cal.HsTiaCfg.HstiaCtia = AppBIACfg.CtiaSel;
hsrtia_cal.HsTiaCfg.HstiaRtiaSel = AppBIACfg.HstiaRtiaSel;
hsrtia_cal.SysClkFreq = AppBIACfg.SysClkFreq;
if(AppBIACfg.SweepCfg.SweepEn == bTRUE)
{
uint32_t i;
AppBIACfg.SweepCfg.SweepIndex = 0;  /* Reset index */
for(i=0;i<AppBIACfg.SweepCfg.SweepPoints;i++)
{
printf("Freq:%.2f,Mag:%.2f,Phase:%fDegree\n", hsrtia_cal.fFreq, AppBIACfg.RtiaCalTable[i],
AppBIACfg.RtiaCalTable[i]*180/MATH_PI);
}
AppBIACfg.RtiaCurrValue[AppBIACfg.SweepCfg.SweepIndex] = AppBIACfg.RtiaCalTable[i];
AppBIACfg.RtiaCurrValue[AppBIACfg.SweepCfg.SweepIndex] = AppBIACfg.RtiaCalTable[i];
AppBIACfg.SweepCfg.SweepIndex = 0;  /* Reset index */
}
else
{
hsrtia_cal.fFreq = AppBIACfg.SinFreq; 