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AD5934 Datasheet(PDF) 17 Page  Analog Devices 

AD5934 Datasheet(HTML) 17 Page  Analog Devices 
17 / 31 page Data Sheet AD5934 Rev. E  Page 17 of 31 GAIN FACTOR TEMPERATURE VARIATION The typical impedance error variation with temperature is in the order of 30 ppm/°C. Figure 21 shows an impedance profile with a variation in temperature for 100 kΩ impedance using a 2point gain factor calibration. 101.5 98.5 54 66 FREQUENCY (kHz) 101.0 100.5 100.0 99.5 99.0 56 58 60 62 64 +125°C +25°C –40°C VDD = 3.3V CALIBRATION FREQUENCY = 60kHz MEASURED CALIBRATION IMPEDANCE = 100kΩ Figure 21. Impedance Profile Variation with Temperature Using a 2Point Gain Factor Calibration IMPEDANCE ERROR Refer to Circuit Note CN0217 on the AD5933 product page, which highlights a method to improve accuracy. The EVAL AD5933EBZ board can be used to evaluate the AD5934 performance. MEASURING THE PHASE ACROSS AN IMPEDANCE The AD5934 returns a complex output code made up of a separate real and imaginary components. The real component is stored at Register Address 0x94 and Register Address 0x95, and the imaginary component is stored at Register Address 0x96 and Register Address 0x97 after each sweep measurement. These correspond to the real and imaginary components of the DFT and not the resistive and reactive components of the impedance under test. For example, it is a common misconception to assume that if a user was analyzing a series RC circuit that the real value stored in Register Address 0x94 and Register Address 0x95 and the imaginary value stored in Register Address 0x96 and Register Address 0x97 would correspond to the resistance and capacitive reactance, respectfully. However, this is incorrect because the magnitude of the impedance (Z) can be calculated by calculating the magnitude of the real and imaginary components of the DFT given by the following formula: 2 2 I R Magnitude + = After each measurement, multiply it by the calibration term and invert the product. Therefore, the magnitude of the impedance is given by the following formula: Magnitude Factor Gain Impedance × = 1 Where the gain factor is given by Magnitude Impedance 1 Code Admittance Factor Gain = = The user must calibrate the AD5934 system for a known impedance range to determine the gain factor before any valid measurement can take place. Therefore, the user must know the impedance limits of the complex impedance (ZUNKNOWN) for the sweep frequency range of interest. The gain factor is simply determined by placing a known impedance between the input/ output of the AD5934 and measuring the resulting magnitude of the code. The AD5934 system gain settings need to be chosen to place the excitation signal in the linear region of the onboard ADC. Because the AD5934 returns a complex output code made up of real and imaginary components, the user is also able to calculate the phase of the response signal through the signal path of the AD5934. The phase is given by the following formula: Phase (rads) = tan−1(I/R) (3) The phase measured by Equation 3 accounts for the phase shift introduced to the DDS output signal as it passes through the internal amplifiers on the transmit and receive side of the AD5934, along with the lowpass filter, and also the impedance connected between the VOUT and VIN pins of the AD5934. The parameters of interest for many users are the magnitude of the impedance (ZUNKNOWN) and the impedance phase (ZØ).The measurement of the impedance phase (ZØ) is a 2step process. The first step involves calculating the AD5934 system phase. The AD5934 system phase can be calculated by placing a resistor across the VOUT and VIN pins of the AD5934 and calculating the phase (using Equation 3) after each measurement point in the sweep. By placing a resistor across the VOUT and VIN pins, there is no additional phase lead or lag introduced to the AD5934 signal path, and the resulting phase is due entirely to the internal poles of the AD5934, that is, the system phase. Once the system phase is calibrated using a resistor, the second step involves calculating the phase of any unknown impedance can be calculated by inserting the unknown impedance between the VIN and VOUT terminals of the AD5934 and recalculating the new phase (including the phase due to the impedance) using the same formula. The phase of the unknown impedance (ZØ) is given by ZØ = (Φunknown − system ∇ ) where: system ∇ is the phase of the system with a calibration resistor connected between VIN and VOUT. Φunknown is the phase of the system with the unknown impedance connected between VIN and VOUT. ZØ is the phase due to the impedance, that is, the impedance phase. 
