EVAL-AD5933EBZ Datasheet, PDF (26/32 Pages) Analog Devices – Evaluation Board for the 1 MSPS 12-Bit Impedance Converter Network Analyzer

EVAL-AD5933EB
Measuring Higher Excitation Frequencies
The AD5933 is specified to a typical system accuracy of 0.5%
within the frequency range of 1 kHz up to 100 kHz (assuming
the AD5933 system is calibrated correctly for the impedance
range being tested).
The lower frequency limit is determined by the value of the
system clock frequency connected to the external clock pin
(MCLK) of the AD5933. The lower limit can be reduced by
scaling the system clock (see Measuring Lower Excitation
Frequencies).
The upper frequency limit of the system is due to the finite
bandwidth of the internal amplifiers coupled with the effects of the
low-pass filter pole locations (for example, 200 kHz and 300 kHz),
which are used to roll off any noise signals from corrupting the
DFT output on the receive side of the AD5933. Therefore, the
AD5933 has a finite frequency response similar to that shown
in Figure 33.
20
0
–20
–40
–60
100
1k
10k
100k
1M
SYSTEM BANDWIDTH (Hz)
Figure 33. Typical AD5933 System Bandwidth
Using the AD5933 to analyze frequencies above 100 kHz
introduces errors in the impedance profile if the sweep span is
too large. This is due to the effect of the increased roll-off in the
finite frequency response of the system for frequencies above
100 kHz. However, if the user is performing a sweep with a
frequency above 100 kHz, it is important to ensure that the
sweep range is as small as possible, for example, 120 kHz to
122 kHz. The impedance error from the calibration frequency
is approximately linear over a small frequency range. The user
can remove any linear errors introduced by performing an end-
point or multipoint calibration (see the AD5933 data sheet for
further details on end-point calibration).
Preliminary Technical Data
Measuring the Phase Across an Impedance
The AD5933 returns a complex output code composed of separate
real and imaginary components. The real component is stored
at Register Addresses 94h and 95h, and the imaginary component
is stored at Register Addresses 96h and 97h after each sweep
measurement. These correspond to the real and imaginary
components of the DFT, not to the resistive and reactive
components of the impedance being tested.
For example, it is a common misconception to assume that if a
customer is analyzing a series RC circuit, the real value stored in
94h and 95h and the imaginary value stored at 96h and 97h corres-
pond to the resistance and capacitive reactance, respectfully. This is
incorrect. However, the magnitude of the impedance (|Z|) can
be determined by first calculating the magnitude of the real and
imaginary components of the DFT by using the following formula:
Magnitude = R² + I²
Next multiply by the calibration term (see the Gain Factor
Calculation section in the AD5933 data sheet) and invert the
product gives the impedance. Therefore, the magnitude of the
impedance is given by the following formula:
Impedance (| Z |) =
1
Gain Factor × Magnitude
The gain factor is given by the following formula:
Gain
Factor
=
⎜⎛
Admittance
⎟⎞
=
⎜⎜⎝⎛
1
Impedance
⎟⎟⎠⎞
⎝ Code ⎠ Magnitude
Before a valid measurement can occur, the user must calibrate
the AD5933 system for a known impedance range to determine
the gain factor. Therefore, the user of the AD5933 must know
the impedance limits of the complex impedance (ZUNKNOWN) for
the sweep frequency range of interest. The gain factor is
determined by placing a known impedance between the input
and output of the AD5933, and then measuring the resulting
magnitude of the code. The AD5933 system gain settings need
to be chosen so that the excitation signal is in the linear region
of the on-board ADC. (Refer to the data sheet for further details.)
Rev. PrC | Page 26 of 32