Digilent PmodIA™ Impedance
Analyzer Reference Manual
Revision: November 01, 2012
Note: This document applies to REV A of the board
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Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners.
Overview
The PmodIA is an impedance analyzer built
around the Analog Devices AD5933
Impedance Converter Network Analyzer.
Features include:
• I
2
C communication interface
• capable of measuring impedances
ranging from 100Ω to 10 MΩ.
• programmable frequency sweep: start
frequency, step increment, and number
of steps.
• programmable gain amplifier
• external clock generation optional
Functional Description
The PmodIA uses an 8-pin connector that
allows for I
2
C communication. There is also a
1-pin connector that allows you to select either
a high or low impedance measurement.
The PmodIA measures impedance by emitting
an AC voltage at a known frequency and
sampling the frequency response to identify
the unknown impedance value. You can
access the voltage output via the SMA
connector J3. SMA connector J4 captures the
response.
Please see the AD5933 data sheet, available
from www.analog.com, for more detailed
information on controlling the analyzer.
I
2
C Interface
The PmodIA acts as a slave device using I
2
C
communication protocol. The I
2
C interface
standard uses two signal lines. These are I
2
C
data and I
2
C clock. These signals map to the
serial data (SDA) and serial
clock (SCL) respectively on the PmodIA. (See
Table 1.) The following instructions explain
how to read and write to the device.
You must consider two protocols when writing
to the PmodIA: the write byte/command byte
and the block write. Writing a single byte from
the master to the slave requires the master to
initiate a start condition and send the 7-bit
slave address. You must hold the read/write bit
low to write to the slave device successfully.
The PmodIA should set the slave address as
0001101 (0x0D) upon startup. After the slave
acknowledges it’s address, the master must
send the address of the register it wants to
write to. Once the slave acknowledges receipt
of this address, the master will send a single
data byte that the slave should acknowledge
with a return bit. The master should then issue
a stop condition.
You can also use this protocol to set a pointer
for a register address. After the master sends
the slave address and write bit, and the slave
responds with an acknowledge bit, the master
sends a pointer command byte (10110000, or,
0xB0). The slave will assert an acknowledge
bit and then the master will send the address
of the register to point to in memory. The next
time the device reads from or writes data to a
register, it will occur at this address.
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Note: The pointer must be set prior to using
block write or block read protocols.
You can perform a block write protocol in a
similar fashion to setting a pointer. Send the
block write command (10100000, or, 0xA0) in
place of the pointer command, and the number
of bytes being sent (represented as a byte) will
take the place of the register address with
subsequent data bytes being zero indexed.
Use the same two protocols when reading data
from the PmodIA: receive byte and block read.
Table 1. Interface Connector Signal Description
Clock Source
The PmodIA has an internal oscillator that
generates a 16.776MHz clock to run the
device. You can use an external clock by
loading IC4 on the PmodIA and setting bit 3 in
the control register (register address 0x80 and
0x81).
The PmodIA schematic provides a list of
recommended oscillators. The schematic is
available from the PmodIA product page at
www.digilentinc.com.
Setting up a Frequency Sweep
The electrical impedance, of a circuit can
vary over a range of frequencies. The PmodIA
allows you to easily set up a frequency sweep
to find the impedance characteristics of a
circuit.
First, you must set up an I
2
C interface between
the host board and the PmodIA. The PmodIA
requires three pieces of information to perform
a frequency sweep: a starting frequency, the
number of steps in the sweep, and the
frequency increment after each step. The
starting frequency and the increment per step
parameters are stored as 24-bit words. The
number of steps parameter is stored as a 9-bit
word.
You can program the peak-to-peak voltage of
the output frequency in the sweep by setting
bits 10 and 9 in the control register.
Once the circuit has been excited, it takes
some time to reach its steady state. You can
program a settling time for each point in the
frequency sweep by writing a value to register
addresses 0x8A and 0x8B. This value
represents the number of output frequency
periods that the analog-to-digital converter will
ignore before it starts sampling the frequency
response. (See Table 2 for a list of registers
and their corresponding parameters.)
You can calculate the 24-bit word to store at
the register addresses for the start frequency
and the increment per step parameters using
the start frequency code and frequency
increment code equations below. You can
also find these equations and more information
in the AD5933 data sheet.
Connector J1 – I
2
C Communications
Pin Signal Description
1, 2 SCL I
2
C clock
3, 4 SDA I
2
C data
5, 6 GND Power supply Ground
7, 8 VCC Power supply (3.3V/5V)
Register Address Parameter
0x80, 0x81
Control register (Bit-10
and Bit-9 set peak-to-
peak voltage for the
output frequency).
0x82, 0x83, 0x84 Start frequency (Hz)
0x85, 0x86, 0x87 Increment per step (Hz)
0x88, 0x89 Number of steps in sweep
0x8A, 0x8B Settling time (Number of
output frequency periods)
Frequency Sweep Parameter Storage Registers
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Once you have set these parameters, perform
the following steps to start the frequency
sweep (paraphrased from the AD5933 data
sheet):
1) Enter standby mode by sending the
standby command to the control
register.
2) Enter the initialize mode by sending an
initialize with start frequency command
to the control register. This allows the
circuit being measured to reach its
steady state.
3) Start the frequency sweep by sending
the start frequency sweep command to
the control register.
Impedance Calculations
The analog-to-digital converter samples the
frequency response from unknown
impedances at up to 1MSPS with 12-bit
resolution for every point in the frequency
sweep. Before storing the measurements, the
PmodIA performs a Discrete Fourier Transform
(DFT) on the sampled data (1,024 samples for
each frequency step). Two registers store the
DFT result: the Real Register, and the
Imaginary Register.
Electrical impedance contains both real and
imaginary numbers. In Cartesian form, you can
express impedance with the equation:
Where Real is the real component, Imaginary is
the imaginary component, and is an
imaginary number (equivalent to , in
mathematics). You can also represent
impedance in polar form:
Where is the magnitude and θ is the
phase angle:
The PmodIA does not perform any
calculations. After each DFT, the master
device must read the values in the Real and
Imaginary registers.
In order to calculate the true impedance you
must take into account the gain. You can find
an example gain factor calculation in the
AD9533 data sheet.
Temperature Readings
The PmodIA has a self-contained, 13-bit
temperature sensor to monitor device
temperature. Please refer to the AD5933 data
sheet for more information on controlling this
module.
Register Addresses
The AD5933 data sheet has a complete table
of register addresses.
