DIGILENT PmodIA With External Clock Microcontroller Boards User Manual
- June 13, 2024
- DIGILENT
Table of Contents
PmodIA™ Reference Manual
Revised April 15, 2016
This manual applies to the PmodIA rev. A
Overview
The PmodIA is an impedance analyzer built around the Analog Devices AD5933 12-bit Impedance Converter Network Analyzer. Features include:
- Impedance analyzer with 12-bit impedance converter
- Measure impedance values ranging from 100Ω to 10 MΩ.
- Programmable frequency sweep
- Programmable gain amplifier
- Optional external clock generation
- Small PCB size for flexible designs 1.6 in × 0.8 in (4.1 cm × 2.0 cm)
- 2×4-pin port with I²C interface
- Follows Digilent Interface Specification
- Library and example code available in resource center
The PmodIA.
Functional Description
The PmodIA utilizes Analog Devices AD5933 with its on-board frequency
generator and analog-to-digital converter (ADC) to be able to excite an
external unknown impedance at a known frequency. This known frequency is sent
out through one of the SMA connectors. The frequency response is captured by
the other SMA connector and sent to the ADC and a discrete Fourier transform
(DFT) is performed on sampled data, storing the real and imaginary parts of
the solution in the on-chip data registers. The magnitude of the unknown
impedance as well as the relative phase of the impedance at each point in the
generated frequency sweep can be calculated from these two data words.
1.1 I² C Interface
The PmodIA acts as a slave device using I² C communication protocol. The I² C
interface standard uses two signal lines. These are I² C data and I² 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
7bit 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 its 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.
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.
Connector J1 – I² C Communications
Pin| Signal| Description
1, 2| SCL| I² C clock
3, 4| SDA| I² C data
5, 6| GND| Power supply Ground
7, 8| VCC| Power supply (3.3V/5V)
1.2 Clock Source
The PmodIA has an internal oscillator that generates a 16.776 MHz 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.
1.3 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 I² 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. The peak to peak voltage
needs to be set appropriately in relation to the impedance test. This is to
avoid the internal op-amps from trying to deliver an output voltage or current
beyond their maximum capability. It is recommended that when using the 20-ohm
feedback resistor to set the peak to peak voltage to either the 200mV or 400mV
and when using the 100K-ohm feedback resistor, set the peak to peak voltage at
1V.
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.)
| 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)
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.
Once you have set these parameters, perform the following steps to start the frequency sweep (paraphrased from the AD5933 data sheet):
-
Enter standby mode by sending the standby command to the control register.
-
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. -
Start the frequency sweep by sending the start frequency sweep command to the control register.
1.4 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:
z = Real + j ∗Imaginary
Where Real is the real component, Imaginary is the imaginary component, and ? is an imaginary number (equivalent to i = √−1, in mathematics). You can also represent impedance in polar form:
Impedance = |z|∠θ
Where |Z| 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.
1.5 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.
1.6 Register Addresses
The AD5933 data sheet has a complete table of register addresses.
Physical Dimensions
The pins on the pin header are spaced 100 mil apart. The PCB is 1.6 inches long on the sides parallel to the pins on the pin header and 0.8 inches long on the sides perpendicular to the pin header.
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