Digilent 410-246 User Manual

Page 3

PmodIA™ Reference Manual

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First, you must set up an I

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)

Table 2. Frequency sweep parameter storage registers.

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):

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.