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35
ERES35105
User’s Manual
7.5.4
E
XCITATION
F
REQUENCY
(0
X
04)
This shift register select line is connected to an Analog Devices AD9833 Programmable Waveform Generator. This Generator uses a 28-bit
phase accumulator based on a 20 MHz clock. To calculate the value to write to the 28-bit FREQ register, use the following formula:
𝐹𝑅𝐸𝑄 =
(𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦)(2
28
)
20 𝑀𝐻𝑧
Output Data
To set the output frequency, several writes to SR_DATA_OUT are required. These must be performed in order. In this list, the pipe represents
a bit-wise or operation, and the square brackets represent the bits in the FREQ register.
1.
0x2000
2.
0x4000 | FREQ[13:0]
3.
0x4000 | FREQ[27:14]
Input Data
The Programmable Waveform Generator does not provide any read data.
7.5.5
E
XCITATION
A
MPLITUDE
(0
X
05)
This shift register select line is connected to an Intersil ISL22424 digital potentiometer. Wiper 0 of the potentiometer is used to adjust the
amplitude of the Channel 0 Excitation. Wiper 1 of the potentiometer is used to adjust the amplitude of the Channel 1 Excitation.
Output Data
The Table below shows the value to write to the SR_DATA_OUT register to issue various commands.
Table 30: Excitation Amplitude Shift Register
Data
Name
Description
0x60C0
Set to Non-Volatile
All subsequent writes to the wiper position are only stored in non-volatile
memory. This should be done prior to adjusting the wiper, i.e. at board
open.
0xC0XX
Write Channel 0 Wiper
Output amplitude. See Table 30 below. Values may be interpolated.
0xC1YY
Write Channel 1 Wiper
Output amplitude. See Table 30 below. Values may be interpolated.
0x8000
Read Channel 0 Wiper
Reads the current wiper position.
0x8100
Read Channel 1 Wiper
Reads the current wiper position.
Table 31: Excitation Amplitude Values
Value
Amplitude (V
RMS
)
Amplitude (V
p-p
)
0x00
0.0
0.0
0x3B
2.0
5.7
0x95
5.0
14.1
0xD1
7.0
19.8
Input Data
The lower 8 bits of the SR_DATA_IN register contain the wiper position after a read command is issued.
