AD5232 Analog Devices, AD5232 Datasheet - Page 10

• Manufacturer Part Number: AD5232
• Description: 2-Channel/ 256-Position Digital Potentiometer
• Manufacturer: Analog Devices

ADVANCED CONTROL MODES
The AD5232 digital potentiometer contains a set of user program-
ming features to address the wide applications available to these
universal adjustment devices. Key programming features include:
Independently Programmable Read and Write to all registers.
Increment and Decrement Commands
The increment and decrement commands (#14, #15, #6, #7)
are useful for the basic servo adjustment application. This com-
mand simplifies microcontroller software coding by eliminating
the need to perform a readback of the current wiper position,
then add one to the register contents using the microcontroller’s
adder. The microcontroller simply sends an increment command
(#14) to the digital POT, which will automatically move the
wiper to the next resistance segment position. The master incre-
ment command (#15) will move all POT wipers by one position
from their present position to the next resistor segment position.
The direction of movement is referenced to Terminal B. Thus
each increment #15 command will move the wiper tap position
farther away from Terminal B.
Logarithmic Taper Mode Adjustment
Programming instructions allow a decrement and an increment
wiper position control by individual POT or in a ganged POT
arrangement where both wiper positions are changed at the
same time. These settings are activated by the 6 dB decrement
and 6 dB increment instructions #4 and #5 and #12 and #13
respectively. For example, starting with the wiper connected to
Terminal B executing nine increment instructions (#12) would
move the wiper in +6 dB steps from the 0% of R
position to the 100% of R
potentiometer. The 6 dB increment instruction doubles the
value of the RDAC register contents each time the command is
executed. When the wiper position is greater than midscale, the
last 6 dB increment instruction will cause the wiper to go to the
Full-Scale 255 code position. Any additional +6 dB instruction
will no longer change the wiper position from full scale (RDAC
register code = 255).
Figure 6 illustrates the operation of the 6 dB shifting function
on the individual RDAC register data bits for the 8-bit AD5232
example. Each line going down the table represents a successive
shift operation. Very important: the left shift #12 and #13 com-
mands were modified so that if the data in the RDAC register is
equal to zero and the data is left shifted, it is then set to code 1.
AD5232
Simultaneous refresh of all RDAC wiper registers from
corresponding internal EEMEM registers.
Increment and Decrement instructions for each RDAC wiper
register.
Left and right bit shift of all RDAC wiper registers to achieve
6 dB level changes.
Nonvolatile storage of the present scratch pad RDAC register
values into the corresponding EEMEM register.
Fourteen extra bytes of user-addressable electrical-erasable memory.
BA
position of the AD5232 8-Bit
BA
(B terminal)
Also the left shift commands were modified so that if the data in
the RDAC register is greater than or equal to midscale and the
data is left shifted then the data in the RDAC register is set to
full-scale. This makes the left shift function as close to ideally
logarithmic as is possible.
The right shift #4 and #5 commands will be ideal only if the
LSB is zero (i.e., ideal logarithmic–no error). If the LSB is a
one then the right shift function generates a linear half LSB
error, which translates to a code dependent logarithmic error
for odd codes only as shown in the attached plots, (see Figure
5). The plot shows the errors of the odd codes for the AD5232.
Actual conformance to a logarithmic curve between the data
contents in the RDAC register and the wiper position for each
Right Shift #4 and #5 command execution contains an error
only for the odd codes. Even codes are ideal except zero right
shift or greater than half-scale left shift. The graph in Figure 7
shows plots of Log_Error [i.e., 20 log 10 (error/code)]. For
example, code 3 Log_Error = 20
which is the worst case. The plot of Log_Error is more signifi-
cant at the lower codes.
–10
–20
–30
–40
–50
–60
0
0
(+6 dB)
SHIFT
LEFT
20
LOG_ERROR (CODE) FOR 8-BIT
40
60
LEFT SHIFT
0000 0000
0000 0001
0000 0010
0000 0100
0000 1000
0001 0000
0010 0000
0100 0000
1000 0000
1111 1111
1111 1111
CODE, FROM 1 TO 255 BY 2
80 100
120 140 160 180 200 220 240 260
RIGHT SHIFT
1111 1111
0111 1111
0011 1111
0001 1111
0000 1111
0000 0111
0000 0011
0000 0001
0000 0000
0000 0000
0000 0000
log 10 (0.5/3) = –15.56 dB,
(–6 dB)
RIGHT
SHIFT