Data Sheet
ADVANCED CONTROL MODES
The
digital potentiometer contains a set of user program-
ming features to address the wide variety of applications avail-
able to these universal adjustment devices. Key programming
features include the following:
•
•
•
•
•
•
Independently programmable read and write to all
registers
Simultaneous refresh of all RDAC wiper registers from
corresponding internal EEMEM registers
Increment and decrement command 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 RDACx
register values into the corresponding EEMEMx register
Fourteen extra bytes of user-addressable, electrical erasable
memory
AD5232
tional 6 dB instruction does not change the wiper position from
full scale (RDACx register code = 255).
on the individual RDACx register data bits for the 8-bit
example. Each line going down the table represents a successive
shift operation. Note that the Left Shift 12 and Left Shift 13 com-
mand instructions were modified so that if the data in the RDACx
register is equal to 0 and is left shifted, it is then set to Code 1.
In addition, the left shift commands were modified so that if the
data in the RDAC register is greater than or equal to midscale and
is left shifted, the data is then set to full scale. This makes the left
shift function as close to ideally logarithmic as possible.
The Right Shift 4 and Right Shift 5 command instructions are
ideal only if the LSB is 0 (that is, ideal logarithmic, with no error).
If the LSB is a 1, the right shift function generates a linear half-
LSB error that translates to a code-dependent logarithmic error
for odd codes only, as shown in Figure 38. The plot shows the
errors of the odd codes.
LEFT SHIFT RIGHT SHIFT
1111 1111
0000 0000
0111 1111
0000 0001
0011 1111
0000 0010
0000 0100
0001 1111
RIGHT SHIFT
0000 1000
0000 0111
(–6dB)
0001 0000
0000 0011
0010 0000
0000 0001
0100 0000
0000 0000
0000 0000
1000 0000
0000 0000
1111 1111
1111 1111
0000 0000
Increment and Decrement Commands
The increment and decrement command instructions (Command
Instruction 14, Command Instruction 15, Command Instruction 6,
and Command Instruction 7) are useful for the basic servo adjust-
ment application. These commands simplify microcontroller
software coding by eliminating the need to perform a readback
of the current wiper position and then add a 1 to the register
contents using the microcontroller adder. The microcontroller
sends an increment command instruction (Command Instruc-
tion 14) to the digital potentiometer, which automatically moves
the wiper to the next resistance segment position. The master
increment command instruction (Command Instruction 15)
moves all potentiometer 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 Command
Instruction 15 moves the wiper tap position farther from
Terminal B.
LEFT SHIFT
(+6dB)
Figure 37. Detail Left and Right Shift Function
Logarithmic Taper Mode Adjustment
Programming instructions allow decrement and increment wiper
position control by an individual potentiometer or in a ganged
potentiometer arrangement, where both wiper positions are
changed at the same time. These settings are activated by the
6 dB decrement and 6 dB increment command instructions
(Command Instruction 4 and Command Instruction 5, and
Command Instruction 12 and Command Instruction 13,
respectively). For example, starting with the wiper connected
to Terminal B, executing nine increment instructions (Command
Instruction 12) moves the wiper in 6 dB steps from the 0% of the
R
BA
(Terminal B) position to the 100% of the R
BA
position of the
8-bit potentiometer. The 6 dB increment instruction
doubles the value of the RDACx register contents each time the
command is executed. When the wiper position is greater than
midscale, the last 6 dB increment command instruction causes
the wiper to go to the full-scale 255 code position. Any addi-
Actual conformance to a logarithmic curve between the data
contents in the RDACx register and the wiper position for each
Right Shift 4 and Right Shift 5 command execution contains an
error only for the odd codes. The even codes are ideal, with the
exception of zero right shift or greater than half-scale left shift.
(error/code). For example, Code 3 Log_Error = 20 × log10 (0.5/3)
= −15.56 dB, which is the worst case. The plot of Log_Error is
more signifi-cant at the lower codes.
0
–10
–20
GAIN (dB)
–30
LOG_ERROR (CODE) FOR 8-BIT
–40
–50
0
20
40
60
80 100 120 140 160 180 200 220 240 260
CODE, FROM 1 TO 255 BY 2
Figure 38. Plot of Log_Error Conformance for Odd Codes Only
(Even Codes Are Ideal)
Rev. C | Page 17 of 24
02618-038
–60
02618-037
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