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Data Sheet

AD5232

Rev. C | Page 17 of 24

ADVANCED CONTROL MODES

The AD5232 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

•

Fourteen extra bytes of user-addressable, electrical erasable

memory

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.

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

RBA (Terminal B) position to the 100% of the RBA position of the

AD5232 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-

tional 6 dB instruction does not change the wiper position from

full scale (RDACx register code = 255).

Figure 37 illustrates the operation of the 6 dB shifting function

on the individual RDACx register data bits for the 8-bit AD5232

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

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

0000 0000

0000 0001

0000 0010

0000 0100

0000 1000

0001 0000

0010 0000

0100 0000

1000 0000

1111 1111

RIGHT SHIFT

1111 1111

0111 1111

0011 1111

0001 1111

0000 0111

0000 0011

0000 0001

0000 0000

LEFT SHIFT

(+6dB)

RIGHT SHIFT

(–6dB)

Figure 37. Detail Left and Right Shift Function

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.

Figure 38 shows plots of Log_Error, that is, 20 × log10

(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.

–10

–20

–30

–40

–50

–60

80 100 120 140 160 180 200 220 240 260

CODE, FROM 1 TO 255 BY 2

LOG_ERROR (CODE) FOR 8-BIT

Figure 38. Plot of Log_Error Conformance for Odd Codes Only

(Even Codes Are Ideal)

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