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AD5231

Data Sheet

Rev. D | Page 18 of 28

ADVANCED CONTROL MODES

The AD5231 digital potentiometer includes a set of user

programming features to address the wide number of

applications for these universal adjustment devices.

Key programming features include:

• Scratchpad programming to any desirable values

• Nonvolatile memory storage of the scratchpad RDAC register

value in the EEMEM register

• Increment and decrement instructions for the RDAC wiper

• Left and right bit shift of the RDAC wiper register to achieve

±6 dB level changes

• 28 extra bytes of user-addressable nonvolatile memory

Linear Increment and Decrement Instructions

The increment and decrement instructions (14, 15, 6, and 7) are

useful for linear step-adjustment applications. These commands

simplify microcontroller software coding by allowing the

controller to send just an increment or decrement command to

the device.

For an increment command, executing Instruction 14 with the

proper address automatically moves the wiper to the next

resistance segment position. Instruction 15 performs the same

function, except that the address does not need to be specified.

Logarithmic Taper Mode Adjustment

Four programming instructions produce logarithmic taper

increment and decrement of the wiper. These settings are

activated by the 6 dB increment and 6 dB decrement

instructions (12, 13, 4, and 5). For example, starting at zero

scale, executing the increment Instruction 12 eleven times

moves the wiper in 6 dB per step from 0% to full scale, RAB. The

6 dB increment instruction doubles the value of the RDAC

wiper position is near the maximum setting, the last 6 dB

increment instruction causes the wiper to go to the full-scale

1023 code position. Further 6 dB per increment instructions do

not change the wiper position beyond its full scale.

The 6 dB step increments and 6 dB step decrements are

achieved by shifting the bit internally to the left or right,

respectively. The following information explains the nonideal

±6 dB step adjustment under certain conditions. Table 8

illustrates the operation of the shifting function on the RDAC

operation. Note that the left-shift 12 and 13 instructions were

modified such that, if the data in the RDAC register is equal to

zero and the data is shifted left, the RDAC register is then set to

Code 1. Similarly, if the data in the RDAC register is greater

than or equal to midscale and the data is shifted left, then the

data in the RDAC register is automatically set to full scale. This

makes the left-shift function as ideal a logarithmic adjustment

as possible.

The right-shift 4 and 5 instructions are ideal only if the LSB is 0

(ideal logarithmic = no error). If the LSB is 1, the right-shift

function generates a linear half-LSB error, which translates to

a number-of-bits dependent logarithmic error, as shown in

Figure 42. The plot shows the error of the odd numbers of bits

for the AD5231.

Table 8. Detail Left-Shift and Right-Shift Functions

for 6 dB Step Increment and Decrement

Left-Shift

(+6 dB/step)

Left-Shift

Right-Shift

(–6 dB/step)

00 0000 0000

11 1111 1111

00 0000 0001

01 1111 1111

00 0000 0010

00 1111 1111

00 0000 0100

00 0111 1111

00 0000 1000

00 0011 1111

00 0001 0000

00 0001 1111

00 0010 0000

00 0000 1111

00 0100 0000

00 0000 0111

00 1000 0000

00 0000 0011

01 0000 0000

00 0000 0001

10 0000 0000

00 0000 0000

11 1111 1111

00 0000 0000

11 1111 1111

00 0000 0000

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 odd numbers of bits. Even numbers of bits are ideal. The

graph in Figure 42 shows plots of Log_Error [20 × log10

(error/code)] for the AD5231. 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 significant at the lower codes.

CODE (From 1 to 1023 by 2.0 × 103)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

–40

–20

–60

–80

Figure 42. Plot of Log_Error Conformance for Odd Numbers of Bits Only

(Even Numbers of Bits Are Ideal)

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