REV. 0

–10–

AD5232

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.

0000 0000

0000 0001

0000 0010

0000 0100

0000 1000

0001 0000

0010 0000

0100 0000

1000 0000

1111 1111

1111 1111

LEFT SHIFT

1111 1111

0111 1111

0011 1111

0001 1111

0000 1111

0000 0111

0000 0011

0000 0001

0000 0000

0000 0000

0000 0000

RIGHT SHIFT

LEFT

SHIFT

(+6 dB)

RIGHT

SHIFT

(–6 dB)

Figure 6. Detail Left and Right Shift Function for the

8-Bit 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

× log 10 (0.5/3) = –15.56 dB,

which is the worst case. The plot of Log_Error is more signifi-

cant at the lower codes.

CODE, FROM 1 TO 255 BY 2

20

40

60

80 100

0

120 140 160 180 200 220 240 260

0

–10

–20

–30

–40

–50

–60

LOG_ERROR (CODE) FOR 8-BIT

Figure 7. Plot of Log_Error Conformance for Odd Codes

Only (Even Codes Are Ideal)

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.

• 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.

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

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.