CAT5171

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7

Basic Operation

The CAT5171 is a 256−position digitally controlled

potentiometer. When power is first applied, the wiper

assumes a mid−scale position. Once the power supply is

interface.

Programming: Variable Resistor

Rheostat Mode

nominal value of 50 k

W or 100 kW and has 256 contact

points accessed by the wiper terminal, plus the B terminal

contact. Data in the 8−bit Wiper register is decoded to select

one of these 256 possible settings.

The wiper’s first connection is at the B terminal,

corresponding to control position 0x00. Ideally this would

present a 0

W between the Wiper and B, but just as with a

mechanical rheostat there is a small amount of contact

resistance to be considered, there is a wiper resistance

wiper output with its respective contact point. In CAT5171

this ‘contact’ resistance is typically 50

W. Thus a connection

setting of 0x00 yields a minimum resistance of 50

W

between terminals W and B.

For a 100 k

W device, the second connection, or the first tap

point, corresponds to 441

+ 50

W) for data 0x01. The third connection is the next tap

point, is 831

W (2 x 390.6 + 50 W) for data 0x02, and so on.

Figure 11 shows a simplified equivalent circuit where the

last resistor string will not be accessed; therefore, there is

1 LSB less of the nominal resistance at full scale in addition

to the wiper resistance.

Figure 11. CAT5171 Equivalent DPP Circuit

Wiper

Register

and

Decoder

A

W

B

The equation for determining the digitally programmed

output resistance between W and B is

256

(eq. 1)

where D is the decimal equivalent of the binary code loaded

on resistance of the internal switch.

the indicated Wiper register codes:

D (Dec.)

Output State

255

99,559

128

50,050

Midscale

1

441

1 LSB

0

50

Zero Scale

(Wiper Contact Resistance)

Be aware that in the zero−scale position, the wiper

resistance of 50

W is still present. Current flow between W

and B in this condition should be limited to a maximum

pulsed current of no more than 20 mA. Failure to heed this

restriction can cause degradation or possible destruction of

the internal switch contact.

Similar to the mechanical potentiometer, the resistance of

the DPP (Digitally Programmed Potentiometer) between the

wiper W and terminal A also produces a digitally controlled

used, the B terminal can be opened. Setting the resistance

decreases as the data loaded in the latch increases in value.

The general equation for this operation is

256 * D

256

(eq. 2)

Wiper register codes.

D (Dec.)

Output State

255

441

Full Scale

128

50,050

Midscale

1

99,659

1 LSB

0

100,050

Zero Scale

Typical device to device resistance matching is lot

dependent and may vary by up to

±20%.