AD5204/AD5206

Rev. C | Page 13 of 20

PROGRAMMING THE VARIABLE RESISTOR

RHEOSTAT OPERATION

The nominal resistance of the RDAC between Terminal A and

Terminal B is available with values of 10 kΩ, 50 kΩ, and 100 kΩ.

The last digits of the part number determine the nominal

resistance value; for example, 10 kΩ = 10 and 100 kΩ = 100.

accessed by the wiper terminal, plus Terminal B contact. The

8-bit data-word in the RDAC latch is decoded to select one of

the 256 possible settings. The first connection of the wiper starts

at Terminal B for the 0x00 data. This Terminal B connection has a

wiper contact resistance of 45 Ω. The second connection (for a

third connection is the next tap point, representing 78 + 45 =

123 Ω for the 0x02 data. Each LSB data value increase moves

the wiper up the resistor ladder until the last tap point is

reached at 10,006 Ω. The wiper does not directly connect to

Terminal A. See Figure 21 for a simplified diagram of the

equivalent RDAC circuit.

The general transfer equation determining the digitally

programmed output resistance between the Wx and Bx

terminals is

(1)

where Dx is the data contained in the 8-bit RDACx latch, and

output resistance values are set as outlined in Table 7 for the

RDAC latch codes (applies to the 10 kΩ potentiometer).

Table 7. Output Resistance Values for the RDAC Latch Codes—

D (Dec)

Output State

255

10006

Full scale

128

5045

Midscale (PR = 0 condition)

1

84

1 LSB

0

45

Zero scale (wiper contact resistance)

In the zero-scale condition, a finite total wiper resistance of 45 Ω

is present. Regardless of which setting the part is operating in,

care should be taken to limit the current between Terminal A to

Terminal B, Wiper W to Terminal A, and Wiper W to Terminal

B, to the maximum continuous current of ±5.65 mA(10 kΩ) or

±1.35 mA(50 kΩ and 100 kΩ) or pulse current of ±20 mA.

Otherwise, degradation or possible destruction of the internal

switch contact, can occur.

Like the mechanical potentiometer that the RDAC replaces,

the RDAC is completely symmetrical. The resistance between

Wiper W and Terminal A produces a digitally controlled

starts at a maximum value of resistance and decreases as the

data loaded to the latch is increased in value. The general

transfer equation for this operation is

(2)

where Dx is the data contained in the 8-bit RDACx latch, and

the output resistance values outlined in Table 8 are set for the

RDAC latch codes.

Table 8. Output Resistance Values for the RDAC Latch Codes—

D (DEC)

Output State

255

84

Full scale

128

5045

Midscale (PR = 0 condition)

1

10006

1 LSB

0

10045

Zero scale

to within ±1%. However, device-to-device matching is process

terms of temperature has a 700 ppm/°C temperature coefficient.