AD5171BRJ50-R2 datasheet(13/24 Pages) AD

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AD5171

Rev. D | Page 13 of 24

VARIABLE RESISTANCE AND VOLTAGE FOR

RHEOSTAT MODE

If only the W-to-B or W-to-A terminals are used as variable

resistors, the unused terminal can be opened or shorted with

Terminal W. This operation is called rheostat mode (see Figure 26).

Figure 26. Rheostat Mode Configuration

The nominal resistance (RAB) of the RDAC has 64 contact points

accessed by the wiper terminal, plus Terminal B contact if RWB is

considered. The 6-bit data in the RDAC latch is decoded to

select one of the 64 settings. Assuming that a 10 kΩ part is used,

the first connection of the wiper starts at Terminal B for Data 0x00.

Such a connection yields a minimum of 60 Ω resistance between

Terminal W and Terminal B due to the 60 Ω wiper contact

resistance. The second connection is the first tap point, which

corresponds to 219 Ω (RWB = 1 × RAB/63 + RW) for Data 0x01,

and so on. Each LSB data value increase moves the wiper up

the resistor ladder until the last tap point is reached at 10,060 Ω

(63 × RAB/63 + RW). Figure 27 shows a simplified diagram of the

equivalent RDAC circuit. The general equation determining RWB is

)

(

(1)

where:

D is the decimal equivalent of the 6-bit binary code.

RAB is the end-to-end resistance.

RW is the wiper resistance contributed by the on-resistance of

the internal switch.

Table 6. RWB vs. Codes: RAB = 10 kΩ; Terminal A Open

D (Dec)

RWB (Ω)

Output State

10060

Full-scale (RAB + RW)

5139

Midscale

219

1 LSB

Zero-scale (wiper contact resistance)

Because a finite wiper resistance of 60 Ω is present in the zero-

scale condition, care should be taken to limit the current flow

between Terminal W and Terminal B in this state to a maximum

pulse current 20 mA. Otherwise, degradation or possible

destruction of the internal switch contact can occur.

Similar to the mechanical potentiometer, the resistance of the

RDAC between the wiper (Terminal W) and Terminal A also

produces a complementary resistance, RWA. When these terminals

are used, Terminal B can be opened or shorted to Terminal W.

Setting the resistance value for RWA starts at a maximum value

of resistance and decreases as the data loaded in the latch

increases in value. The general equation for this operation is

−

)

(

(2)

Table 7. RWA vs. Codes: RAB = 10 kΩ; Terminal B Open

D (Dec)

RWA (Ω)

Output State

Full-scale

4980

Midscale

9901

1 LSB

10060

Zero-scale

The typical distribution of the resistance tolerance from device

to device is process-lot dependent; it is possible to have ±30%

tolerance.

RDAC

LATCH

AND

DECODER

Figure 27. AD5171 Equivalent RDAC Circuit

VARIABLE RESISTANCE AND VOLTAGE FOR

POTENTIOMETER MODE

If all three terminals are used, the operation is called the

potentiometer mode. The most common configuration is the

voltage divider operation (see Figure 28).

Figure 28. Potentiometer Mode Configuration

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