Datasheet
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).
A
W
B
A
W
B
A
W
B
0
3437-050
Figure 26. Rheostat Mode Configuration
The nominal resistance (R
AB
) of the RDAC has 64 contact points
accessed by the wiper terminal, plus Terminal B contact if R
WB
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 wip er contac t
resistance. The second connection is the first tap point, which
corresponds to 219 Ω (R
WB
= 1 × R
AB
/63 + R
W
) 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 × R
AB
/63 + R
W
). Figure 27 shows a simplified diagram of the
equivalent RDAC circuit. The general equation determining R
WB
is
W
AB
WB
RR
D
DR +×=
63
)(
(1)
where:
D is the decimal equivalent of the 6-bit binary code.
R
AB
is the end-to-end resistance.
R
W
is the wiper resistance contributed by the on-resistance of
the internal switch.
Table 6. R
WB
vs. Codes: R
AB
= 10 kΩ; Terminal A Open
D (Dec) R
WB
(Ω) Output State
63 10060 Full-scale (R
AB
+ R
W
)
32 5139 Midscale
1 219 1 LSB
0 60 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
bet ween Termina l W and Terminal B in this st ate 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, R
WA
. When these terminals
are used, Terminal B can be opened or shorted to Terminal W.
Setting the resistance value for R
WA
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
W
ABWA
RR
D
DR +×
−
=
63
63
)(
(2)
Table 7. R
WA
vs. Codes: R
AB
= 10 kΩ; Terminal B Open
D (Dec) R
WA
(Ω) Output State
63 60 Full-scale
32 4980 Midscale
1 9901
1 LSB
0 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.
R
S
R
S
R
S
A
W
B
D5
D4
D3
D2
D1
D0
RDAC
LATCH
AND
DECODER
03437-026
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).
A
W
B
V
I
V
O
0
3437-051
Figure 28. Potentiometer Mode Configuration