ad5171 Analog Devices, Inc., ad5171 Datasheet - Page 14

• Manufacturer Part Number: ad5171
• Description: 64-position Otp Digital Potentiometer
• Manufacturer: Analog Devices, Inc.

AD5171
POWER-UP/POWER-DOWN SEQUENCES
Similarly, because of the ESD protection diodes, it is important
to power V
Terminal B, and Terminal W. Otherwise, the diode is forward-
biased such that V
the remainder of the users’ circuits. The ideal power-up
sequence is in the following order: GND, V
and V
digital inputs is not important as long as they are powered after
V
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 (Figure 30).
The nominal resistance (R
accessed by the wiper terminal, plus the B terminal contact if
R
to select one of the 64 settings. Assuming that a 10 kΩ part is
used, the wiper’s first connection 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 (R
0x01, and so on. Each LSB data value increase moves the wiper
up the resistor ladder until the last tap point is reached at
10060 Ω (63 × R
diagram of the equivalent RDAC circuit. The general equation
determining R
where:
D is the decimal equivalent of the 6-bit binary code.
R
R
the internal switch.
Table 5. R
D (Dec)
63
32
1
0
WB
AB
W
DD
is the wiper resistance contributed by the on-resistance of
. Similarly, V
is the end-to-end resistance.
is considered. The 6-bit data in the RDAC latch is decoded
R
A
WB
/V
A
B
(
B
D
WB
/V
DD
)
W
R
10060
5139
219
60
=
vs. Codes:R
first before applying any voltages to Terminal A,
. The order of powering V
WB
WB
W
Figure 30. Rheostat Mode Configuration
63
D
AB
DD
is
(Ω)
×
/63 + R
DD
should be powered down last.
R
is powered unintentionally and may affect
AB
+
A
B
Output State
Full-scale (R
Midscale
1 LSB
Zero-scale (wiper contact resistance)
AB
W
AB
R
). Figure 31 shows a simplified
W
) of the RDAC has 64 contact points
= 10 kΩ; Terminal A Open
WB
W
= 1 × R
AB
+ R
A
AB
, V
W
A
B
DD
/63 + R
)
B
, digital inputs,
, V
W
W
, and the
W
) for data
Rev. B | Page 14 of 24
(1)
Since 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, R
terminals are used, Terminal B can be opened or shorted to
Terminal W. Setting the resistance value for R
maximum value of resistance and decreases as the data loaded
in the latch increases in value. The general equation for this
operation is
Table 6. R
D (Dec)
63
32
1
0
The typical distribution of the resistance tolerance from device
to device is process-lot dependent; it is possible to have ±30%
tolerance.
R
WA
(
D
WA
)
=
R
60
4980
9901
10060
vs. Codes: R
Figure 31. AD5171 Equivalent RDAC Circuit
WA
63
63
(Ω)
−
D5
D4
D3
D2
D1
D0
DECODER
LATCH
D
RDAC
AND
×
R
AB
Output State
Full-scale
Midscale
1 LSB
Zero-scale
AB
+
=10 kΩ;Terminal B Open
R
R
R
W
R
S
S
S
WA
. When these
A
W
B
WA
starts at a
(2)