AD5170 Analog Devices, AD5170 Datasheet - Page 13

• Manufacturer Part Number: AD5170
• Description: 256-Position Two-Time Programmable I2C Digital Potentiometer
• Manufacturer: Analog Devices

The general equation that determines the digitally programmed
output resistance between W and B is
where D is the decimal equivalent of the binary code loaded in
the 8-bit RDAC register, R
R
the internal switch.
In summary, if R
circuited, the output resistance R
codes, as shown in Table 5.
Table 5. Codes and Corresponding R
D (Dec.)
255
128
1
0
Note that in the zero-scale condition, a finite wiper resistance of
100 Ω is present. Care should be taken to limit the current flow
between W and B in this state to a maximum pulse current of
no more than 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 W and terminal A also produces a
digitally controlled complementary resistance, R
terminals are used, the B terminal can be opened. Setting the
resistance value for R
and decreases as the data loaded in the latch increases in value.
The general equation for this operation is
For R
following output resistance R
as shown in Table 6.
Table 6. Codes and Corresponding R
D (Dec.)
255
128
1
0
R
W
WA
is the wiper resistance contributed by the on resistance of
(
D
R
AB
)
WB
= 10 kΩ and the B terminal open-circuited, the
=
(
D
256
)
R
9,961
5,060
139
100
128
=
WB
–
128
D
D
(Ω)
AB
R
139
5,060
9,961
10,060
×
×
= 10 kΩ and the A terminal is open-
WA
R
R
WA
AB
AB
(Ω)
Output State
Full Scale (R
Midscale
1 LSB
Zero Scale (Wiper Contact Resistance)
+
starts at a maximum value of resistance
+
2
AB
2
×
×
is the end-to-end resistance, and
R
WA
R
W
W
is set for the RDAC latch codes,
WB
AB
is set for the RDAC latch
Output State
Full Scale
Midscale
1 LSB
Zero Scale
– 1 LSB + R
WB
WA
Resistance
Resistance
W
WA
)
. When these
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Typical device-to-device matching is process lot dependent and
may vary by up to ±30%. Since the resistance element is pro-
cessed using thin film technology, the change in R
temperature has a very low 35 ppm/°C temperature coefficient.
PROGRAMMING THE POTENTIOMETER DIVIDER
Voltage Output Operation
The digital potentiometer easily generates a voltage divider at
wiper-to-B and wiper-to-A proportional to the input voltage at
A-to-B. Unlike the polarity of V
positive, voltage across A–B, W–A, and W–B can be at either
polarity.
If ignoring the effect of the wiper resistance for approximation,
connecting the A terminal to 5 V and the B terminal to ground
produces an output voltage at the wiper-to-B starting at 0 V up
to 1 LSB less than 5 V. Each LSB of voltage is equal to the
voltage applied across Terminal AB divided by the 256 positions
of the potentiometer divider. The general equation defining the
output voltage at V
voltage applied to Terminals A and B is
For a more accurate calculation, which includes the effect of
wiper resistance, V
Operation of the digital potentiometer in the divider mode
results in a more accurate operation over temperature. Unlike
the rheostat mode, the output voltage is dependent mainly on
the ratio of the internal resistors R
lute values. Thus, the temperature drift reduces to 15 ppm/°C.
V
V
W
W
(
(
D
D
)
)
=
Figure 33. Potentiometer Mode Configuration
=
R
256
D
WB
R
AB
W
W
(
V
D
can be found as
with respect to ground for any valid input
V
A
)
I
V
+
A
256
+
256
A
B
R
−
WA
R
W
D
DD
AB
(
V
D
WA
to GND, which must be
B
)
V
and R
V
O
B
WB
and not the abso-
AB
with
AD5170
(3)
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