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AD5112 Datasheet(PDF) 25 Page - Analog Devices

Part No. AD5112
Description  Single-Channel, 128-/64-/32-Position, I2C, ±8% Resistor Tolerance, Nonvolatile Digital Potentiometer
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD5112 Datasheet(HTML) 25 Page - Analog Devices

 
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Data Sheet
AD5110/AD5112/AD5114
Rev. 0 | Page 25 of 28
Similar to the mechanical potentiometer, the resistance of
the RDAC between the W terminal and the A terminal also
produces a digitally controlled complementary resistance, RWA.
RWA also gives a maximum of 8% absolute resistance error. RWA
starts at the maximum resistance value and decreases as the
data loaded into the latch increases. The general equations for
this operation are
AD5110:
W
AB
AW
R
R
R
+
=
Bottom scale (0xFF) (7)
W
AB
AW
R
R
D
D
R
+
×
=
128
128
)
(
From 0x00 to 0x7F (8)
TS
AW
R
R
=
Top scale (0x80) (9)
AD5112:
W
AB
AW
R
R
R
+
=
Bottom scale (0xFF) (10)
W
AB
AW
R
R
D
D
R
+
×
=
64
64
)
(
From 0x00 to 0x3F (11)
TS
AW
R
R
=
Top scale (0x40) (12)
AD5114:
W
AB
AW
R
R
R
+
=
Bottom scale (0xFF) (13)
W
AB
AW
R
R
D
D
R
+
×
=
32
32
)
(
From 0x00 to 0x1F (14)
TS
AW
R
R
=
Top scale (0x20) (15)
where:
D is the decimal equivalent of the binary code in the 5-/6-/7-bit
RDAC register.
RAB is the end-to-end resistance.
RW is the wiper resistance.
RTS is the wiper resistance at top scale.
In the bottom-scale condition or top-scale condition, a finite
total wiper resistance of 45 Ω is present. Regardless of which
setting the part is operating in, take care to limit the current
between Terminal A to Terminal B, Terminal W to Terminal A,
and Terminal W to Terminal B, to the maximum continuous
current of ±6 mA or to the pulse current specified in Table 6.
Otherwise, degradation or possible destruction of the internal
switch contact can occur.
Calculating the Actual End-to-End Resistance
The resistance tolerance is stored in the internal memory
during factory testing. The actual end-to-end resistance can,
therefore, be calculated, which is valuable for calibration,
tolerance matching, and precision applications.
The resistance tolerance in percentage is stored in fixed-point
format, using an 8-bit sign magnitude binary. The data can be
read back by executing Command 6 and setting Bit DB0 (A0).
The MSB is the sign bit (0 = − and 1 = +) and the next four bits
are the integer part, the fractional part is represented by the
three LSBs, as shown in Table 11.
Table 11. Tolerance Format
Data Byte
DB7
DB6
DB5
DB4
DB3
DB2
DB1
DB0
Sign
24
23
22
21
.
2-1
2-2
2-3
For example, if RAB = 10 kΩ and the data readback shows
01010010, the end-to-end resistance can be calculated as,
if,
DB[7] is 0 = negative
DB[6:3] is 1010 = 10
DB[2:0] is 010 = 2 × 2−3 = 0.25
then,
tolerance = −10.25% and, therefore, RAB = 8.975 kΩ
PROGRAMMING THE POTENTIOMETER DIVIDER
Voltage Output Operation
The digital potentiometer easily generates a voltage divider at
wiper-to-B and wiper-to-A that is proportional to the input
voltage at A to B, as shown in Figure 48. Unlike the polarity of
VDD to GND, which must be positive, voltage across A-to-B, W-
to-A, and W-to-B can be at either polarity.
A
VI
W
B
VO
Figure 48. Potentiometer Mode Configuration
Connecting Terminal A to 5 V and Terminal B to ground
produces an output voltage at the Wiper W to Terminal B
ranging from 0 V to 5 V. The general equation defining the
output voltage at VW with respect to ground for any valid input
voltage applied to Terminal A and Terminal B, is:
B
AB
AW
A
AB
WB
W
V
R
D
R
V
R
D
R
D
V
×
+
×
=
)
(
)
(
)
(
(16)
where:
RWB(D) can be obtained from Equation 1 to Equation 6.
RAW(D) can be obtained from Equation 7 to Equation 15.
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, RAW and RWB, and not the
absolute values. Therefore, the temperature drift reduces to
5 ppm/°C.


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