ad5258 Analog Devices, Inc., ad5258 Datasheet - Page 14

• Manufacturer Part Number: ad5258
• Description: Nonvolatile, I2c-compatible 64-position, Digital Potentiometer
• Manufacturer: Analog Devices, Inc.

AD5258
THEORY OF OPERATION
The AD5258 is a 64-position digitally controlled variable
resistor (VR) device. The wipers default value prior to pro-
gramming the EEPROM is midscale.
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The nominal resistance (R
and Terminal B is available in 1 kΩ, 10 kΩ, 50 kΩ, and 100 kΩ.
The nominal resistance of the VR has 64 contact points accessed
by the wiper terminal. The 6-bit data in the RDAC latch is
decoded to select one of 64 possible settings.
The general equation determining the digitally programmed
output resistance between Wiper W and Terminal B is
where:
D is the decimal equivalent of the binary code loaded in the
6-bit RDAC register.
R
R
each internal switch.
Note that in the zero-scale condition, there is a relatively
low value finite wiper resistance. Care should be taken to
limit the current flow between Wiper W and Terminal B in
this state to a maximum pulse current of no more than 20 mA.
Otherwise, degradation or destruction of the internal switch
contact may occur.
AB
W
is the wiper resistance contributed by the on resistance of
is the end-to-end resistance.
R
WB
( )
D
A
B
=
Figure 37. AD5258 Equivalent RDAC Circuit
64
Figure 36. Rheostat Mode Configuration
D
D5
D4
D3
D2
D1
D0
DECODER
LATCH
W
RDAC
×
AND
R
AB
+
A
B
AB
2
) of the RDAC between Terminal A
×
R
R
R
R
S
S
S
S
R
W
W
A
B
A
W
B
W
Rev. A | Page 14 of 24
(1)
Similar to the mechanical potentiometer, the resistance of the
RDAC between Wiper W and Terminal A produces a digitally
controlled complementary resistance, R
setting for R
decreases as the data loaded in the latch increases in value.
The general equation for this operation is
Typical device-to-device matching is process lot dependent and
may vary by up to ±30%. For this reason, resistance tolerance is
stored in the EEPROM such that the user will know the actual
R
PROGRAMMING THE POTENTIOMETER DIVIDER
Voltage Output Operation
The digital potentiometer easily generates a voltage divider at
Wiper W-to-Terminal B and Wiper W-to-Terminal A propor-
tional to the input voltage at Terminal A-to-Terminal B. Unlike
the polarity of V
across Terminal A-to-Terminal B, Wiper W-to-Terminal A,
and Wiper W-to-Terminal 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 Wiper W-to-Terminal B starting
at 0 V up to 1 LSB less than 5 V. The general equation defining
the output voltage at V
input voltage applied to Terminal A and Terminal B is
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 internal resistors (R
lute values.
AB
within 0.1%.
V
V
R
W
WA
W
(
( )
D
D
( )
D
)
WA
=
=
Figure 38. Potentiometer Mode Configuration
W
=
) is
R
64
starts at a maximum value of resistance and
D
64
WB
DD
R
V
AB
64
-to-GND, which must be positive, voltage
(
−
D
A
D
)
+
V
W
V
×
I
64
A
with respect to ground for any valid
R
+
64
−
AB
R
D
WA
A
B
R
+
WA
V
AB
2
(
B
D
W
×
and R
)
R
V
W
B
V
WA
WB
O
. The resistance value
) and not the abso-
(2)
(3)
(4)