EVAL-AD5254EBZ Analog Devices Inc, EVAL-AD5254EBZ Datasheet - Page 24

• Manufacturer Part Number: EVAL-AD5254EBZ
• Description: Evaluation Board
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD5254EBZ

• Main Purpose: Digital Potentiometer
• Utilized Ic / Part: AD5245
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Secondary Attributes: -
• Embedded: -
• Primary Attributes: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD5253/AD5254
DIGITAL POTENTIOMETER OPERATION
The structure of the RDAC is designed to emulate the
performance of a mechanical potentiometer. The RDAC
contains a string of resistor segments with an array of analog
switches that act as the wiper connection to the resistor array.
The number of points is the resolution of the device. For
example, the AD5253/AD5254 emulate 64/256 connection
points with 64/256 equal resistance, R
provide better than 1.5%/0.4% resolution.
Figure 41 provides an equivalent diagram of the connections
between the three terminals that make up one channel of the
RDAC. Switches SW
switches SW(0) to SW(2
the setting decoded from the data bit). Because the switches are
nonideal, there is a 75 Ω wiper resistance, R
is a function of supply voltage and temperature: Lower supply
voltages and higher temperatures result in higher wiper
resistances. Consideration of wiper resistance dynamics is
important in applications in which accurate prediction of
output resistance is required.
R
DIGITAL
CIRCUITRY
OMIITTED FOR
CLARITY
REGISTER
S
DECODER
= R
WIPER
RDAC
AND
Figure 41. Equivalent RDAC Structure
AB
/2
A
N
and SW
N–1
R
R
R
) can be on at a time (determined by
S
S
S
B
are always on, but only one of
SW (2
SW (2
SW(1)
SW(0)
SW
SW
N
N
A
B
– 1)
– 2)
S
, allowing them to
W
A
B
W
X
X
X
. Wiper resistance
Rev. B | Page 24 of 32
PROGRAMMABLE RHEOSTAT OPERATION
If either the W-to-B or W-to-A terminal is used as a variable
resistor, the unused terminal can be opened or shorted with W;
such operation is called rheostat mode (see Figure 42). The
resistance tolerance can range ±20%.
The nominal resistance of the AD5253/AD5254 has 64/256
contact points accessed by the wiper terminal, plus the B
terminal contact. The 6-/8-bit data-word in the RDAC register
is decoded to select one of the 64/256 settings. The wiper’s first
connection starts at the B terminal for Data 0x00. This B termi-
nal connection has a wiper contact resistance, R
regardless of the nominal resistance. The second connection
(the AD5253 10 kΩ part) is the first tap point where R
(R
Each LSB data value increase moves the wiper up the resistor
ladder until the last tap point is reached at R
Figure 41 for a simplified diagram of the equivalent RDAC circuit.
The general equation that determines the digitally programmed
output resistance between W and B is
where:
D is the decimal equivalent of the data contained in the
RDAC latch.
R
AB
WB
is the nominal end-to-end resistance.
AD5253: RWB(D) = (D/64) × RAB + 75 Ω
AD5254: RWB(D) = (D/256) × RAB + 75 Ω
= R
A
B
AB
/64 + R
W
Figure 42. Rheostat Mode Configuration
W
= 156 Ω + 75 Ω) for Data 0x01, and so on.
A
B
W
A
B
WB
= 9893 Ω. See
W
, of 75 Ω,
W
WB
= 231 Ω
(1)
(2)