EVAL-AD5290EBZ Analog Devices Inc, EVAL-AD5290EBZ Datasheet - Page 15

• Manufacturer Part Number: EVAL-AD5290EBZ
• Description: Eval Board For AD5290
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD5290EBZ

• Main Purpose: Digital Potentiometer
• Utilized Ic / Part: AD5290
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Secondary Attributes: -
• Embedded: -
• Primary Attributes: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

THEORY OF OPERATION
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The part operates in the rheostat mode when only two termi-
nals are used as a variable resistor. The unused terminal can
be floating or tied to the W terminal as shown in Figure 26.
The nominal resistance between Terminal A and Terminal B,
R
ance and has 256 tap points accessed by the wiper terminal.
The 8-bit data in the RDAC latch is decoded to select one of
the 256 possible settings. Figure 27 shows a simplified RDAC
structure.
In order to achieve optimum cost performance, Analog Devices
has patented the RDAC segmentation architecture for all the
digital potentiometers. In particular, the AD5290 employs a
3-stage segmentation approach as shown in Figure 27. As
a result, the general equation determining the digitally
programmed output resistance between the W terminal
and B terminal is
AB
8-BIT ADDRESS
, is available in 10 kΩ, 50 kΩ, and 100 kΩ with ±30% toler-
DECODER
R
WB
A
B
(
D
)
=
Figure 27. AD5290 Simplified RDAC Circuit.
W
Figure 26. Rheostat Mode Configuration
256
(R
D
S
4R
4R
4R
4R
4R
= Step Resistor, R
S
S
S
S
S
×
A
B
R
R
AB
W
A
B
+
3
×
2R
2R
2R
2R
R
W
W
W
S
S
S
S
= Wiper Resistor)
R
W
A
B
R
R
S
S
W
R
W
W
Rev. B | Page 15 of 20
(1)
where:
D is the decimal equivalent of the binary code loaded in
the 8-bit RDAC register from 0 to 255.
R
R
resistance of an internal switch.
The AD5290 wiper switch is designed with the transmission
gate CMOS topology and with the gate voltage derived from
V
temperature. Contrary to the temperature coefficient of the R
which is only 35 ppm/°C, the temperature coefficient of the wiper
resistance is significantly higher because the wiper resistance
doubles from 25°C to 125°C. As a result, the user must take into
consideration the contribution of RW on the desirable
resistance. On the other hand, the wiper resistance is insensitive
to the tap point potential. As a result, RW remains relatively flat
at a given V
Assuming that an ideal 10 kΩ part is used, the wiper’s first
connection starts at the B terminal for the programming code
of 0x00 where SWB is closed. The minimum resistance between
Terminal W and Terminal B is, therefore, generally 150 Ω. The
second connection is the first tap point, which corresponds to
189 Ω ( R
and so on. Each LSB data value increase moves the wiper up the
resistor ladder until the last tap point is reached at 10,110 Ω.
In the zero-scale condition, a finite total wiper resistance of
150 Ω is present. Regardless of which setting the part is oper-
ating in, care should be taken to limit the current between
the A terminal to B terminal, W terminal to A terminal, and
W terminal to B terminal, to the maximum dc current of 5 mA
or pulse current of 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 W terminal and the A terminal also
produces a digitally controlled complementary resistance, R
R
the data loaded into the latch increases. The general equation
for this operation is
AB
W
WA
DD
is one of the wiper resistances contributed by the on
is the end-to-end resistance.
. The wiper resistance, RW, is a function of V
starts at the maximum resistance value and decreases as
R
WA
WB
(
D
DD
= 1/256 × R
)
=
and temperature at various codes.
256
256
−
D
AB
×
R
+ 3 R
AB
+
W
3
= 39 Ω + 150 Ω) for code 0x01,
×
R
W
DD
and
AD5290
WA
.
AB
(2)
,