AD5260BRU50-REEL7 View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD5260BRU50-REEL7 Datasheet PDF : 20 Pages
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AD5260/AD5262
Ax
SHDN
RS
D7
RS
D6
D5
D4
RS
D3
D2
D1
D0
Wx
RDAC
LATCH
AND
DECODE
RS
Bx
DIGITAL CIRCUITRY
OMITTED FOR CLARITY
RS = RAB/2N
Figure 11. Simplified RDAC Architecture
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The nominal resistances of the RDAC between terminals A and B
are available with values of 20 kW, 50 kW, and 200 kW. The final
three digits of the part number determine the nominal resistance
value, e.g., 20 kW = 20; 50 kW = 50; 200 kW = 200. The nominal
resistance (RAB) of the VR has 256 contact points accessed by the
wiper terminal, plus the B terminal contact. The 8-bit data in the
RDAC latch is decoded to select one of the 256 possible settings.
Assuming a 20 kW part is used, the wiper’s first connection starts
at the B terminal for data 00H. Since there is a 60 W wiper contact
resistance, such connection yields a minimum of 60 W resistance
between terminals W and B. The second connection is the first tap
point corresponds to 138 W (RWB = RAB/256 ϩ RW = 78 W ϩ 60 W)
for data 01H. The third connection is the next tap point represent-
ing 216 W (78 ϫ 2 ϩ 60) for data 02H and so on. Each LSB data
value increase moves the wiper up the resistor ladder until the last
tap point is reached at 19982 W [RAB Ϫ 1 LSB ϩ RW]. The wiper
does not directly connect to the B terminal. See Figure 11 for a
simplified diagram of the equivalent RDAC circuit.
The general equation determining the digitally programmed
output resistance between W and B is:
() D
R WB D = 256 ¥ R AB + R W
(1)
where D is the decimal equivalent of the binary code which is
loaded in the 8-bit RDAC register, and RAB is the nominal end-
to-end resistance.
For example, RAB = 20 kW, when VB = 0 V and A–terminal is
open circuit, the following output resistance values RWB will be
set for the following RDAC latch codes. The result will be the
same if terminal A is tied to W:
D
(DEC)
256
128
1
0
RWB
(W)
19982
10060
138
60
Output State
Full-Scale (RAB – 1 LSB + RW)
Mid-Scale
1 LSB
Zero-Scale (wiper contact resistance)
Note that in the zero-scale condition a finite wiper resistance of
60 W is present. Care should be taken to limit the current flow
between W and B in this state to no more than 20 mA to avoid
degradation or possible destruction of the internal switches.
Like the mechanical potentiometer the RDAC replaces, the
AD5260/AD5262 parts are totally symmetrical. The resistance
between the wiper W and terminal A also produces a digitally
controlled complementary resistance RWA. Figure 12 shows the
symmetrical programmability of the various terminal connections.
When RWA is used, the B–terminal can be let floating or tied to the
wiper. Setting the resistance value for RWA starts at a maximum
value of resistance and decreases as the data loaded in the latch
is increased in value. The general equation for this operation is:
( ) RWA
D
=
256 - D
256
¥
R AB
+
RW
(2)
For example, RAB = 20 kW, when VA = 0 V and B–terminal is open,
the following output resistance RWA will be set for the following
RDAC latch codes. The result will be the same if terminal B is
tied to W:
D
(DEC)
256
128
1
0
RWA
(W)
60
10060
19982
20060
Output State
Full-Scale
Mid-Scale
1 LSB
Zero-Scale
20
RWA
16
RWB
12
8
4
RAB = 20K⍀
0
0
64
128
192
256
D – CODE in decimal
Figure 12. AD5260/AD5262 Equivalent RDAC Circuit
The typical distribution of the nominal resistance RAB from
channel to channel matches within ±1%. Device-to-device match-
ing is process lot dependent with the worst case of ±30% variation.
On the other hand, since the resistance element is processed in
thin film technology, the change in RAB with temperature has a
low 35 ppm/∞C temperature coefficient.
REV. 0
–9–
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