Programming The Variable Resistor; Programming The Potentiometer Divider - Analog Devices AD5235 Manual

AD5235

PROGRAMMING THE VARIABLE RESISTOR

Rheostat Operation
The nominal resistance of the RDAC between Terminals A
and B, R , is available with 25 kΩ and 250 kΩ with
AB
1024 positions (10-bit resolution). The final digits of the part
number determine the nominal resistance value, for example,
25 kΩ = 25; 250 kΩ = 250.
The 10-bit data-word in the RDAC latch is decoded to select
one of the 1024 possible settings. The following discussion
describes the calculation of resistance R
25 kΩ part. The wiper's first connection starts at Terminal B for
data 0x000. R (0) is 50 Ω because of the wiper resistance, and
WB
it is independent of the nominal resistance. The second
connection is the first tap point where R
50 Ω = 74.4 Ω for data 0x001. The third connection is the next
tap point representing R (2) = 48.8 Ω + 50 Ω = 98.8 Ω for data
WB
0x002, and so on. Each LSB data value increase moves the wiper
up the resistor ladder until the last tap point is reached at
R (1023) = 25026 Ω. See Figure 43 for a simplified diagram of
WB
the equivalent RDAC circuit. When R
be left floating or tied to the wiper.
100
R
WA
75
50
25
0
0 256
CODE (Decimal)
Figure 44. R (D) and R
WA
The general equation that determines the programmed output
resistance between Wx and Bx is
D
= ×
R ( D ) R
WA AB
1024
where:
D is the decimal equivalent of the data contained in the RDAC
register.
R is the nominal resistance between Terminals A and B.
AB
R is the wiper resistance.
W
For example, the output resistance values in Table 12 are set for
the given RDAC latch codes (applies to R
potentiometers).
at different codes of a
WB
(1) becomes 24.4 Ω +
WB
is used, Terminal A can
WB
R
WB
512 768 1023
(D) vs. Decimal Code
WB
+
R
W
= 25 kΩ digital
AB
Table 12. R
WB
D (DEC) R
WB
1023 25,026
512 12,550
1 74.4
0 50
Note that, in the zero-scale condition, a finite wiper resistance
of 50 Ω 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 that the RDAC replaces, the
AD5235 part is totally symmetrical. The resistance between
Wiper W and Terminal A also produces a digitally controlled
complementary resistance, R
cal programmability of the various terminal connections. When
R is used, Terminal B can be left floating or tied to the wiper.
WA
Setting the resistance value for R
of resistance and decreases as the data loaded in the latch is
increased in value.
The general transfer equation for this operation is
=
R ( D )
WA
For example, the output resistance values in Table 13 are set for
the given RDAC latch codes (applies to R
potentiometers).
Table 13. R (D) at Selected Codes for R
WA
D (DEC) R
WA
1023 74.4
512 12,550
1 25,026
0 25,050
The typical distribution of R
±0.2% within the same package. Device-to-device matching is
process lot dependent upon the worst case of ±30% variation.
However, the change in R
temperature coefficient.
(1)

PROGRAMMING THE POTENTIOMETER DIVIDER

Voltage Output Operation
The digital potentiometer can be configured to generate an
output voltage at the wiper terminal that is proportional to the
input voltages applied to Terminals A and B. For example,
connecting Terminal A to 5 V and Terminal B to ground
produces an output voltage at the wiper that can be any value
from 0 V to 5 V. Each LSB of voltage is equal to the voltage
applied across Terminal AB divided by the 2
resolution of the potentiometer divider.
Rev. B | Page 20 of 28
(D) at Selected Codes for R
(D) (Ω) Output State
Full scale
Midscale
1 LSB
Zero scale (wiper contact resistor)
. Figure 44 shows the symmetri-
WA
starts at a maximum value
WA
−
1024 D
× +
R R
AB W
1024
AB
AB
(D) (Ω) Output State
Full scale
Midscale
1 LSB
Zero scale (wiper contact resistance)
from channel to channel is
AB
with temperature has a 35 ppm/°C
AB
= 25 kΩ
AB
(2)
= 25 kΩ digital
= 25 kΩ
N
position