Datasheet

Data Sheet AD5232
Rev. C | Page 19 of 24
Table 11. Nominal Individual Segment Resistor Values (Ω)
Device
Resolution
Segmented Resistor Size
for R
AB
End-to-End Values
10 kΩ
Version
50 kΩ
Version
100 kΩ
Version
8-Bit 78.10 390.5 781.0
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The nominal resistances of the RDACx between Terminal A and
Terminal B are available with values of 10 kΩ, 50 kΩ, and 100 kΩ.
The final digits of the part number determine the nominal
resistance value; for example, 10 kΩ = 10; 100 kΩ = 100. The
nominal resistance (R
AB
) of the AD5232 VR has 256 contact
points accessed by Wiper Terminal W, plus the Terminal B contact.
The 8-bit data-word in the RDACx latch is decoded to select
one of the 256 possible settings.
The general transfer equation, which determines the digitally
programmed output resistance between Wx and Bx, is
W
AB
WB
RR
D
DR 
256
)(
(1)
where:
D is the decimal equivalent of the data contained in the RDACx
register.
R
AB
is the nominal resistance between Terminal A and Terminal B.
R
W
is the wiper resistance.
Table 12 lists the output resistance values that are set for the
RDACx latch codes shown for 8-bit, 10 kΩ potentiometers.
Table 12. Nominal Resistance Value at Selected Codes for
R
AB
= 10 kΩ
D (Dec) R
WB
(D) (Ω) Output State
255 10011 Full scale
128 5050 Midscale
1 89 1 LSB
0 50 Zero scale
1
(wiper contact resistance)
1
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 Wx and Bx
in this state to a maximum continuous value of 2 mA to avoid degradation
or possible destruction of the internal switch metallization. Intermittent
current operation to 20 mA is allowed.
Like the mechanical potentiometer that the RDACx replaces,
the AD5232 parts are totally symmetrical. The resistance between
the Wiper Terminal W and Terminal A a l s o produces a dig itally
controlled resistance, R
WA
. Figure 41 shows the symmetrical
programmability of the various terminal connections.
100
75
50
25
0
0 64 128 192 258
CODE (Decimal)
PERCENT OF NOMINAL
END-TO-END RESISTANCE (% R
AB
)
02618-041
R
WB
R
WA
Figure 41. Symmetrical RDAC Operation
When these terminals are used, Terminal B should be tied to
the wiper. Setting the resistance value for R
WA
starts at a maximum
value of resistance and decreases as the data loaded in the latch
is increased in value. The general transfer equation for this
operation is
W
ABWA
RR
D
DR 


256
256
)(
(2)
where:
D is the decimal equivalent of the data contained in the RDAC
register.
R
AB
is the nominal resistance between Terminal A and Terminal B.
R
W
is the wiper resistance.
Table 13 lists the output resistance values that are set for the
RDACx latch codes shown for 8-bit, 10 kΩ potentiometers.
Table 13. Nominal Resistance Value at Selected Codes for
R
AB
= 10 kΩ
D (Dec) R
WA
(D) (Ω) Output State
255 89 Full scale
128 5050 Midscale
1 10011 1 LSB
0 10050 Zero scale
The multichannel AD5232 has a ±0.2% typical distribution of
internal channel-to-channel R
BA
match. Device-to-device matching
is dependent on process lot and exhibits a −40% to +20% variation.
The change in R
BA
with temperature has a 600 ppm/°C temperature
coefficient.