Datasheet
AD5233
Rev. B | Page 21 of 32
The general equation that determines the programmed output
resistance between W and B is
W
AB
WB
RR
D
DR +×=
64
)(
(1)
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.
For example, the output resistance values in Table 10 are set for
the given RDAC latch codes with V
DD
= 5 V (applies to R
AB
=
10 kΩ digital potentiometers).
Table 10. R
WB
(D) at Selected Codes for R
AB
= 10 kΩ
D (Decimal) R
WB
(D) (Ω) Output State
63 9858 Full scale
32 5015 Midscale
1 171 1 LSB
0 15 Zero scale (wiper contact resistor)
Note that in the zero-scale condition a finite wiper resistance of
15 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
AD5233 part is totally symmetrical. The resistance between
Wiper W and Terminal A also produces a digitally controlled
complementary resistance, R
WA
. Figure 44 shows the symmetrical
programmability of the various terminal connections. When
R
WA
is used, Terminal B can be left floating or 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 +×
−
=
64
64
)(
(2)
For example, the output resistance values in Table 11 are set for
the RDAC latch codes with V
DD
= 5 V (applies to R
AB
= 10 kΩ
digital potentiometers).
Table 11. R
WA
(D) at Selected Codes for R
AB
= 10 kΩ
D (Decimal) R
WA
(D) (Ω) Output State
63 171 Full scale
32 5015 Midscale
1 9858 1 LSB
0 10015 Zero scale
Channel-to-channel R
AB
matching is better than 1%. The
change in R
AB
with temperature has a 600 ppm/°C temperature
coefficient.
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 Terminal A and Terminal B. For
example, con ne cting Terminal A to 5 V and Ter minal 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 A and Terminal B divided
by the 2
N
position resolution of the potentiometer divider.
Because AD5233 can also be supplied by dual supplies, the
general equation defining the output voltage at V
W
with respect
to ground for any given input voltages applied to the A and B
terminals is
B
AB
W
VV
D
DV +×=
64
)(
(3)
Equation 3 assumes that V
W
is buffered so that the effect of
wiper resistance is minimized. Operation of the digital potenti-
ometer in divider mode results in more accurate operation over
temperature. Here, the output voltage is dependent on the ratio
of the internal resistors and not the absolute value; therefore,
the drift improves to 15 ppm/°C. There is no voltage polarity
restriction among the A, B, and W terminals as long as the
terminal voltage (V
TERM
) stays within V
SS
< V
TERM
< V
DD
.
PROGRAMMING EXAMPLES
The following programming examples illustrate a typical
sequence of events for various features of the AD5233. See
Table 7 for the instructions and data-word format. The
instruction numbers, addresses, and data appearing at the
SDI and SDO pins are in hexadecimal format.
Table 12. Scratchpad Programming
SDI SDO Action
0xB010 0xXXXX
Writes Data 0x10 into RDAC register,
Wiper W1 moves to ¼ full-scale position.
Table 13. Incrementing RDAC1 Followed by Storing the
Wiper Setting to EEMEM1
SDI SDO Action
0xB010 0xXXXX
Writes Data 0x10 into RDAC register,
Wiper W1 moves to ¼ full-scale
position.
0xE0XX 0xB010
Increments the RDAC register by one
to 0x11.
0xE0XX 0xE0XX
Increments the RDAC register by one
to 0x12. Continues until desired wiper
position is reached.
0x20XX 0xXXXX
Stores the RDAC register data into
EEMEM1. Optionally tie WP
to GND to
protect EEMEM values.