Datasheet

Data Sheet ADN2850
Rev. E | Page 19 of 28
PROGRAMMING THE VARIABLE RESISTOR
The nominal resistance of the RDAC between Terminal W and
Terminal B, R
WB
, is available with 25 kΩ and 250 kΩ with
1024 positions (10-bit resolution). The final digits of the part
number determine the nominal resistance value, for example,
25 kΩ = 24.4 Ω; 250 kΩ = 244 Ω.
The 10-bit data-word in the RDAC latch is decoded to select one of
the 1024 possible settings. The following description provides the
calculation of resistance, R
WB
, at different codes of a 25 kΩ part.
The first connection of the wiper starts at Terminal B for
Data 0x000. R
WB
(0) is 30 Ω because of the wiper resistance, and
it is independent of the nominal resistance. The second connection
is the first tap point where R
WB
(1) becomes 24.4 Ω + 30 Ω = 54.4 Ω
for Data 0x001. The third connection is the next tap point
representing R
WB
(2) = 48.8 Ω + 30 Ω = 78.8 Ω for Data 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
WB
(1023) =
25006 Ω. See Figure 32 for a simplified diagram of the equivalent
RDAC circuit.
CODE (Decimal)
100
75
0
0 1023256
R
WB
(D) (% R
WF
)
512 768
50
25
R
WB
02660-045
Figure 33. R
WB
(D) vs. Decimal Code
The general equation that determines the programmed output
resistance between Terminal Bx and Terminal Wx is
WNOMWBWB
RR
D
DR +×=
_
1024
)(
(1)
where:
D is the decimal equivalent of the data contained in the RDAC
register.
R
WB_NOM
is the nominal resistance value
R
W
is the wiper resistance.
Table 13. R
WB
(D) at Selected Codes for R
WB_NOM
= 25 kΩ
D (Dec) R
WB
(D) (Ω) Output State
1023
25,006 Full scale
512 12,530 Midscale
1
54.4 1 LSB
0
30
Zero scale (wiper contact resistor)
Note that, in the zero-scale condition, a finite wiper resistance
of 30 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.
The typical distribution of R
WB_NOM
from channel to channel is
±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
WB
at full scale with temperature has a
35 ppm/°C temperature coefficient.
PROGRAMMING EXAMPLES
The following programming examples illustrate a typical sequence
of events for various features of the ADN2850. See Table 8 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 14. Scratchpad Programming
SDI SDO Action
0xB00100
0xXXXXXX Writes Data 0x100 into RDAC1 register,
Wiper W1 moves to 1/4 full-scale
position.
0xB10200
0xB00100 Loads Data 0x200 into RDAC2 register,
Wiper W2 moves to 1/2 full-scale
position.
Table 15. Incrementing RDAC Followed by Storing the
Wiper Setting to EEMEM
SDI
SDO
Action
0xB00100 0xXXXXXX Writes Data 0x100 into RDAC1
register, Wiper W1 moves to 1/4 full-
scale position.
0xE0XXXX
0xB00100 Increments RDAC1 register by one to
0x101.
0xE0XXXX
0xE0XXXX Increments RDAC1 register by one to
0x102. Continue until desired wiper
position is reached.
0x20XXXX
0xXXXXXX Stores RDAC2 register data into
EEMEM1. Optionally, tie
AA
WP
EE
AA to GND to
protect EEMEM values.
The EEMEM values for the RDACs can be restored by power-
on, by strobing the
AA
PR
EE
AA pin, or by the two commands shown in
Table 16.
Table 16. Restoring the EEMEM Values to RDAC Registers
SDI SDO Action
0x10XXXX
0xXXXXXX Restores the EEMEM1 value to the
RDAC1 register.