EVAL-AD5235EBZ Analog Devices Inc, EVAL-AD5235EBZ Datasheet - Page 22

• Manufacturer Part Number: EVAL-AD5235EBZ
• Description: BOARD EVALUATION FOR AD5235
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD5235EBZ

• Main Purpose: Digital Potentiometer
• Utilized Ic / Part: AD5235
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Secondary Attributes: -
• Embedded: -
• Primary Attributes: -

AD5235
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The nominal resistance of the RDAC between Terminal A
and Terminal B, R
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
part. The first connection of the wiper starts at Terminal B for
Data 0x000. R
it is independent of the nominal resistance. The second connection
is the first tap point where R
for Data 0x001. The third connection is the next tap point
representing R
and so on. Each LSB data value increase moves the wiper up the
resistor ladder until the last tap point is reached at R
25006 Ω. See Figure 45 for a simplified diagram of the equivalent
RDAC circuit. When R
floating or tied to the wiper.
The general equation that determines the programmed output
resistance between Terminal Bx and Terminal Wx is
where:
D is the decimal equivalent of the data contained in the RDAC
register.
R
R
For example, the output resistance values in Table 12 are set for
the given RDAC latch codes (applies to R
potentiometers).
AB
W
is the wiper resistance.
is the nominal resistance between Terminal A and Terminal B.
R
100
WB
75
50
25
0
0
(
D
)
Figure 46. R
WB
=
WB
(0) is 30 Ω because of the wiper resistance, and
1024
R
(2) = 48.8 Ω + 30 Ω = 78.8 Ω for Data 0x002,
WA
D
AB
, is available with 25 kΩ and 250 kΩ with
256
×
WA
R
WB
(D) and R
AB
is used, Terminal A can be left
WB
+
CODE (Decimal)
WB
(1) becomes 24.4 Ω + 30 Ω = 54.4 Ω
R
W
, at different codes of a 25 kΩ
512
WB
(D) vs. Decimal Code
AB
= 25 kΩ digital
768
R
WB
WB
(1023) =
1023
Rev. D | Page 22 of 32
(1)
Table 12. R
D (Dec)
1023
512
1
0
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 symmetrical. The resistance between Wiper W
and Terminal A also produces a digitally controlled complementary
resistance, R
programmability of the various terminal connections. When R
is used, Terminal B can be left floating or tied to the wiper.
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
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 (Dec)
1023
512
1
0
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.
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,
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 A to Terminal B divided by the 2
position resolution of the potentiometer divider.
R
WA
(
D
R
54.4
12,530
25,006
25,030
WB
R
25,006
12,530
54.4
30
WA
WA
)
WA
WB
=
. Figure 46 shows the symmetrical
(D) at Selected Codes for R
(D) at Selected Codes for R
(D) (Ω)
(D) (Ω)
1024
1024
−
D
×
AB
Output State
Full scale
Midscale
1 LSB
Zero scale (wiper contact resistance)
Output State
Full scale
Midscale
1 LSB
Zero scale (wiper contact resistor)
R
with temperature has a 35 ppm/°C
AB
AB
from channel to channel is
+
WA
R
W
starts at a maximum value
AB
AB
AB
= 25 kΩ digital
= 25 kΩ
= 25 kΩ
N
WA
(2)