AD5273 Analog Devices, AD5273 Datasheet - Page 17

• Manufacturer Part Number: AD5273
• Description: 64-Position OTP Digital Potentiometer
• Manufacturer: Analog Devices

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Resistance Scaling
The AD5273 offers 1 k , 10 k , 50 k , and 100 k nominal
resistances. For users who need to optimize the resolution with
an arbitrary full-range resistance, the following techniques can
be the solutions. Applicable only to the voltage divider mode, by
paralleling a discrete resistor as shown in Figure 19, a propor-
tionately lower voltage appears at terminal A–B. This translates
into a fi ner degree of precision because the step size at terminal
W will be smaller. The voltage can be found as:
Figure 19 shows that the digital potentiometer changes steps lin-
early. On the other hand, log taper adjustment is usually preferred in
applications like volume control. Figure 20 shows another way of
resistance scaling. In this circuit, the smaller the R2 with respect
to R
The wiper voltage is simply:
REV. 0
SDA1
SCL1
V
DD1
Figure 18. Level Shift for Different Voltage Operation
AB
V
V
CONTROLLER
W
W
Figure 20. Resistor Scaling with Log Adjustment
Characteristics
, the more the pseudo log taper characteristic it behaves.
= 2.5V
Figure 19. Lowering the Nominal Resistance
( )
( )
D
D
2.5V
=
=
Rp
R
R
WA
(
3
R
+
AB
( )
R
D
R
WB
AB
/ /
+
Rp
R
( )
S
/ /
R
R1
D
R2
2
B
M1
A
WB
R
V
)
G
I
/ /
2
W
( )
V
R
D
¥
DD
R2
R3
R1
D
2
A
B
256
S
/ /
D
M2
G
R
W
V
2
¥
O
V
¥
D
Rp
DD
V
I
AD5273
2.7V–5.5V
Rp
V
DD2
= 5V
SDA2
SCL2
(10)
(9)
–17–
Resolution Enhancement
Borrowed from ADI’s patented RDAC segmentation technique,
users can confi gure three AD5273s to double the resolution. (See
Figure 21.) First, U3 must be paralleled with a discrete resistor
R
may see that adjusting U1 and U2 together forms the coarse 6-bit
adjustment and adjusting U3 alone forms the fi ner 6-bit adjust-
ment. As a result, the effective resolution becomes 12-bit.
Figure 21. Double the Resolution in Rheostat Mode Operation
RDAC CIRCUIT SIMULATION MODEL
The internal parasitic capacitances and the external capacitive
loads dominate the ac characteristics of the digital potentiometers.
Confi gured as a potentiometer divider, the –3 dB bandwidth of
the AD5273 (1 k resistor) measures 6 MHz at half scale. TPCs
15–18 provide the large signal BODE plot characteristics of the
four available resistor versions 1 k , 10 k , 50 k , and 100 k .
Figure 22 shows a parasitic simulation model. The code following
Figure 22 provides a macro model net list for the 1 k device:
Macro Model Net List for RDAC
.PARAM D = 63, RDAC = 1E3
*
.SUBCKT DPOT (A,W,B)
*
CA
RWA
CW
RWB
CB
*
.ENDS DPOT
P
that is chosen to be equal to a step resistance (R
Figure 22. Circuit Simulation Model for RDAC = 1 k
A
A
W
W
B
0
W
0
B
0
25pF
C
ADJUSTMENT
A
A
U1
U2
COARSE
25E-12
{(1-D/63)*RDAC+60}
55E-12
{D/63*RDAC+60}
25E-12
B1
A2
B2
A1
R
1k
P
W
W1
W2
C
U3
W
55pF
ADJUSTMENT
A3
B3
FINE
W3
B
C
25pF
B
AD5273
P
= R
AB
/64). We