ad5171 Analog Devices, Inc., ad5171 Datasheet - Page 20

• Manufacturer Part Number: ad5171
• Description: 64-position Otp Digital Potentiometer
• Manufacturer: Analog Devices, Inc.

AD5171
For log taper adjustment, such as volume control, Figure 45
shows another way of resistance scaling. In this circuit, the
smaller the R2 with respect to R
pseudo log taper characteristic. The wiper voltage is simply
RESOLUTION ENHANCEMENT
The resolution can be doubled in the potentiometer mode of
operation by using three digital potentiometers. Borrowed from
ADI’s patented RDAC segmentation technique, users can con-
figure three AD5171s (Figure 46) to double the resolution. First,
U3 must be parallel with a discrete resistor, R
to be equal to a step resistance (R
U2 together forms the coarse 6-bit adjustment, and adjusting U3
alone forms the finer 6-bit adjustment. As a result, the effective
resolution becomes 12-bit.
V
Figure 45. Resistor Scaling with Log Adjustment Characteristics
W
(
D
)
=
R
WA
(
Figure 46. Doubling the Resolution
R
ADJUSTMENT
WB
+
COARSE
U1
U2
R
||
WB
A1
A2
B2
B1
R
R1
V
B
A
2
I
||
W1
W2
)
R
R
2
P
W
R2
ADJUSTMENT
×
AB
V
P
, the more it behaves like the
U3
I
FINE
= R
V
O
A3
B3
AB
W3
/64). Adjusting U1 and
P
, which is chosen
Rev. B | Page 20 of 24
(6)
RDAC CIRCUIT SIMULATION MODEL
The internal parasitic capacitances and the external capacitive
loads dominate the ac characteristics of the digital potentio-
meters. Configured as a potentiometer divider, the –3 dB
bandwidth of the AD5171 (5 kΩ resistor) measures 1.5 MHz at
half scale. Figure 14 to Figure 17 provide the large signal BODE
plot characteristics of the four available resistor versions: 5 kΩ,
10 kΩ, 50 kΩ, and 100 kΩ. A parasitic simulation model is
shown in Figure 47. Listing 1 provides a macro model net list
for the 10 kΩ device.
Listing 1. Macro Model Net List for RDAC
.PARAM D=64, RDAC=10E3
*
.SUBCKT DPOT (A,W,B)
*
CA
RWA
CW
RWB
CB
*
.ENDS DPOT
A
A
W
W
B
Figure 47. Circuit Simulation Model for RDAC = 10 kΩ
A
0
W
0
B
0
C
25pF
A
W
RDAC
10kΩ
25E-12
{(1-D/64)*RDAC+60}
55E-12
{D/64*RDAC+60}
25E-12
55pF
C
W
C
25pF
B
B