Datasheet

AD5171
Rev. D | Page 20 of 24
For log taper adjustment, such as volume control, Figure 44
shows another way of resistance scaling. In this circuit, the
smaller the R2 with respect to R
AB
, the more it behaves like the
pseudo log taper characteristic. The wiper voltage is simply
I
WB
WA
WB
W
V
2RRR
2RR
DV ×
+
=
||
)||(
)( (6)
V
I
R1
B
A
R2
V
O
W
0
3437-044
Figure 44. Resistor Scaling with Log Adjustment Characteristics
RESOLUTION ENHANCEMENT
The resolution can be doubled in the potentiometer mode of
operation by using three digital potentiometers. Borrowed from
the Analog Devices patented RDAC segmentation technique,
users can configure three AD5171s to double the resolution (see
Figure 45). First, U3 must be parallel with a discrete resistor, R
P
,
which is chosen to be equal to a step resistance (R
P
= R
AB
/64).
Adjusting U1 and 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.
U1
A1
B1
W1
U2
A2
B2
W3
W2
U3
A3
B3
R
P
COARSE
ADJUSTMENT
FINE
ADJUSTMENT
03437-045
Figure 45. Doubling the Resolution
RDAC CIRCUIT SIMULATION MODEL
The internal parasitic capacitances and the external capacitive
loads dominate the ac characteristics of the digital potentiometers.
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 46. Listing 1 provides a macro model net list
for the 10 kΩ device.
55pF
C
A
25pF
C
B
25pF
A
B
RDAC
10k
W
C
W
03437-046
Figure 46. Circuit Simulation Model for RDAC = 10 kΩ
Listing 1. Macro Model Net List for RDAC
.PARAM D=64, RDAC=10E3
*
.SUBCKT DPOT (A,W,B)
*
CA A 0 25E-12
RWA A W {(1-D/64)*RDAC+60}
CW W 0 55E-12
RWB W B {D/64*RDAC+60}
CB B 0 25E-12
*
.ENDS DPOT