Datasheet

AD5273
Rev. H | Page 20 of 24
Depending on the op amp GBP, reducing the feedback resistor
may extend the zero’s frequency far enough to overcome the
problem. A better approach is to include a compensation
capacitor, C2, to cancel the effect caused by C1. Optimum
compensation occurs when R1 × C1 = R2 × C2, but this is not
an option because of the variation of R2. As a result, users can
use the relationship described and scale C2 as if R2 were at its
maximum value. However, doing so may overcompensate by
slowing down the settling time when R2 is set to low values. To
avoid this problem, C2 should be found empirically for a given
application. In general, setting C2 in the range of a few picofarads
to no more than a few tenths of a picofarad is usually adequate
for compensation.
There is also a Terminal W capacitance connected to the output
(not shown); its effect on stability is less significant; therefore,
compensation is not necessary unless the op amp is driving a
large capacitive load.
PROGRAMMABLE LOW-PASS FILTER
In ADC applications, it is common to include an antialiasing
filter to band-limit the sampling signal. To minimize various
system redesigns, users can use two 1 kΩ AD5273s to construct
a generic second-order Sallen-Key low-pass filter. Because the
AD5273 is a single-supply device, the input must be dc offset
when an ac signal is applied to avoid clipping at ground. This is
illustrated in Figure 50. The design equations are
2
2
2
O
O
O
I
O
S
Q
S
V
V
ω+
ω
+
ω
=
(6)
1R2C1C2R
O
1
=ω
(7)
R2C2R1C1
Q
11
+=
(8)
Users can first select some convenient values for the capacitors.
To achieve maximally flat bandwidth where Q = 0.707, let C1 be
twice the size of C2 and let R1 = R2. As a result, R1 and R2 can
be adjusted to the same setting to achieve the desired bandwidth.
V
O
AD8601
+2.5V
U1
–2.5V
V+
V–
C1
C
R1
R2
A
B
W
A
B
W
C2
C
ADJUSTED TO
SAME SETTINGS
V
I
03224-050
Figure 50. Sallen Key Low-Pass Filter
LEVEL SHIFT FOR DIFFERENT VOLTAGES
OPERATION
If the SCL and SDA signals come from a low voltage logic
controller and are below the minimum V
IH
level (0.7 × V
DD
),
level-shift the signals for successful read/write communication
between the AD5273 and the controller. Figure 51 shows one of
the implementations. For example, when SDA1 is 2.5 V, M1
turns off, and SDA2 becomes 5 V. When SDA1 is 0 V, M1 turns
on, and SDA2 approaches 0 V. As a result, proper level-shifting
is established. M1 and M2 should be low threshold N-Channel
power MOSFETs, such as FDV301N.
2.5V
CONTROLLER
2.7V–5.5V
AD5273
Rp Rp Rp Rp
V
DD1
= 2.5V
V
DD2
= 5
V
G
G
S
D
M1
S
D
M2
S
DA1
S
CL1
SDA2
SCL2
03224-051
Figure 51. Level Shift for Different Voltages Operation
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 AD5273 (1 kΩ resistor) measures 6 MHz at
half scale. Figure 17 to Figure 20 provide the large signal BODE
plot characteristics of the four available resistor versions: 1 kΩ,
10 kΩ, 50 kΩ, and 100 kΩ. Figure 52 shows a parasitic simula-
tion model. The code following Figure 52 provides a macro
model net list for the 1 kΩ device.
55pF
C
A
2
5pF
C
B
25pF
AB
1kΩ
W
C
W
03224-055
Figure 52. Circuit Simulation Model for RDAC = 1 kΩ
Macro Model Net List for RDAC
.PARAM D = 63, RDAC = 1E3
*
.SUBCKT DPOT (A,W,B)
*
CA A 0 25E-12
RWA A W {(1-D/63)*RDAC+60}
CW W 0 55E-12
RWB W B {D/63*RDAC+60}
CB B 0 25E-12
*
.ENDS DPOT