AD820
Low Power Three-Pole Sallen Key Low-Pass Filter
R1
100k⍀
R2
100k⍀
The AD820’s high input impedance makes it a good selection
for active filters. High value resistors can be used to construct
low frequency filters with capacitors much less than 1 µF. The
AD820’s picoamp level input currents contribute minimal dc
errors.
+V
S
+V
0.01F
S
7
0.01F
3
2
A
7
C
3
2
6
A2
V
6
IN
FULL-WAVE
RECTIFIED OUTPUT
Figure 46 shows an example, a 10 Hz three-pole Sallen Key
Filter. The high value used for R1 minimizes interaction with
signal source resistance. Pole placement in this version of the
filter minimizes the Q associated with the two-pole section of
the filter. This eliminates any peaking of the noise contribution
of resistors R1, R2, and R3, thus minimizing the inherent out-
put voltage noise of the filter.
A1
AD820
4
AD820
4
B
HALF-WAVE
RECTIFIED OUTPUT
C2
0.022F
A
+V
S
0.01F
R2
243k⍀
R3
243k⍀
R1
243k⍀
7
3
2
B
C
V
IN
C3
0.022F
C1
6
0.022F
AD820
V
OUT
0.01F
4
–V
S
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
Figure 44. Single Supply Half- and Full-Wave Rectifier
4.5 Volt Low Dropout, Low Power Reference
The rail-to-rail performance of the AD820 can be used to pro-
vide low dropout performance for low power reference circuits
powered with a single low voltage supply. Figure 45 shows a
4.5 volt reference using the AD820 and the AD680, a low power
2.5 volt bandgap reference. R2 and R3 set up the required gain
of 1.8 to develop the 4.5 volt output. R1 and C2 form a low-
pass RC filter to reduce the noise contribution of the AD680.
+2.5V
OUTPUT
–100
0.1
1
10
100
1k
+4.5V
FREQUENCY – Hz
OUTPUT
U2
6
+5V
AD820
Figure 46. 10 Hz Sallen Key Low-Pass Filter
R2
80k⍀
7
4
(20k⍀)
2
+2.5V ؎10mV
3
2
C3
3
6
U1
10F/25V
AD680
R1
100k⍀
R3
100k⍀
(25k⍀)
C2
0.1F FILM
4
C1
0.1F
REF
COMMON
Figure 45. Single Supply 4.5 Volt Low Dropout Reference
With a 1 mA load, this reference maintains the 4.5 volt output
with a supply voltage down to 4.7 volts. The amplitude of the
recovery transient for a 1 mA to 10 mA step change in load
current is under 20 mV, and settles out in a few microseconds.
Output voltage noise is less than 10 µV rms in a 25 kHz noise
bandwidth.
REV. B
–15–
ADI [ ADI ]
