AD820
Rev. H | Page 18 of 24
noninverting input of Amplifier A2 sees the ground level output
of A1; therefore, A2 operates as a unity-gain inverter. The output at
Node C is then a full-wave rectified version of the input. Node B is
a buffered half-wave rectified version of the input. Input voltages
up to ±18 V can be rectified, depending on the voltage supply used.
00873-045
A1
–
+
–
+
+V
S
0.01µF
R1
100kΩ
R2
100kΩ
AD820
FULL-WAVE
RECTIFIED OUPUT
V
IN
3
A
C
2
4
7
6
–
+
–
+
HALF-WAVE
RECTIFIED OUPUT
–
+
B
A
B
C
100
90
10
0%
A2
+V
S
0.01µF
AD820
3
2
4
7
6
Figure 43. Single-Supply Half- and Full-Wave Rectifier
4.5 V LOW DROPOUT, LOW POWER REFERENCE
The rail-to-rail performance of the AD820 can be used to
provide low dropout performance for low power reference
circuits powered with a single low voltage supply. Figure 44
shows a 4.5 V reference using the AD820 and the AD680, a low
power 2.5 V band gap reference. R2 and R3 set up the required
gain of 1.8 to develop the 4.5 V output. R1 and C2 form a low-
pass RC filter to reduce the noise contribution of the AD680.
00873-046
R2
90kΩ
(20kΩ)
R1
100kΩ
R3
100kΩ
(25kΩ)
U2
AD820
–
+
2.5V
OUTPUT
4.5V
OUTPUT
5V
REF
COMMON
C3
10µF/25V
U1
AD680
C2
0.1µF FILM
3 2
4
4
7
6
2
63
2.5V ± 10mV
C1
0.1µF
Figure 44. Single Supply 4.5 V Low Dropout Reference
With a 1 mA load, this reference maintains the 4.5 V output
with a supply voltage down to 4.7 V. 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.
LOW POWER, 3-POLE, SALLEN KEY LOW-PASS
FILTER
The high input impedance of the AD820 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 picoamp level input currents contribute
minimal dc errors.
Figure 45 shows an example of 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 Resistor R1, Resistor R2, and Resistor R3, thus minimizing
the inherent output voltage noise of the filter.
AD820
–
+
+V
S
–V
S
0.01µF
0.01µF
V
OUT
3
2
4
7
6
–
+
R3
243kΩ
C3
0.022µF
–
+
V
IN
R2
243kΩ
R1
243kΩ
C1
0.022µF
C2
0.022µF
0
–100
0.1 1k
FREQUENCY (Hz)
FILTER GAIN RESPONSE (dB)
00873-047
1 10 100
–10
–20
–30
–40
–50
–60
–70
–80
–90
Figure 45. 10 Hz Sallen Key Low-Pass Filter
