Datasheet
Table Of Contents
- Specifications
- Pinout
- Package drawings
- Ordering Guide
- Features
- Applications
- Product Description
- Absolute Maximum Ratings
- Typical Characteristics
- APPLICATION NOTES Input Characteristics
- OUTPUT CHARACTERISTICS
- OFFSET VOLTAGE ADJUSTMENT
- DIAGRAMS
- Unity-Gain Follower
- Unity-Gain Follower
- Gain of Two Inverter
- Total Noise vs. Source Impedance
- Extending Unity Gain Follower Capacitive Load Capability Beyond 350 pF
- Offset Null
- Capacitive Load Tolerance vs. Noise Gain
- Single Supply Half- and Full-Wave Rectifier
- Single Supply 4.5 Volt Low Dropout Reference
- 10 Hz Sallen Key Low-Pass Filter
AD820
REV. A
–15–
10
90
100
0%
2V
2V
200µs
2V
A
B
C
3
2
6
7
4
0.01µF
V
IN
+V
S
AD820
HALF-WAVE
RECTIFIED OUTPUT
FULL-WAVE
RECTIFIED OUTPUT
A1
R2
100kΩ
R1
100kΩ
3
2
6
7
4
0.01µF
+V
S
AD820
A2
A
B
C
Figure 42. 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 43 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.
U1
AD680
7
6
4
23
+2.5V
OUTPUT
+4.5V
OUTPUT
REF
COMMON
+5V
R2
80k
(20k)
R3
100k
(25k)
C3
10µF/25V
C2
1µF FILM
C1
0.1µF
+2.5V ± 10mV
2
3
4
6
R1
100k
U2
AD820
Figure 43. 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.
Low Power Three-Pole Sallen Key Low-Pass Filter
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.
Figure 44 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
output voltage noise of the filter.
0
–50
–100
11k100100.1
–40
–30
–20
–10
–90
–80
–70
–60
FREQUENCY – Hz
FILTER GAIN RESPONSE – dB
V
OUT
–V
S
R1
243k
R2
243k
R3
243k
C1
0.022µF
C3
0.022µF
C2
0.022µF
3
2
6
7
4
0.01µF
+V
S
AD820
0.01µF
V
IN
Figure 44. 10 Hz Sallen Key Low-Pass Filter