AD820A Analog Devices, AD820A Datasheet - Page 14

• Manufacturer Part Number: AD820A
• Manufacturer: Analog Devices

AD820
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 21 shows a 4.5 V reference
using the AD820 and the AD680, a low power 2.5 V bandgap
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.
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.
V
C1
0.1 F
5V
IN
3
A
AD680
U1
A
3
2
B
C
2
4
A1
+V
7
6
100k
S
4
R1
0.01 F
AD820
2.5V
6
100k
R1
10mV
AD820
U2
3
2
7
C2
0.1 F FILM
A2
3
+V
7
100k
R2
S
4
6
0.01 F
AD820
2
4
6
R2
80k
(20k )
R3
100k
(25k )
FULL-WAVE
RECTIFIED OUTPUT
HALF-WAVE
RECTIFIED OUTPUT
C3
10 F/25V
C
B
4.5V
OUTPUT
2.5V
OUTPUT
REF
COMMON
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 22 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.
V
IN
–100
–10
–20
–30
–40
–50
–60
–70
–80
–90
0.022 F
243k
0
R1
0.1
C1
243k
R2
1
243k
R3
FREQUENCY – Hz
C3
0.022 F
10
0.022 F
C2
3
2
AD820
+V
7
–V
4
S
S
100
0.01 F
0.01 F
6
1k
V
OUT