AD820

REV. B

–15–

R1

100k

AD820

2

3

6

FULL-WAVE

RECTIFIED OUTPUT

HALF-WAVE

RECTIFIED OUTPUT

R2

100k

A

C

B

AD820

0.01 F

3

2

4

7

6

A1

A2

0.01 F

7

4

A

B

C

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

3

4

R2

80k

(20k )

U1

AD680

REF

COMMON

+4.5V

OUTPUT

+2.5V

OUTPUT

R3

100k

(25k )

C3

10 F/25V

3

6

7

2

C1

0.1 F

C2

0.1 F FILM

U2

AD820

+5V

+2.5V

10mV

6

R1

100k

4

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.

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 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.

FREQUENCY – Hz

0.1

0

–10

–100

–20

–30

–40

–50

–60

–70

–80

–90

1

10

100

1k

C2

0.022 F

0.01 F

0.01 F

3

2

4

7

6

AD820

R1

243k

C3

0.022 F

C1

0.022 F

R2

243k

R3

243k

Figure 46. 10 Hz Sallen Key Low-Pass Filter