AD820 datasheet(19/24 Pages) AD

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AD820

Rev. E | Page 19 of 24

APPLICATIONS

SINGLE SUPPLY HALF-WAVE AND FULL-WAVE

RECTIFIERS

An AD820 configured as a unity-gain follower and operated

with a single supply can be used as a simple half-wave rectifier.

The AD820 inputs maintain picoamp level input currents even

when driven well below the negative supply. The rectifier puts

that behavior to good use, maintaining an input impedance of

over 1011 Ω for input voltages from 1 V from the positive supply

to 20 V below the negative supply.

The full- and half-wave rectifier shown in Figure 44 operates as

follows: when VIN is above ground, R1 is bootstrapped through

the unity-gain follower, A1, and the loop of amplifier A2. This

forces the inputs of A2 to be equal; thus, no current flows

through R1 or R2, and the circuit output tracks the input. When

VIN is below ground, the output of A1 is forced to ground. The

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.

–

+VS

0.01µF

100kΩ

AD820

FULL-WAVE

RECTIFIED OUPUT

VIN

+VS

0.01µF

AD820

–

HALF-WAVE

RECTIFIED OUPUT

–

+ B

100

Figure 44. 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 45

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.

90kΩ

(20kΩ)

100kΩ

(25kΩ)

AD820

–

2.5V

OUTPUT

4.5V

OUTPUT

REF

COMMON

10µF/25V

AD680

0.1µF FILM

2.5V ± 10mV

0.1µF

Figure 45. 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.

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