AD820ARMZ-R7 datasheet(18/24 Pages) AD

18 / 24 page

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.

–

+VS

0.01µF

100kΩ

AD820

FULL-WAVE

RECTIFIED OUPUT

VIN

–

HALF-WAVE

RECTIFIED OUPUT

–

100

+VS

0.01µF

AD820

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.

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 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

–

+VS

–VS

0.01µF

VOUT

–

243kΩ

0.022µF

–

VIN

243kΩ

0.022µF

–100

0.1

FREQUENCY (Hz)

100

–10

–20

–30

–40

–50

–60

–70

–80

–90

Figure 45. 10 Hz Sallen Key Low-Pass Filter

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