ADA4622-2ACPZ-R7 datasheet(30/34 Pages) AD

30 / 34 page

ADA4622-1/ADA4622-2

Data Sheet

Rev. B | Page 30 of 34

PEAK DETECTOR

A peak detector captures the peak value of a signal and

produces an output equal to it. By taking advantage of the dc

precision and super low input bias current of the JFET input

amplifiers, such as the ADA4622-1/ADA4622-2, a highly

accurate peak detector can be built, as shown in Figure 98.

ADA4622-1/

ADA4622-2

ADA4622-1/

ADA4622-2

VCC

VIN

–

VEE

50pF

1µF

1kΩ

10kΩ

1N448

1N4148

+PEAK

1N4148

Figure 98. Positive Peak Detector

In this application, D3 and D4 act as unidirectional current

switches that open when the output is kept constant in hold mode.

To detect a positive peak, U2A drives C3 through D3 and drives

D4 until C3 is charged to a voltage equal to the input peak value.

Feedback from the output of the U2B (positive peak) through

R6 limits the output voltage of U2A. After detecting the peak,

the output of U2A swings low but is clamped by D2. D3 reverses

bias and the common node of D3, D4, and R7 is held to a voltage

equal to positive peak by R7. The voltage across D4 is 0 V;

therefore, the leakage is small. The bias current of U2B is also

small. With almost no leakage, C3 has a long hold time.

The ADA4622-1/ADA4622-2, shown in Figure 98, are a perfect

fit for building a peak detector because U1 requires dc precision

and high output current during fast peaks, and U2 requires low

input bias current (IB) to minimize capacitance discharge between

peaks. A low leakage and low dielectric absorption capacitor, such

as polystyrene or polypropylene, is required for C3. Reversing the

diode directions causes the circuit to detect negative peaks.

MULTIPLEXING INPUTS

By using the ADA4622-1 DISABLE input, it is possible to

multiplex two inputs to a single output by using the circuit shown

in Figure 99. If the gain configuration or filter configuration of

the two amplifiers is different, and a common single input to

both amplifiers is used, this configuration can control selectable

gain or selectable frequency response at the output.

V+

–

ADA4622-1

V–

VOUT

–

V–

DISABLE

Figure 99. Multiplexed Input Circuit

Figure 100 shows the output response when multiplexing two

input signals. The input to the first amplifier is a 4 V p-p,

200 kHz sine wave; the input to the second amplifier is an

8 V p-p, 100 kHz sine wave.

OUTPUT

VLOGIC

TIME

Figure 100. Multiplexed Output

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