ADA4622-2ACPZ-R7 datasheet(32/36 Pages) AD

32 / 36 page

Data Sheet

ADA4622-1/ADA4622-2/ADA4622-4

APPLICATIONS INFORMATION

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Rev. F | 32 of 36

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/ADA4622-4, a highly accurate peak

detector can be built, as shown in Figure 99.

Figure 99. 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, U1 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 U2 (positive peak) through R6

limits the output voltage of U1. After detecting the peak, the output

of U1 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 U2 is also small. With almost

no leakage, C3 has a long hold time.

The ADA4622-1/ADA4622-2/ADA4622-4, shown in Figure 99, are a

perfect fit for building a peak detector because U1 requires dc pre-

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

low input bias current (IB) to minimize capacitance discharge be-

tween 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

100. 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.

Figure 100. Multiplexed Input Circuit

Figure 101 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.

Figure 101. Multiplexed Output

FULL WAVE RECTIFIER

Figure 102 shows the circuit of a full wave rectifier using two

ADA4622-1 op amps in single-supply operation. The circuit is com-

posed of a voltage follower (U1) and a second stage amplifier (U2)

that combine the output of the first stage amplifier and the inverted

version of the input signal. U1 follows the input during the positive

half cycle and clamps the negative going input signal to ground,

producing a half wave signal at VHW. The following equation defines

the circuit transfer function:

VFW = (1+ R3/R2)VHW − (R3/R2) × VIN

where:

VFW is the full wave output from U1.

R3 and R2 are the feedback resistors shown in Figure 102.

VHW is the half wave output from U1.

VIN is the input voltage.

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