SMP04

–11–

REV. D

POSITIVE AND NEGATIVE PEAK DETECTOR WITH

HOLD CONTROL (Figure 21)

In this application the top amplifier (Amplifier A) is the positive

peak detector and the bottom amplifier (Amplifier B) is the

negative peak detector. Operation can be analyzed as follows:

Assume that the

S/H switch is closed. As a positive increasing

the feedback loop around Amplifier A and the SMP04, causing

the output to track the input. Conversely, in the negative peak

holding the last most negative input voltage on the SMP04.

voltage will be held until the input has a more positive voltage

than the previously held peak voltage, or a reset condition is

applied.

An optional HOLD control can be used by applying a logic HIGH

to the

PD/H inputs. This HOLD mode further reduces leakage

peak hold.

NEGATIVE

DGND

–5V

+5V

1/2 OP221

1/2 OP221

–5V

+5V

POSITIVE

AMPLIFIER A

AMPLIFIER B

R1

20k

1N914

1/2 SMP04

R2

100

D

S

SD214

( 3.5V)

RESET

PD/H

POSITIVE

PD/H

NEGATIVE

SD214

R3

20k

1N914

R4

100

D

S

G

G

Figure 21. Positive and Negative Peak Detector with Hold Control

GAIN OF 10 SAMPLE-AND-HOLD (Figure 22)

This application places the SMP04 in a feedback loop of an

amplifier. Because the SMP04 has no sign inversion and the

amplifier has very high open-loop gain, the gain of the circuit is set

by the ratio of the sum of the source and feedback resistances

+12V

1/4 OP490

+12V

0V TO

10V

1/4 SMP04

1k

0V TO

1.0V

S/H

100k

1N914

8.66k

340

Figure 22. Gain of 10 Sample-and-Hold Amplifier

to the source resistance. When a logic LOW is applied to the

S/H control input, the loop is closed around the OP490,

yielding a gain of 10 (in the example shown) amplifier. When

the

S/H control goes HIGH, the loop opens and the SMP04

holds the last sampled voltage. The loop remains open and the

output is unaffected by the input until a logic LOW is reapplied

to the

S/H control. The pair of back-to-back diodes from the

output of the op amp to the output of the track-and-hold pre-

vents the op amp from saturating when the track-and-hold is in

the hold mode and the loop is open.