AN2451 datasheet(45/55 Pages) STMICROELECTRONICS

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AN2451

Application ideas

Doc ID 12791 Rev 3

45/55

Figure 44.

Peak detector electrical schematic

The schematic above is based on a simple diode-capacitor (D1-C1) circuit improved with an

LM393 comparator so that:

●

The comparator eliminates the diode reverse voltage.

●

The feedback network (R3/R2) introduces a gain of 4 to improve performance against

low amplitude signals.

In the end this circuit gives, on DC_OUT line, a DC voltage proportional to the AC peak-to-

peak level at the input. Figure 45 shows the measured behavior of this circuit with a given

pure sinusoidal waveform at the input. The DC_OUT signal should be converted through an

integrated A/D converter by the application microcontroller.

Figure 45.

Measured DC_OUT Vs. AC_IN peak detector response

7.4

Non-isolated coupling

A possible alternative solution for line coupling is a non-isolated circuit. An example is

shown in Figure 46, in which the transformer T2 has been substituted with the shunt

inductor L7. The value of the inductor has been chosen to give about 100 dB rejection to the

mains voltage through a C23-L7 high pass filter. Advantages arising from non-isolated

Rx_IN

100k

18k

82k

4.7k

U1A

LM393

1N4148

100n

DC_OUT

500

1000

1500

2000

2500

3000

200

400

600

800

1000

1200

AC_IN [mVpp]

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