ST ST7540, Application Note

Page: 45 / 55

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
R1
100k
R2
18k
R3
82k
R4
4.7k
+
-
U1A
LM393
3
1
8
4
2
5V
D1
1N4148
C1
100n
DC_OUT
0
500
1000
1500
2000
2500
3000
0 200 400 600 800 1000 1200
AC_IN [mVpp]
DC
_
O
U
T
[m
V
]