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Problems with Isolated Voltage Sensor ACPL-C87

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trungkstn

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Hi, I have some problem with ACPL-C87 (http://www.avagotech.com/docs/AV02-3563EN).
I intend use ACPL-C87 for measuring voltage with scheme like this
AC (220V/50Hz) --> Bridge rectity --> resistive divider (R1 = 1.5 MOhm + R2 = 10 kOhm) --> ACPL-C87.
Based on datasheet of ACPL-C87, I use resitor 10 kOhm. Voltage on R2 about 2V peak to peak, 1/2 cycle 50Hz. But when i supply 5V for IC ACPL-C87, voltage on R2 i measured reach to 4V peak to peak 1/2 cycle 50Hz.
I don't know why voltage on R2 changed?
Can you plain and show me a solution to solve this problem?
Thank you.
 

schmitt trigger

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Because you incorrectly calculated the peak to peak voltage as Vrms * sqrt(2),
and it should be 2 * Vrms * sqrt(2).

I ignore the internal circuit details details of the device, but most likely when it is not powered up, it clamps the input voltage.
 

trungkstn

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But i use bridge rectifier, so peak to peak only Vrms * sqrt(2). I used oscilloscope to show wave on display. ACPL-C87 have Input Impedance about 1GOhm (very large). The problem only happen when power up ACPL-C87.
The schematic like below
2.png1.png
I don't have LR8, so i use 5V from kit STM32F1 to supply for ACPL-C87.
 

schmitt trigger

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You can provide all the information at once to allow us to assist you, or you can provide little pieces of information at the time, and have us running in the dark making wild guesses.

Where are the scope waveforms for both conditions??
Also show what the waveform looks like prior to the voltage divider, both conditions.
 

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