How to convert (-3.3 to 3.3VDC) to (0 to 3VDC) ?? i.e. voltage shifter

[yzou_ua]

I have a AC voltage to be feedback to ADC of controller. So how can I shift the voltage level to the 0~3.3VDC then?? I tried a two op-amp(LM301) circ. but not working well...
Could anyone help me out? Thx alot!

 

[papunblg]

Sir,
Please have a look
**broken link removed**
AD636: Low Level, True RMS-to-DC Converter
AD636 | Low Level, True RMS-to-DC Converter | RMS to DC Converters | Special Linear Functions | Analog Devices

Page 12 contains a complete circuit of a low power ac volt meter (image 17) which is very accurate
thanks

 

[erikl]

I have a AC voltage to be feedback to ADC of controller. So how can I shift the voltage level to the 0~3.3VDC then?

Simply divide the ac voltage by 2 and couple it via capacitor to VDD/2 = 1.65V of your ADC ,
(or by 3.3/1.5 and couple it via capacitor to VDD/2 = 1.5V) :

• R3=R4 , if R4 is the input resistance of your ADC, or R3=R4 « input resistance of your ADC
• R1=R2 « R4
• C1 ≥ 1/pi*f_min*R4 ; f_min = lower frequency limit of your ac voltage

 

[rauol]

search for any Energy meter application note, from Ti.com , the front end has an accurate level shifter

 

[BradtheRad]

If simple is desirable then this is the simplest method which I think anyone can come up with.

Install a resistance inline with the input. Install a resistance to the positive supply.

Best to start with high resistance. Adjust to obtain desired swing at the output.

This method depends on the surrounding circuitry maintaining stable resistance. If not then the voltage can wander outside safe specs. Which would make post #3 a safer method to use.