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Electronics => Beginners => Topic started by: anishkgt on March 26, 2018, 03:07:32 pm

Title: working with opamp and low voltage AC
Post by: anishkgt on March 26, 2018, 03:07:32 pm
Hi All,

Just as learning experiment with opamp, would it be possible to follow an ac signal where when it is changed from 1 to 2vac i would get a similar dc .

(https://www.avrfreaks.net/sites/default/files/DS1Z_QuickPrint6.png)
Title: Re: working with opamp and low voltage AC
Post by: rstofer on March 26, 2018, 06:17:12 pm
Your question isn't very clear (to me...).  Are you trying to get an amplifier with a gain of 2x or are you trying to rectify a signal to get a DC value?

For the op amp, an x2 gain is possible as long as your output doesn't swing near the +- rails.  For a single supply circuit, you will need a bias circuit to slide the AC signal up such that it doesn't go negative with respect to ground.  All of this is covered in Dave's op amp videos.

Or Google for 'op amp videos'.

If you want to create a DC level, use a bridge rectifier.  You can amplify the signal before you rectify it and you can even rectify it with op amps:

http://www.site.uottawa.ca/~rhabash/ELG4135L8.pdf (http://www.site.uottawa.ca/~rhabash/ELG4135L8.pdf)
Title: Re: working with opamp and low voltage AC
Post by: anishkgt on March 27, 2018, 08:29:06 am
I had the precision rectofier in mind but i wanted some advise if that would really follow the AC peak voltage or at least show any variation at the output when the ac voltage varied. The level shift something like this is what i also have in mind.

Which of this would get a noticable change on the output. with level shifting i would need to sample the ac signal and the rectified would be smooth for the ADC to read.
Title: Re: working with opamp and low voltage AC
Post by: Zero999 on March 27, 2018, 01:13:03 pm
It's possible to build a precision rectifier, which doesn't need a negative supply rail, or the signal to be shifted to within the power rails.

(http://cds.linear.com/image/resized/457_circuit_1.jpg)
http://www.linear.com/solutions/1608 (http://www.linear.com/solutions/1608)

If the input signal is too higher voltage, then it's easy to scale it down with resistors. If it's too lower voltage, then an amplifier can be added after the precision rectifier.
Title: Re: working with opamp and low voltage AC
Post by: Rigolon on March 27, 2018, 01:32:10 pm
I believe that the precision rectifier Hero999 posted it's one of the best choices out there for what you want. I would also advice you to use a RC low pass filter before your ADC, just to make the DC signal a bit more smooth.
Title: Re: working with opamp and low voltage AC
Post by: anishkgt on March 27, 2018, 02:16:33 pm
Quote from: Hero999 on March 27, 2018, 01:13:03 pm
It's possible to build a precision rectifier, which doesn't need a negative supply rail, or the signal to be shifted to within the power rails.

(http://cds.linear.com/image/resized/457_circuit_1.jpg)
http://www.linear.com/solutions/1608 (http://www.linear.com/solutions/1608)

If the input signal is too higher voltage, then it's easy to scale it down with resistors. If it's too lower voltage, then an amplifier can be added after the precision rectifier.
An amplifier after the follower or can we use the follower in an amplifier config ?


Www.Georgehobby.wordpress.com

Equipments: DSO104z, Hakko FX888D, Brymen BM869s
Title: Re: working with opamp and low voltage AC
Post by: anishkgt on March 27, 2018, 02:18:10 pm
Quote from: Rigolon on March 27, 2018, 01:32:10 pm
I believe that the precision rectifier Hero999 posted it's one of the best choices out there for what you want. I would also advice you to use a RC low pass filter before your ADC, just to make the DC signal a bit more smooth.
RC Filter with 1K and 17pf is what I’ve noticed on the scope. Will post a pic when am back.


Www.Georgehobby.wordpress.com

Equipments: DSO104z, Hakko FX888D, Brymen BM869s
Title: Re: working with opamp and low voltage AC
Post by: Zero999 on March 27, 2018, 10:31:24 pm
Quote from: anishkgt on March 27, 2018, 02:18:10 pm
Quote from: Rigolon on March 27, 2018, 01:32:10 pm
I believe that the precision rectifier Hero999 posted it's one of the best choices out there for what you want. I would also advice you to use a RC low pass filter before your ADC, just to make the DC signal a bit more smooth.
RC Filter with 1K and 17pf is what I’ve noticed on the scope. Will post a pic when am back.
That's a lower cut-off of 9.36MHz, on a 50Hz signal, which won't be noticeable.

Quote from: anishkgt on March 27, 2018, 02:16:33 pm
Quote from: Hero999 on March 27, 2018, 01:13:03 pm
It's possible to build a precision rectifier, which doesn't need a negative supply rail, or the signal to be shifted to within the power rails.

(http://cds.linear.com/image/resized/457_circuit_1.jpg)
http://www.linear.com/solutions/1608 (http://www.linear.com/solutions/1608)

If the input signal is too higher voltage, then it's easy to scale it down with resistors. If it's too lower voltage, then an amplifier can be added after the precision rectifier.
An amplifier after the follower or can we use the follower in an amplifier config ?


Www.Georgehobby.wordpress.com (http://Www.Georgehobby.wordpress.com)

Equipments: DSO104z, Hakko FX888D, Brymen BM869s
The follower could be replaced with a non-inverting amplifier, of course.