Need help to my problem. I tried solve this out long time but now i decided call for help.
I have ac coupled signal going to op amp positive input. Lm358 supply is +5V. My problem is that signal need to be rectified and it works with low voltages. When op amp positive input signal goes lower than ~-0,550v output shoots max voltage.. If under -0.55v output is normal half wave rectified signal.
What causes this?
C1 = 0.1uF
R1 = 10k
Circuit
Input signal
good output
bad output
Thank you
-Wils0n
I don't know if it's just me, but I can't see your images.
My problem is that signal need to be rectified and it works with low voltages. When op amp positive input signal goes lower than ~-0,550v output shoots max voltage.. If under -0.55v output is normal half wave rectified signal.
Whenever the +input is lower (less positive or more negative) than the -input, the output should "try" to go negative. But since you only have a positive power supply and the output can't go negative it should go as close to ground as it can go.
It should never "invert"... That is, the output shouldn't go positive any time when the +input is negative relative to the -input.
His images aren't working because he entered the address of the dropbox page for his file, instead of a direct link to the file (if you can even do that from dropbox).
It sounds like you're applying a negative voltage (-0.55v) to the input pin. Per LM358 datasheet, you can't do that:
The input common-mode voltage of either input signal voltage should not be allowed to go negative by more than 0.3 V (at 25°C). The
upper end of the common-mode voltage range is V
- −1.5 V (at 25°C), but either or both inputs can go to 32 V without damage (26 V for
LM2904), independent of the magnitude of V+
So you're using the chip in a manner which they specifically caution against in the datasheet - no surprise that it ain't working!
Generally speaking, unless the chip is designed to expect a negative voltage somewhere (usually in combination with a dual rail supply), exposing it to such will result in undesired behavior.
What do you want to rectify. Audio?
Half-wave (positive or negative) or full wave.
Leo..
I think the way to address this is use either an opamp rectification circuit, or do rectification in
software using AC-coupling to a mid-rail voltage divider. Driving the opamp input too far negative
risks frying it.
DC offset the input signal??
Still can't see the image. You can't just give the link, you must select "share".
DrAzzy:
His images aren't working because he entered the address of the dropbox page for his file, instead of a direct link to the file (if you can even do that from dropbox).
...
Yes you can!
A typical dropbox link looks like this:
Remove everything at the end from the question mark on, then replace the "www" with dl. Go to the resulting URL. The URL will change (automagically) to something like this:
https://dl.dropboxusercontent.com/s/a214vqxvovktl6h/circuit.jpg
and that is the actual image file's URL, which you can use with [img] tags. I do it all the time. This has worked with every version of Dropbox since the beginning.
Thank you ChrisTenone my pics should work now!
I think this why it's acting weirdly.
The input common-mode voltage of either input signal voltage should not be allowed to go negative by more than 0.3 V (at 25°C). The
upper end of the common-mode voltage range is V
- −1.5 V (at 25°C), but either or both inputs can go to 32 V without damage (26 V for
LM2904), independent of the magnitude of V+
I have ~12-13v supply where i need detect very small voltage changes (+-200mV to 1V) and convert those changes to 0-5v digital signal to arduino. My plan was that 12v to ac coupling and then to half rectifier (positive) and then slightly amplify the signal then make 0-5v trigger with op amp.
Maybe i should lift op amps voltage potential so that op amp negative supply voltage is like 1V.
Then my input signal is able to go low as -1V?
-Wils0n
Connect a small capacitor (10nF) between pin-4 and pin-8.
kesirajus:
Connect a small capacitor (10nF) between pin-4 and pin-8.
Yes. This only remove noise from supply?
Wils0n:
Yes. This only remove noise from supply?
Maybe, might stop RF parasitic oscillations too - the datasheet says 10nF, you use 10nF.
The thing you're seeing on the scope is called phase-reversal.
Happens when you drive an opamp beyond it's common mode range.
http://www.njr.com/semicon/design_support/faq/10088.html
Google "opamp full-wave rectifier circuit", and look at images.
Leo..
Wawa:
The thing you're seeing on the scope is called phase-reversal.
Happens when you drive an opamp beyond it's common mode range.
http://www.njr.com/semicon/design_support/faq/10088.htmlGoogle "opamp full-wave rectifier circuit", and look at images.
Leo..
Thank you alot Wawa! Finally found explanation to my problem.