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OpAmp producing negative voltage

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toozie

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Howdy,
I am using a national semi **broken link removed** (though only using one of the two amps on it) as a microphone preamp. I followed the datasheet to a T, but I feel like I am missing something vital. I am using the parts that the schematic spells out (seen in the schematic attachment).

If I power the circuit with 5V and have "nothing" going through the mic, I see Vcc/2 at the V+ and V- pins like I would expect and ~0V on the output.

If I start making noise into the mic, I see the V- pin start to fluctuate (which I can believe) and I start getting a voltage out of the output. But if I scope the output, I see the voltage swinging above an below 0V (sometimes as high/low as 3.3V/-3.3V, but always centered at 0V.

At first I thought this was normal, so I started putting a DC offset on the output to make the output stay positive (mostly). But the more I read the datasheet (I am not really an analog guy), the more I realized that I should never be getting a negative voltage out (and the mic was already being biased to Vcc through the 5V resistor). As I talk into the mic, the signal looks like what I would expect, except it is centered around 0V (almost looks like a mirror above/below the 0V line).

Does anyone else with op-amp (pre-amp) experience see where I might be going wrong and/or how I can start debugging this (I have replaced the IC, checked voltages and resistances), I am too the limit of my analog experience.

Thanks!
~J
 

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toozie said:
Howdy,
I am using a national semi **broken link removed** (though only using one of the two amps on it) as a microphone preamp. I followed the datasheet to a T, but I feel like I am missing something vital. I am using the parts that the schematic spells out (seen in the schematic attachment).

If I power the circuit with 5V and have "nothing" going through the mic, I see Vcc/2 at the V+ and V- pins like I would expect and ~0V on the output.

If I start making noise into the mic, I see the V- pin start to fluctuate (which I can believe) and I start getting a voltage out of the output. But if I scope the output, I see the voltage swinging above an below 0V (sometimes as high/low as 3.3V/-3.3V, but always centered at 0V.

At first I thought this was normal, so I started putting a DC offset on the output to make the output stay positive (mostly). But the more I read the datasheet (I am not really an analog guy), the more I realized that I should never be getting a negative voltage out (and the mic was already being biased to Vcc through the 5V resistor). As I talk into the mic, the signal looks like what I would expect, except it is centered around 0V (almost looks like a mirror above/below the 0V line).

Does anyone else with op-amp (pre-amp) experience see where I might be going wrong and/or how I can start debugging this (I have replaced the IC, checked voltages and resistances), I am too the limit of my analog experience.

Thanks!
~J

I'm not sure what the problem is...

The signal is centered around zero volts because you have a DC blocking capacitor at the output, and it will surely swing in both directions. Put the scope before the cap and you'll see that it won't swing below 0V. You want this DC blocking cap, because you have a single power supply op amp amplifying an AC signal.
 
Analog said:
I'm not sure what the problem is...

The signal is centered around zero volts because you have a DC blocking capacitor at the output, and it will surely swing in both directions. Put the scope before the cap and you'll see that it won't swing below 0V. You want this DC blocking cap, because you have a single power supply op amp amplifying an AC signal.

Really?! So it would appear (based off of the little info I gave you), that the amp is working as one would expect? So if I see it is purely positive before the cap (and assume it is working), I can go back to adding in my DC coupling (VCC/2 via two 10k resistors) post the cap to make sure everything stays positive going into my ADC?

~J
 
toozie said:
I can go back to adding in my DC coupling (VCC/2 via two 10k resistors) post the cap to make sure everything stays positive going into my ADC?
~J
No, just get rid of C2 and connect the output of the OpAmp to your ADC.
 
Hello Toozie,
A capacitor decouples the DC component from the AC component of any signal. For your particular case it would be better if your signal keeps its DC component before it goes into the ADC.
best of luck!

Antonio
 
toozie said:
Really?! So it would appear (based off of the little info I gave you), that the amp is working as one would expect? So if I see it is purely positive before the cap (and assume it is working), I can go back to adding in my DC coupling (VCC/2 via two 10k resistors) post the cap to make sure everything stays positive going into my ADC?

~J
Get rid of the final cap so that the ADC sees only positive, unless, of course, its a bipolar ADC. Do you have enough gain for the ADC to get a decent voltage swing?
 
Youall are absolutely right! I guess I was over thinking it. I knew that the cap would be getting rid of the DC bias, but I guess I hadn't thought that things could swing negative with it in there. Thanks to everyone for helping me past that. I checked it on a scope today and it looked fine (once I removed the cap), it is exactly how I would have expected it to be when I first wrote the post! Youall can take th day off now!

Do you have enough gain for the ADC to get a decent voltage swing?
I think so. It is a 50:1 gain, and I actually think it might be a little too much. With the mic a foot (or so) from my mouth, I can see a slight rippled on the scope (with 1V/div) while talking normal, but as I get louder, or pull it closer, I can see that I am hitting the rails. I guess I will have to play with it and see.

~J
 
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