Simple op amp circuit very noisy

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I made a simple voltage follower on a breadboard with an audio op amp (LME49726) and I'm getting so much noise that the audio signal is barely intelligible at the output. This is the datasheet of the op amp: http://www.ti.com/lit/ds/symlink...

The positive rail is  at +3V and negative rail is Gnd. The audio signal is connected to the same Gnd as the op amp. I checked the connections and I'm not sure how else to troubleshoot because it's such a simple circuit...any suggestions?

 

Last Edited: Mon. Oct 17, 2016 - 06:09 PM
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An audio signal is AC isn't it? So it goes +ve and -ve. How will the output of your op-amp go -ve with no -ve supply rail?

#1 This forum helps those that help themselves

#2 All grounds are not created equal

#3 How have you proved that your chip is running at xxMHz?

#4 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand." - Heater's ex-boss

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Post your Schematic...even if hand drawn and a photo taken so we can see how you have things connected.

 

Jim

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Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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The signal MUST be between 0V and 3V at all times. In practice, depending on the op-amp, you can only get to within a few 10s of mV to ground and the same for Vcc. If I read the specs correctly, the input signal must stay above 0.1V and below (3V-1.6V) = +1.4V. This is set by the input common mode range. Almost all of the examples appear to be in inverting mode where the common mode voltage is set at 3.0V/2 but that is outside of the common mode range using a 3V supply!

 

Even though it says it will operate down to a supply voltage of 2.5V, the operating conditions appear to be VERY limited and VERY challenging below about 3.5V.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

Last Edited: Mon. Oct 17, 2016 - 06:56 PM
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need a bias of vcc/2

      o
      |
     [ ] R
   C  |
o-) (-+----- to plus input
      |
     [ ] R
      |
      V
      

 

Imagecraft compiler user

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Ahh thanks for pointing this out, it does make sense that an AC audio signal would need to have a negative rail for the negative portion of the AC signal...how does this microphone pre-amplifier work if it doesn't have a negative rail? https://www.sparkfun.com/datashe...

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R1 and R2 bias the opamp to VCC/2 (just as pointed out by Bob in post #5), while C1 provides a high pass filter for the signal so that the amp sees an input signal centered around VCC/2.

David (aka frog_jr)

Last Edited: Mon. Oct 17, 2016 - 11:46 PM
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If you look at the datasheet for the ADMP401, you'll find this:

Output DC Offset  0.8V

That means the AC signal is centered around 0.8V, and it never goes negative since it has a common ground with the connected op amp.  If you have a signal that does go negative in relation to your gorund, you can always just apply your own DC bias to ensure the signal stays within your voltage range, like so: http://electronics.stackexchange.com/questions/14404/dc-biasing-audio-signal

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I haven't dealt with op amps in a long time, thank you for the refresher laugh...I'll experiment now. I'm just concerned by ka7ehk's comment above about it being a problem that the operating voltage is 3V...although I plan to use 3.3V later on.

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My ultimate circuit is a summing amplifier to mix different signals. I applied a VCC/2 bias at the positive op amp terminal and it works! The audio quality isn't as good as it could be though...how do you choose the input capacitor value? I am setting all resistors to the same value of 10k so it just adds the signals without any gain. What else can cause noise/distortion?

 

 

Update: The audio sounded better when I connected Gnd of the audio signal to VCC/2...

Last Edited: Tue. Oct 18, 2016 - 04:12 AM
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What is Vout connecting to?

#1 This forum helps those that help themselves

#2 All grounds are not created equal

#3 How have you proved that your chip is running at xxMHz?

#4 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand." - Heater's ex-boss

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Sounds better because you have to connect to mid-rail (voltage divider output) whatever was connected to ground in the original circuit (probably even the output ground, I think), not just the opamp +IN. This includes the input ground. Maybe you should buffer the divider output with another opamp (configured as buffer) and see if things improve. If the output ground also needs to be connected to mid rail, you will need a buffer for sure (probably another, identical opamp to handle the same current).

You need to remove the DC bias of the output somehow, either connect output ground to a mid-rail buffer, or place an output capacitor. I really don't know much about audio, so I might be wrong.

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The datasheet says that part is two op amps in one package.  How have you connected the unused amplifier?  If you leave the 2nd amplifier pins floating strange and mysterious things may happen.

Letting the smoke out since 1978

 

 

 

 

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In that mixing amp, the - input needs to be biased to vcc/2, not grounded. Got a scope and a signal generator? (mic, guitar, mp3 player). Look at the output with the scope and turn the music up/down to keep the output from clipping. I guess you could gator clip the output to the line in on a stereo and listen to it. When it clips it should sound real groady.

 

Imagecraft compiler user

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I think you need a breadboard, an op-amp cookbook, an O'scope, and some time working with real world signals.

 

The circuit below isn't a mixer, but it demonstrates several concepts.

 

It uses rail-to-rail, 3V op-amps.

 

It uses a unipolar power supply, (0/3V, not +/- X volts).

 

It uses a 1.5 V Virtual Ground.

 

It has two "AC Coupling" stages, that block any DC offset and keep the signal centered at 1.5 V, ("Ground").

 

It includes several gain stages and filter stages.

 

JC

 

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In the schematic in msg #10, you cannot connect the + input to ground. That is outside of the input common mode range which is between +0.6V and Vcc - 1.6V. For starters, make a voltage divider that will make +1V from +3V and bias the + input with that. You will find a huge difference.

 

The 3 signal inputs are capacitor coupled, so the - input will float along to what ever value the + input is biased at. With no signal, the output will also be at the same voltage (or very close to it). 

 

Once you have that working, you have the challenge of optimizing the bias to fall within the INPUT common mode range AND to get the largest output swing you can.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Sorry for the confusion, the image I posted to #10 was something I found on a search...I am biasing the positive op amp terminal at VCC/2. Here's my quick sketch of the way I have it connected now.

 

 

 

 

Not as sophisticated as DocJC's circuit lol...is this design too simplified? The first stage is a summing amplifier, which has an inverted output so I thought a second inverting stage would be needed to de-invert the signal...but it doesn't seem to make a difference in the sound! Why is that? I'm not sure if the second op amp is necessary.

 

Also, I don't want the signals V1 and V2 to be affected at all, so it's unity gain. Should the input signals also need to be biased at VCC/2 instead of being connected to Gnd? 

 

How do you choose the capacitor value? The 0.1 uF caps seem to be working fine...

 

Any other suggestions? Thank you!

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The second stage provides no gain, so unless you need a signal that is NOT inverted with respect to the input(s), I see no reason for it. 

 

For now, I would use two 10K resistors in the top half of the  bias divider. That would make the bias voltage 3V * 10K/( ( 10K + 10K) + 10K) = 1V. The 1.5V you have now is outside of the input common mode range.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Our ears can't hear the difference between an audio signal and that same signal that has been inverted 180 degrees.  That's why the outputs of the inverted op-amp and the 2nd (unity-gain) op-amp sound the same.

 

One trick that does bring out audio differences with signal inversion is to take the Left and Right outputs of old pop songs from the 1960's and 1970s, invert only the Left channel, and then mix the inverted left and unchanged right together (as in the voice/drums/guitar mixer above).  Then the audio that is common to both channels [ that appears in the center of the head with stereo headphones] gets cancelled out.  This leaves a mono signal with the only the sounds unique to each of the left and right channels.   Sometimes both channels are delayed by the same amount (about 5-10 milliseconds) and then one channel gets delayed @ 0.5milli seconds more.  This helps adjust what L/R mix of the sound gets cancelled.

 

Another way to remix old stereo records is to have one channel be the addition of the left and right signals with a 50% attenuation.  The other channel is Left channel - Right channel with some signal gain.  With headphones,  instruments that were in the center of the head are now re-positioned to center/left or center/right.
 

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As far as choosing the input capacitor values, the C value and the 10K input resistor set the low corner frequency (1/(2*pi*R*C) ). A 0.1uf and 10K give a corner of about 150Hz and that seems a bit high. 1uf would make that 15Hz which is probably lower than you need.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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I don't think you mentioned the Peak-to-Peak voltage of the two input signals.

 

Know that you need enough voltage swing range in the op-amps for the sum of the two signals, and still be within the linear (non-clipped) range of the op-amp, or you will hear the distortion from the clipping of the summed signal.

 

You really need a "better" op-amp that goes rail-to-rail, and you need to carefully specify the input signal amplitudes.

 

JC

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Your capacitively coupled signals would probably benefit from a high value resistor divider on the "output" side of the capacitor, as in my schematic above. 

That resistor divider sets the DC bias for that signal as it feeds into the next stage, (op-amp).

You would like those resistors to be significantly larger than the input resistors feeding the next stage to minimize their impact on that stage's performance, (gain, filter frequency, etc.), unless you want to take the time to calculate in their actual effect.

 

You also might wish to use an active driver for your virtual ground.

With or without an op-amp driver you still ought to put a cap on the virtual driver's resistor divider, again as seen in the above schematic.

It will help to hold the virtual ground voltage stable from fluxuations based upon the current being draw from the virtual ground, and from noise on the V+ supply.

An RC bridge for setting the virtual ground voltage is usually fine for audio work, but one might not use it for precision ADC work, (low frequency drift, etc.).

 

Do you have an O'scope?

 

JC 

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DocJC makes a good point about "stiffening" the virtual ground. You might use the other half of the dual op-amp for that. For many applications, simply adding a capacitor to ground would do it. With 10K resistors in there, I would use at least 1uf. And, I would do that even if buffering with another op-amp.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

Last Edited: Tue. Oct 18, 2016 - 11:09 PM
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I was thinking something like this:

 

Mixer

 

that is, using the second opamp as a buffer to create a virtual ground, or bias, or whatever you call it (here I made it pot adjustable). This is then used to bias +IN and as ground for the output. The inputs are capacitor decoupled, so they can use normal ground or virtual ground, should not matter except for allowable swing.

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The peak to peak voltages fluctuate but the maximum of the first signal is about 1V peak-peak and the second signal maximum is about 200 mV peak-peak:

 

How does it help to have the virtual ground be created by an op amp instead of biasing the positive terminal of the first op amp?

 

That's interesting that you can't hear the difference between an audio signal and its 180 inverted form!

 

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I am wondering if the AMPLIFIER is causing that noise. If the input coupling has too high of a corner frequency, then the sine will be attenuated, leaving all that noise which is at a much higher frequency. I have not stopped to figure out the frequency of the input signal because I don't know whether that time number is per division or for the whole sweep. If it is per division, then 20ms corresponds to 50Hz. Your earlier circuit had an input coupling frequency of about 150Hz so if that is what you used, the signal would be down several db. Thats not enough to cause all of what you are seeing but it could be part of it.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Try disconnecting your input signals and then tying your inputs to your virtual earth and examining your output. The result is due to your choice of opamp and its connections.

 

Ross McKenzie ValuSoft Melbourne Australia

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The time is per division. The second signal that looks really noisy is actually a microphone output. Thanks for the suggestions! I think it's fine after all but I wonder if there's a more ideal op amp for this application, there's so many to choose from! I selected it simply because it's an audio op amp. It seems like some audio products use general purpose op amps so I thought an audio op amp would be super high performance. 

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There are two places where "audio" op-amps are important. 

 

First is low level amplification. Many op-amps have an inordinate amount of low frequency noise (typically called "1/f" or "shot" noise). It tends not to be a problem in many applications but the human ear can detect it as quite annoying. You see this in "microphone amplifiers" and such, where the incoming signal is quite small. 

 

The second area for audio op-amps is output. Typical op-amps have a hard time with the kinds of loads that are prevalent with audio. The worst are low impedance loudspeakers but headphones can also be challenging (depending on the type). So, we have special op-amps that can drive some of these special loads. The spec sheet (that you referenced) talks about "low impedance loads" but their idea of low impedance is 2Kohm, not a 8ohm (like a speaker). The spec sheet DOES talk about use as a low-level (eg, microphone) amplifier so it would seem appropriate for your application. 

 

An important thing to note is that these two situations are rarely satisfied by a single amplifier. The ones that can drive audio loads are rarely good at handling low signal levels, and visa versa.

 

You may note that these are at the opposite ends of an audio amplifier chain. In-between, the use of "audio" amplifiers is less important. For these in-between stages, issues of peak-peak signal capability, power consumption, bandwidth, and such, tend to dominate. 

 

Your challenge, in this application is, I believe, one of proper biasing. Using an op-amp at such low supply voltages requires careful design. Things that you could do in a 12V system simply won't work at 3V. Even 5V is more forgiving than 3!

 

Hope this helps.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Lots of guitar pedals run from a 9V battery.

Imagecraft compiler user

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That allows around 6V ppk signal, or more, depending on the op-amp. That is a whole different ball game from 3V0 or 3V3.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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What is your goal?

If you just want to get this single circuit going a bit, then keep on rowing.

 

If you really want to know how opamps work, then get a decent book.

Ti's application note "opamps for everyone" is a very good one (and free).

 

Via this link you can find a lot of free electronics books.

http://www.freebookcentre.net/El...

 

bobgardner wrote:
Lots of guitar pedals run from a 9V battery.

They do that on purpose to distort the signal even further :)

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

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if its a distortion pedal. But if its a delay or a chorus, the audio signal is fairly hifi. No one has mentioned signal to noise ratio. You need to keep the signal as hot as you can without clipping, so it stays out of the noise. This is harder to do with 5V opamps than it is with +-15V opamps, which have a lot more signal.

 

Imagecraft compiler user

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bobgardner wrote:
You need to keep the signal as hot as you can without clipping, so it stays out of the noise.
The following is a low noise microphone amp with Automatic Gain Control (AGC) on 3V power :

Maxim Integrated

Maxim

MAX9814 Microphone Amplifier with AGC and Low-Noise Microphone Bias

https://www.maximintegrated.com/en/products/analog/audio/MAX9814.html

Integrated Automatic Gain Control Solves Audio Output Level Fluctuations

...

Its specs look good other than PSRR.


Adafruit Industries, Unique & fun DIY electronics and kits

Electret Microphone Amplifier - MAX9814 with Auto Gain Control ID: 1713 - $7.95

https://www.adafruit.com/products/1713

...

This fully assembled and tested board comes with a 20-20KHz electret microphone soldered on.

...

 

"Dare to be naïve." - Buckminster Fuller

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Yeah But.... blues guys dont want no darn AGC fiddlin with their guitar volume unless they turn it on. The carbon based units are superior to the silicon based units.

 

Imagecraft compiler user

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I understand the "common mode input voltage range" now and I have one more question: If I change the supply voltage to 3.3V, the common mode input voltage range is now between 0.1V and 1.7V from the datasheet spec. If I make the bias voltage VCC/2 = 1.65V, would this be okay being below 1.7V, or do the ac audio signals also need to be taken into account?

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Your input signal should be in the range VSS+0.1 to VDD–1.6V but the output can be VSS +0.004 to VDD–0.004 with a load of 2k to VDD/2.  This means that if you want the full rail to rail output the Opamp needs to have gain >1, I'll leave you to do the calculation. 

 

David 

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 or do the ac audio signals also need to be taken into account?

Yes they need to be taken into account!

 

The op-amp doesn't know the difference between an input bias signal or an input audio signal.

The full swing of the input signals, bias and audio, must be within the valid input signal range.

 

The full range of the output signal must be within the valid output signal range, (Rail-to-Rail, or otherwise, based upon the chip).

 

If you use gain, then it is important to reference the "neg" input to the bias voltage, otherwise the gain times the bias voltage will make the output voltage out of range.

 

JC