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Topic: LM358 saturates the output (Read 3846 times)previous topic - next topic

Yoram

Jun 30, 2017, 01:38 pm
Trying to sum two audio signals using LM358 (see attached drawing).

The problem is that the output gives only the lower part of the sine wave (see sketch in the drawing). This happens even if the input signal is very very low.

There is also a D.C voltage of about 3 volts. Can anyone suggest why this should happen. I tried adding a pull-up of 1K and it fixed the issue. But overall I am not happy to design something that I do not understand.

Any help is appreciated

Southpark

#1
Jun 30, 2017, 02:05 pmLast Edit: Jun 30, 2017, 02:33 pm by Southpark
Try a 33 K resistor instead of the 150 K ..... and reduce the AC input voltage levels..... ie. make AC waveforms at the input  smaller.

Or.... if you don't want to make the input AC signals smaller.... then make the 75K resistor smaller.

Circuit theory for ideal op-amps working in negative feedback mode - automatic equalisation of the voltages on its input + and - terminals. If you deliberately make the + terminal equal to roughly half the supply voltage... aka 7.5/2 .... or about 3.75 Volt, then that will lead to a DC level at the output. The basic op-amp tutorials usually teach this.

So if you then have any AC voltage at the output, then that AC will basically ride on the DC level. This means ... 3.75V DC will translate to the 'zero' Volt level of the AC signal. The AC signal will change above and below this 3.75V DC level. If you have the power supply limits... of 0V and 7.5V .... then 'ideal' case output would allow you to reach as high as 7.5V DC, and go as low as 0V DC. But...... for real op-amps, with component voltage drops and things like that inside the op-amp..... you might not be able to go as high as 7.5 V. Instead, you might be able to go as high as 6.5 V ... maybe. Depends on the op-amp.

In your case.....  you made the DC level at the + terminal equal to 2V. Should change your system to make it around 3.75 Volt.

Wawa

#2
Jun 30, 2017, 02:07 pm
Output voltage swing of the LM358 is limited to about 1.5volt below VCC.
So the positive part of the wave is clipping if you try to output more than 3.5volt peak/peak on a 5volt supply.
Only an opamp with rail2rail outputs is able to swing too (almost) VCC and (almost) ground.

DC offset on the output should be about 2volt with that 100k:150k divider at the +input of the opamp.

I assume the output is connected to an analogue Arduino input (not driving something else).
Leo..

Yoram

#3
Jun 30, 2017, 02:37 pm
@Leo

Thanks for the note about the 1.5 gap from the positive rail... I think however, that the saturation starts a lot earlier.

The output is not connected yet to anything. With those resistors,  I tried balancing the DC resistance on both inputs so that the DC differential voltage is close to zero considering the leakage current.
Still the output DC voltage at zero input is not zero and can be something like 2-3V.  .

Yoram

#4
Jun 30, 2017, 02:44 pm
@Wawa

Thanks for the heads-up on the output DC level.

Regarding the input DC level, in reality I changed the divider on the plus signal so that I get a VCC/2. Basically it should have been two 150K but I had to change the resistors to get this mid point as appears on the sketch///

Southpark

#5
Jun 30, 2017, 02:45 pm
Correct. The DC voltage on the output is not meant to be zero.

That's why you have to put a DC blocking capacitor on the output of the op-amp. Look up 'DC blocking capacitor output of op amp'..... in google.

Southpark

#6
Jun 30, 2017, 02:52 pmLast Edit: Jun 30, 2017, 02:58 pm by Southpark
Regarding the input DC level, in reality I changed the divider on the plus signal so that I get a VCC/2. Basically it should have been two 150K but I had to change the resistors to get this mid point as appears on the sketch///
Your diagram has 5V. So it should be something like 33K between 5V and + terminal, followed by 100K between + terminal to ground. Or, alternatively.....  150K between 5V and + terminal, followed by 450K between + terminal to ground.

Yoram

#7
Jun 30, 2017, 03:27 pm
I am not worried about the output dc voltage, I know I can remove it with a cap. The problem is that I am not able to locate this dc level consistently at say vcc/2 (or (vcc-1.5)/2) so that I can get the max swing without saturation...

TomGeorge

Everything runs on smoke, let the smoke out, it stops running....

Southpark

#9
Jun 30, 2017, 03:43 pmLast Edit: Jun 30, 2017, 05:39 pm by Southpark
I am not worried about the output dc voltage, I know I can remove it with a cap. The problem is that I am not able to locate this dc level consistently at say vcc/2 (or (vcc-1.5)/2) so that I can get the max swing without saturation...
That's ok. I was just responding to your comment you made below, where it appears that you assumed that the output DC level should be zero.

Still the output DC voltage at zero input is not zero and can be something like 2-3V.  .
To get the output DC voltage roughly to be at Vcc/2 ..... you could :

option a) use the 7.5 V source, and use two of the same resistors eg ... 10K and 10K to make a voltage divider.... which will be used to set the + terminal to 3.75V

option b) use the 5 V source, and use an appropriate voltage divider to set the + terminal to around 3.75 V.... such as a 33K resistor and a 100K resistor divider.

Your diagram has a 7.5 V source, which will be Vcc. And your diagram also has a 5 V source, which is where you could use the 33K and 100K divider.

And, finally, you won't get saturation if you decrease the gain of amplifier (by, for example, by replacing the 75K resistor with a smaller one)..... or if you use smaller AC input values.

MarkT

#10
Jun 30, 2017, 08:05 pm
Your circuit has a common-mode gain of 3x (well, -3x strictly), no wonder it saturates.  Make the
feedback resistor 22k or so to limit the output excursions, unless the input amplitudes are small.

Normally a virtual ground would be decoupled to ground to reduce noise (ie capacitor from the
non-inverting input to ground).
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

polymorph

#11
Jun 30, 2017, 11:10 pm
What are you doing with the 2nd Op Amp? You cannot just leave it floating. It can go deep into saturation and cause strange effects across the substrate to the other Op Amp section, or go into oscillation. Either can cause the IC to draw excessive amounts of current.

You really need to give us voltages on everything, or we cannot help you.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8

polymorph

#12
Jun 30, 2017, 11:11 pm
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8

Yoram

#13
Jul 01, 2017, 12:01 am
@Polymorph.

I forgot about the second half of the opamp, this could be the culprit. I will look into terminating it properly and post back.

@MarkT
Even if I am using a single input, where in this case the gain is only x1.5, it saturates way before the output signal reaches the rails boundaries.

Southpark

#14
Jul 01, 2017, 01:42 amLast Edit: Jul 01, 2017, 02:01 am by Southpark
Even if I am using a single input, where in this case the gain is only x1.5, it saturates way before the output signal reaches the rails boundaries.
Show your input signal measurements. And show your output signal measurements. This is to make things clear for everybody.

Eg. Set both AC inputs to be zero to begin with. Then increase the amplitude of 1 of the AC signals...... gradually, and slowly. And then, for the case where the output just begins to show onset of clipping --- indicate : what is the peak input AC voltage level (at the node between the 10 microFarad capacitor and the 50K resistor)? And also indicate the output clipping level.

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