removing offset voltage from charging capacitor

Thread Starter

pager48

Joined Nov 25, 2018
160
Buffering a timing capacitor on an ne555 oscillator with an opamp voltage buffer to use its triangle wave shape.

Are a stack of series diodes the only way to remove the offset after the buffer?
 

LesJones

Joined Jan 8, 2017
2,316
You should be able to configure the op amp as a buffer with no offset. You would need to post the schematic so can see what is causing the offset.

Les.
 

Audioguru

Joined Dec 20, 2007
11,251
The poor-looking triangle waveform averages +6V (the offset voltage) which is half the supply voltage that is probably +12V.

The buffer opamp probably also uses the +12V supply and probably is a simple follower with a gain of 1. But the opamp needs an additional negative supply and a bias reference voltage so that its output can average 0V (the new offset voltage).
 

bertus

Joined Apr 5, 2008
20,041
Hello,

The capacitor voltage of the 555 is between 1/3 and 2/3 of the supply voltage.
To remove the offset, you must subtract 1/3 of the power supply voltage.

Bertus
 

iimagine

Joined Dec 20, 2010
388
The real question is, why use an op-amp to buffer a crappy looking waveform rather then just interrogate in the first place ?! :confused:
 

Audioguru

Joined Dec 20, 2007
11,251
The person who started this thread did not say if the output must be 0V to +4V or from -2V to +2V.
 

Thread Starter

pager48

Joined Nov 25, 2018
160
The simplest way should be to use a 2 channel lm358 the fist set as inverting to drop waveshape to baseline the next non inverting to raise it to Vcc. But non inverting only takes negative going voltages.

Without diodes seems there needs to be negative supply.

 

Alec_t

Joined Sep 17, 2013
10,350
You can remove DC bias by taking your output from the buffer via a capacitor. The cap value will depend on wave frequency, load impedance and how much distortion you can tolerate.
 

crutschow

Joined Mar 14, 2008
23,293
Is this what you want?
The offset will track changes in the supply voltage to keep the lower peak at zero volts.
The op amps need to be rail-rail type.

upload_2019-8-21_11-12-47.png
 

crutschow

Joined Mar 14, 2008
23,293
Try an LT1638.
The LT1495 frequency response may have been too low.

I added a pot to allow trimming of tolerance offsets.

I varied the supply voltage from 10V to 14V, showing that the output low peak stays at zero, independent of the voltage change.

upload_2019-8-21_13-31-31.png
 

Thread Starter

pager48

Joined Nov 25, 2018
160
Try an LT1638.
The LT1495 frequency response may have been too low.

I added a pot to allow trimming of tolerance offsets.

I varied the supply voltage from 10V to 14V, showing that the output low peak stays at zero, independent of the voltage change.

View attachment 184469
The LT1638 worked fine. Would these https://www.diodes.com/assets/Datasheets/TLV27x.pdf https://www.diodes.com/assets/Datasheets/TLV27x.pdf work inplace of it?

Opamps have low input current but going straight from the timing capacitor into the rail-rail opamp without any buffers doesn't work.
 

crutschow

Joined Mar 14, 2008
23,293
Those Diodes op amps should work okay.
Opamps have low input current but going straight from the timing capacitor into the rail-rail opamp without any buffers doesn't work.
Why do you say that?
Without what buffer?
If you are referring to the first op amp non-inverting buffer, of course you need that.
 

Alec_t

Joined Sep 17, 2013
10,350
Since you already have opamps, why not ditch the 555 and build the 2-opamp triangle generator shown in the first link suggested by Bertus in post #10? That will give a more linearly sloped triangle and avoid the need for a buffer stage. You would need a third opamp to remove the offset.
Why do you need to eliminate the offset?
 

Thread Starter

pager48

Joined Nov 25, 2018
160
Since you already have opamps, why not ditch the 555 and build the 2-opamp triangle generator shown in the first link suggested by Bertus in post #10? That will give a more linearly sloped triangle and avoid the need for a buffer stage. You would need a third opamp to remove the offset.
Why do you need to eliminate the offset?
The issue is it should be single supply. I did put together a circuit from one of the links but it turned out to be a specific chip. https://www.maximintegrated.com/en/app-notes/index.mvp/id/4362 That ne555 is very scaleable.

Those Diodes op amps should work okay.Why do you say that?
Without what buffer?
If you are referring to the first op amp non-inverting buffer, of course you need that.
In that case the noninverting buffer should be some transistor buffer to save board space.
 

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crutschow

Joined Mar 14, 2008
23,293
In that case the noninverting buffer should be some transistor buffer to save board space.
The TLV272 op amp is a dual in one 8-pin package.
How will using a transistor save board space?
 
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