EEVblog #24 - The secret world of Chopper Amplifiers

A tutorial on the secret world of Chopper (auto-zero) Amplifiers.

Category:

Science & Technology

Tags:

• opamp
• chopper
• amplifier
• auto-zero
• offset
• voltage
• noise
• switch
• thd
• distortion
• maxim
• max4239
• measurement
• demo
• oscilloscope
• gain
• bandwidth
• operational
• how
• it
• works
• tutorial

[Afrotechmods]

Excellent videos but you should be adding better titles. Browsing the related videos and your youtube channel can be troublesome because "EEVBlog #xx" doesn't really tell us what the video is going to be about. Also think about it from an SEO perspective.

[EEVblog]

Thanks.

Yes, I've been thinking about doing this.

What is SEO?

[michaluszynski]

Dear Dave

Based on Your drawing "B" amplifier should have gain=1 (shouldn't there be closed a loop placed between negative input and output) to properly store offset in the output caps

Didn't You forget to place feedback loop switches between "output" and '-" input of "A" amplifier- to obtain gain=1during sampling phase I guess there are also missing switches which disconnect outside world during offset cancellation phase

If my assumptions are wrong please correct me

Regards

Michal Uszynski

[EEVblog]

The drawing is deliberately simplistic to show the basic operation of the composite amplifier structure. So I didn't want to go into more detail, as actual structure can vary between devices. However, it's commonly described as you see it.

[ntjbjhq]

Loved this video! Could you show your test circuit you used to look at the noise on the chopper amp? I'd be interested in seeing that as well.

I look forward to your next video blog!!!

[EEVblog]

it's nothing special, just the MAX4239 connected as a non-inverting amp with a gain of 100. Function gen on the input, scope on the output.

The full circuit for my uCurrent is on my web page. Google "ucurrent" (looks like I can't post links in here)

If you mean the Audio Precision THD graph measurement, that was done with an AP2700 analyser and XLR-Banana test cables. Sorry I don't have that setup any more, nor any photos.

[YANKTON1000]

Hi - #24 - good treatment of a complex subject. I would like to use your audio spectrum analyzer to look at the op-amp noise at 0.001Hz. The reason is that chopper op-amps get jiggy at low-frequencies when used with capacitors. Is it possible to 'trick' out the FFT to look as far down as 0.001Hz?

Charles Gilbert, drcg5@yahoo.com

p.s. The MAX4238 is the lowest-noise chopper at 0.001Hz that I have tested, 20nV rms at 0.001Hz.

[raguaviva]

@EEVblog Search engine optimization

[gvsfgdf]

Dave, as a physics major and massive nerd :D i just wanted to say I really enjoy your videos. Thank you so much for bringing quality material to the table. Keep it up man!

[deb058392]

Hi,

You should change the title of video to autozero amplifier.Chopper amplifier used modulation-demodulation technique for nulling offset.The technique you described is for autozero amplifiers.

[rapsod1911]

There must be few more switches before those 2 amp or during faze B you are not going to sample only Vof , right?

[javedkhan0258]

This video is way better than Myth Buster

[azdinator]

it's just excellent. Thanks a million.

[shodanxx]

@shodanxx oops I was wrong it's not about smps

but I hope you get what I meant

[shodanxx]

@EEVblog this guy is right

often I had trouble finding the other part of a multipart video

and this video should include smps or "switch mode" or something

most people who want to learn about them and aren't familiar with them call them what they call it at wikipedia and that is not chopper but switch mode power supply

(hmm haven't watched the video yet, in fact .. I hope you are in fact talking about SMPSes) ;)

[nbsr1]

Just a word of clarification. This circuit only works when amplifier A is in a closed-loop configuration (which probably is so obvious that Dave didn't put it on the circuit diagram). Only then the input voltage of A equals to its Vos and only then adjusting voltage across C2 nulls Vos(A).