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.
First of all, we don't deserve you at all. Thank you for all your good teaching. When I was 22 or so in the early 80s I worked for an outfit as a test tech that made discrete precision DC amps that had a chopper amp for offset null. To this good day I didn't really get it. It wasn't a requirement of us to understand the theory of operation but just to use the prescribed testing methods to get them out the door. As I recall, they measured strain gauges om missiles in silos in at least one application.
That's actually a CAZ (Commutating Auto-Zeroing) amplifier. Chopper amplifiers are different in concept, by turning the DC input voltage to AC , allowing the amplifier to amplify an AC voltage instead of DC, at the output then the amplified AC signal is converted back to DC to recover the original input signal.
During phase B, how can the amp B measure the offset voltage at the input of amp A?. The offset voltages are internal and can only be measured at output right?
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!!!
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.
Thanks for the video.... There's something i do not understand... In Phase B, op-amp B is nulling-out op-amp A using it's own nulling voltage. What happens if op-amps B and A have different internal offset voltages ???
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
Dear EEVblog, Might I ask some small questions about the phase B?1. It's that Input is always connecting to the output through main Amplifier A. So, in phase B, what stored in C2 might be not only the Vos of main Amplifier A but also with the Input Voltage??? 2. Or only the lower frequency Input might have the chance to be stored in C2??? Because for high frequency(HF) input, C2 works as short circuit and pass the HF input without attenuation.3. So the input signal must be AC signal (chopped DC signal) in this case?Best regards and really look forward to your instruction and recommendationYour tutorial saves my shallowed experience, and really thank you very much!!!
I was also wondering about your first question. www.eetimes.com/document.asp?doc_id=1224795 explains for the same circuit how the total offset gets minimized; it's a bit more complicated than the nulling of opamp-B and involves some math, but it's worth going through it.
I dont think the block diagram shown in this video is accurate. open loop gain of amplifier A and B are quite high - if the offset voltage is ~10mV and gain is 80dB (example) , then the output voltage of the amplifier will saturate during offset cancellation phase. Shouldn't the amplifier be in unity gain negative feedback loop while cancelling the offset to avoid this issue?
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.
nice vid.i was wondering if these would make a good preamp for say a lm386.i currently built a crystal shortwave radio using. a 741. 386 combo.i have noticed dc noise from the 741. but im not sure of the frequency ratings of a chopper type amp.anyway great vid
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.
I guess he knew It all the way. "What is SEO?" means "SEO? Do I look like a marketing dude?!" in this situation. Not that he actually doesn't know what it means.
@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) ;)
Chopper technique is different from Auto-zeroing technique. They are completely different. Differences (1) Chopping eliminates input referred random offset as well as input referred device random noise. Auto-zero eliminates only offset while in-fact it amplifies device noise and makes the op-amp more noisy (2) Mathematical z-domain transfer functions are also different for chopper and auto-zero techniques (3) Chopping requires an internal band-stop filter to eliminate the noise at Fclk and it's harmonics. Auto-zero does not require any such filter More-over, chopping amplifiers themselves can be of fundamentally different types depending on ratio of Fclk and Floop_bw. The video is good. But it's just that chopping requires the audience to have a ton of background on z-transform, random noise, random offset, noise shaping, CMOS Opamp Architectures etc. which defies the whole intent of this video, which is to simplify the topic. This may be of some help: ieeexplore.ieee.org/document/542410
