clintrubber

THD-measurement with diffamp but no filter
« on: January 04, 2005, 09:18:45 AM »
FWIW, I saw an idea in an older issue of Electronic Design for simpler THD-tests.

The article presents an idea with an INA217 but doesn't elaborate much further - nor does it address calbration etc. But still, could be of use:

http://www.electronicdesign.com/Articles/ArticleID/8188/8188.html

Bye,

   Peter


gyraf

THD-measurement with diffamp but no filter
« Reply #1 on: January 04, 2005, 09:43:43 AM »
It's doable - but remember that any phase lag through the device-under-test will (wrongly) show up as distortion.

The HP333 distortion analyzers are going relatively cheaply these days..

Jakob E.
..note to self: don't let Harman run your company..

clintrubber

THD-measurement with diffamp but no filter
« Reply #2 on: January 04, 2005, 10:18:55 AM »
Sure, better to score a ready to go box instead of trying to get this alternative to work.

The idea is nice but was mainly meant just to have a look at it. It sure has some things that have to be addressed. Like the lag you mentioned, and there's the 1:1 requirement. Can be addressed, but...

I don't have one of those cool looking HP's  :cry:  :cry:

but instead this ugly Japanese box:

 :wink:

NewYorkDave

THD-measurement with diffamp but no filter
« Reply #3 on: January 04, 2005, 12:39:22 PM »
If you have a good AC millivoltmeter to use as a detector/indicator, and if you don't mind manual nulling, it's not that hard to make your own THD analyzer. I built one a few years ago. Essentially, it's a tunable Wien-bridge circuit wrapped around a simple op-amp circuit, with overall feedback to sharpen the notch. Matter of fact, mine was TOO sharp, which made it tough to tune. I always meant to go back and tweak the design, but never got around to it.

THD-measurement with diffamp but no filter
« Reply #4 on: January 04, 2005, 01:09:38 PM »
I want an AP!!!! :mad:
Seth

The definition of insanity: "Hey, this thing could be really cool - if I had the money to fix it up, which I don't, but if I did it could be really cool - if I had the time, which I don't, but

PRR

THD-measurement with diffamp but no filter
« Reply #5 on: January 04, 2005, 01:49:34 PM »
That basic idea is old as the hills

> there's the 1:1 requirement.

In most cases, the DUT has gain, so you put a resistive divider on the output and trim for overall unity gain. That means a precision diff-amp is NOT needed, what you really end up with is a diff-amp with different and adjustable gains on each input. You need high internal CMRR, and the classic 4-R diff-amp may not be as good as an inverter-summer.

Also note that if the DUT is inverting, you don't need no diff-amp. Just sum the input and output with adjustable gain to get a null.  

> any phase lag through the device-under-test will (wrongly) show up as distortion.

That is sure true in the classic use, with voltmeter readout. For midband audio, a very small phase-shift will give a null on the fundamental (though it makes the harmonics a mess).

But what is novel here is driving a spectrum analyzer. So I don't think fundamental phase matters. It isn't vital to get a DEEP null. What they are doing is getting "most" of the fundamental out of the result, so the spectrum analyzer input isn't stuffed full of strong fundamental while trying to read a weak residual.

clintrubber

THD-measurement with diffamp but no filter
« Reply #6 on: January 04, 2005, 07:11:21 PM »
Quote
What they are doing is getting "most" of the fundamental out of the result, so the spectrum analyzer input isn't stuffed full of strong fundamental while trying to read a weak residual.


Makes me think a bit of your '67 diffamp-thread: that one being a microscope for AC and getting rid of DC and here it's about getting rid of the fundamental and zooming in on the harmonics.

Quote
That basic idea is old as the hills


Had never seen it for this use - but  :thumb: to EDesign, otherwise I hadn't been aware of it.  :wink:

Quote
> there's the 1:1 requirement.


True, not a problem.

Quote
That means a precision diff-amp is NOT needed


I figure the writer (TI-person) was trying to sell us BB-INA217's  :wink:  :grin:  :shock:

Thanks for the other considerations as well.

Bye,

   Peter


 

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