I was wondering if anyone had thought about using a high CMRR instrumentation amplifier like the AD8429:
AD8429 datasheet and product info | 1 nV/?Hz Low Noise Instrumentation Amplifier | Instrumentation Amplifiers | Analog Devices
as part of a THD test facility. What I mean is to feed an oscillator input to one input of the in amp, feed the output of a DUT to the other input of the in amp, and thus the output would be the distortion which then could be fed to a FFT analyser. Seems like a simple idea to me.
AD8429 datasheet and product info | 1 nV/?Hz Low Noise Instrumentation Amplifier | Instrumentation Amplifiers | Analog Devices
as part of a THD test facility. What I mean is to feed an oscillator input to one input of the in amp, feed the output of a DUT to the other input of the in amp, and thus the output would be the distortion which then could be fed to a FFT analyser. Seems like a simple idea to me.
As above, plus unless you build the rest of the THD analyzer with auto tuning and phase nulling, you'll probably degrade the FFT output as opposed to improving it!
I do like differential tests where you compare the input and output of the DUT, say a power amp, playing music into a load, and the instrumentation amp is perfect for that.
I do like differential tests where you compare the input and output of the DUT, say a power amp, playing music into a load, and the instrumentation amp is perfect for that.
Yes it's simple, if you can adjust the two levels to be exactly the same (for a -100dB distortion resolution, for instance, you need to match levels to 1:100,000 which is about 0.0001dB) and also compensate for the additional phase delay for the signal going through the DUT
Thanks. I had actually realized that I have to match levels, but didn't mention it. I was thinking of providing some kind of tuning indicator that shows when the signals are exactly equal. I had not done the math though, so a 0.0001dB match would probably be really hard to make unless the error was amplified by 100dB or so.
The issues of phase shift I had not considered and would be an obvious problem.
I like this idea though because it would cancel out the distortion components from the osciallator.
Yeah, it's all still a thought bubble in my head right now. I don't know about correcting for phase shifts, so I have some reading to do. Thanks for your reply.
Yess
- Analog Devices AN-245 : Instrumentation amplifiers solve unusual design problems, by Scott Wurcer and Walt Jung - page 10-15. It works very well, and if you're only interested in extending the dynamic range of a spectrum analyzer, then you can relax quite a bit limits on null depth - Bob Cordell's THD magnifier is based on the very same principle.L.
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Yess
L.
Thanks! It looks like I've reinvented the wheel, which is a good thing actually. I have some reading to do!
I've never looked at Bob Cordell's "THD magnifier", so thanks for the tip.
Based on published specs you would have to look hard to beat the THAT1510: http://www.thatcorp.com/1510-1512_Audio_Preamplifier_ICs.shtml
The Burr-Brown (T.I.) INA103, INA163 and INA217 occasionally get mentioned as mic preamps.
Dale
Ah yes, why didn't I think of them?
The thing with in amps is that the CMRR changes with the gain setting. I have to wait and see right now. I have to wait until I get Linear Audio number 0 so I can read Bob's article and hopefully get inspired.
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