Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
Subject: Re: TI's low-cost 20W "filterless" class-D amplifer IC
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Mon, 23 Dec 2002 13:25:31 -0000
NNTP-Posting-Date: Mon, 23 Dec 2002 13:25:34 GMT
Winfield Hill wrote:
> John Woodgate wrote...
>> Kevin Aylward wrote:
>>> So, do you notice the "creative" specmanship. They show graphs
>>> of THD, with 20Khz around 0.01% and 1Khz around 0.06%. Wonder
>>> how they do that :-)
>> It's called 'insufficient bandwidth'.
> Right, a sharp filter in the path of the audio analyzer, justified
> to keep the switching waveforms out of the instrument's sensitive
> circuits. :>) They discuss these filters in Claus Neesgaard's
> slaa114 app note, "Digital Audio Measurements."
> They use AES17 filters, named for "AES Standard Method for Digital
> Audio Engineering—Measurement of Digital Audio Equipment," AES17.
> "Audio Precision (AP) has implemented a new low-pass filter type
> in accordance with AES17 §188.8.131.52 which, in AP’s terminology, is
> referred to as the AES17 filter." If a new-style AP system isn't
> available, an "existing 20-kHz bandwidth-limiting filter, together
> with a passive low-pass filter between the device under test (DUT)
> and the audio analyzer is recommended."
> So it would seem useless to show any data for distortion at 20kHz;
> TI should have shown 5kHz instead, to reveal if the distortion is
> increasing with frequency, e.g. 0.06% at 1kHz and 0.3% at 5kHz.
I favour something like a mixed 19khz and 20khz to get a feel for an
amps real performance at HF. I agree that displaying a 20khz thd on a
20kz filtered system is all but lying.
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.