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Electronics => Beginners => Topic started by: lutkeveld on August 28, 2018, 09:51:31 am

Title: Unexpected waveform class D output
Post by: lutkeveld on August 28, 2018, 09:51:31 am

I wanted to verify the output power of a Texas Instrument class D EVM (TAS5754MDCAEVM)
After hooking up a 4 ohm test load and a 24V 5A power supply I got the following waveform after the LC output filter (see attachment)

If I reduce the output to <1V the output is what I expected (two out of phase sines)
However, when increasing the volume the individual outputs starts to distort, but the differential signal (purple) seems okay.
Is this a problem? Do I measure incorrectly? My speakers dont seem to clip audibly.

Title: Re: Unexpected waveform class D output
Post by: Dat Tran on August 28, 2018, 10:32:08 am

Can you show me your circuit ?
And which channel is output, input on your oscilloscope ?

Title: Re: Unexpected waveform class D output
Post by: lutkeveld on August 28, 2018, 10:37:39 am

The circuit is the TAS5754MDCAEVM, which is a 2x50W max class D amplifier.

I am measuring one channel with a simple DSO. The output is differential, but the measurement is single ended.
That is why I use the 'math function' on the scope to subtract the signals to get the differential output (purple).

Title: Re: Unexpected waveform class D output
Post by: T3sl4co1l on August 28, 2018, 11:25:30 am

Differential works, who cares?

No idea why it's making unbalanced waveforms, you'd have to ask them (but I doubt they'll say why).

Tim