drv134 balanced driver question

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alexc

Well-known member
Joined
Sep 21, 2004
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I've been fooling around with a drv134 balanced driver chip ....  it's in a cheap ebay pcb ...

I measure it in REW  and see very low THD ( triple0 45% )  and THD+N  ( double0 50% )  ...  which is great.

HOWever, when I look with the CRO, I see the output of the drv134 has the expected +6dB level shift, BUT also an additional +1.3dB. 

REW confirms what I measure on the CRO : I send -20dBFS (1 dBu) 1KHz sine  and it returns -12.7 dBFS (+8.3 dBu) - there is a 7.3dB gain increase end-to-end. Six of it is expected but the rest?

Can anyone shed some light on why I seem to have this unexplained +1.3 dB level shift ??

It's not obvious to me  :mad: 

The pcb and schemo is super simple : drv134 chip, a pair of 10uF electros and a a pair of 0.1uF are the only things there!
PSU is good at +/-17V with low noise (lm317/337 regulated)  and there isn't any obvious issues there.

I tried sub'ing the chip with another (us sourced) and it was within 0.1dB.
It is a stereo pcb, and the alternate channel is also within 0.1dB.

wtf?  I think 2.64Vpp unbalanced input should give 2.64Vpp balanced, each-phase-to-ground ...  not 2.84Vpp that I measure!

The spec says 0.1dB gain error, so I'm thinking I have something out of whack.

ANy ideas?
 
alexc said:
wtf?  I think 2.64Vpp unbalanced input should give 2.64Vpp balanced, each-phase-to-ground ...  not 2.84Vpp that I measure!
From datasheet, "The DRV134 converts a single-ended, ground-referenced input to a floating differential output ...".
Error seems the result of your "each-phase-to-ground" measurement.
 
I think it was designed to have +6db at 600 ohm load.  If you drive a high impedance you get more gain.  I remember reading something about it in a product comparison.
 
Thanks for the responses :)

I think it's the '0.1dB for 600ohm loads' thing. I'm driving 10K or so inputs ......  I'll do some terminations and check again!.

I'm fairly certain my ground reference is fairly stable and consistent throughout the system in test, so I don't believe there is any 'floating reference' stuff going on.  In any case

Everything is rock solid and I am cross-checking results from cro, analyser and calc.

 

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