Max signal protection for SGTL5000

toyosm

Well-known member
Hey guys, I'm working on the buffering signal for the SGTL5000. Placed a buffer with variable gain and a BAT54 as overvoltage protection as seen in many designs out there. The issue is that I'm still having some latchup on the codec some times. I'm using this as a guitar pedal, so maximum input signal voltages can go up to 8Vpp if a hot output pedal is placed in front. Measuring with the oscilloscope the signal starts clipping because of the BAT54 (it's supposed to do it) at 3.8Vpp and, according to the SGTL5000 datasheet, the maximum signal input is about 2.8Vpp when powered with 3.3V.
The variable gain on the buffer is intended to adjust this voltage to a safe, non clipping level, but I want to be sure that the signal will not exceed in any case the maximum input voltage for the codec, any ideas how to lower that threshold? maybe instead of going to 3v3 with one of the diodes of the BAT54 having a lower voltage there?.
 

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You say there is a buffer? Could you use a voltage divider as shown here?

BTW maximum rating is VDDA+0.3, i.e. 3.6 V where a damage can start.

Yes, actually there's a voltage divider before to adjust the leel but that won't actually be a good protection if a spike or a greater signal is provided. The idea is to have that input gain adjustment because signals can vary a lot from 200mVpp with vintage pickups for example to 8Vpp with a hot output pedal and have all the excursion of the input gain safe from damaging the codec.

Well, if the max is around 3.6 I'm more in the spot, just have to lower it a bit for safety, don't know actually why I'm having a max of 3.8Vpp there with the clamping schottkys
 
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