THAT1646 question regarding sense capacitors

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syn
Posts: 14
Joined: Sun Aug 12, 2007 3:12 am

THAT1646 question regarding sense capacitors

Post by syn »

Hi
Is it O.K. to use a high quality polypropylene cap in the feedback
loop (1646) or i must use "high quality non-polarized electrolytic" (bipolar el.cap?) capacitor ...
if i have to go with an electrolytic is it cool to use two polarized elctrolitics "back to back" (- ++ -)...
I am confused as THAT app.note claims "high quality non-polarized electrolytic" . I thought that
poly caps, ceramics e.t.c. are non-polarized and elctrolitics are either bi-polar or polar but now i've lost
my compass :( :? :mrgreen:

thanks a lot


Milos
Last edited by syn on Fri Jul 11, 2008 10:34 am, edited 1 time in total.
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mediatechnology
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THAT1646 question regarding sense capacitors

Post by mediatechnology »

Hey Milos!

Those particular caps aren't too critical but need to be 10 uF or so which would rule out, as a practical matter, the use of films. In many cases they are not needed and can be replaced with shorts or opened which disables OutSmarts. They can also be polarized since the DC potential on either side of them is quite small - on the order of millivolts. If you don't want to use non-polar caps you could use back-to-back polarised electrolytics.

The capacitors in the sense lines exist to reduce the gain of OutSmarts at DC and lower the common mode (+ or - output relative to ground) offset. If replaced with shorts, the worst-case DC common-mode offset would be +/-250mV typically 50mV. With caps between force and sense this drops to +/-20mV worst case typically 5mV.

Although the worst-case numbers with them shorted seems high, the differential offset is low. As a practical matter, driving another differential input or input transformer, the common mode offset will be rejected. The SSM2142 and DRV134 also have this characteristic.

We've talked a lot here about using the THAT1646 to drive transformers. If you're using it to drive a transformer OutSmarts can be turned off and the sense inputs left floating with no capacitors used.

The short answer is that they're not critical at all and are not in the primary signal path.

I hope you don't mind me doing it, but I'd like to slightly rename this post so that it's search engine friendly. We get a lot of visits from Google.

Wayne
syn
Posts: 14
Joined: Sun Aug 12, 2007 3:12 am

Re: THAT1646 question regarding sense capacitors

Post by syn »

Wayne
thank you very much, all clear now.
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