Mike

Thanks for the thoughts.
I'm aware of using CMOS inverter s an amp - in this app its not too
good - I'm right on one rail of a 5 volt supply (0 - 0.2 volts to
sense). CMOS works best with voltage in refd to centre point.

339 is a MUCH better comparator (of course) but gain is not  a nice
trick - could make a class D amp (I considered it) but there would be
more problems there. I have used a Schmitt gate as an amplifier
before today - you get  a rail to rail pulse length modulated signal
out which can be low pass filtered - lots of fun.


Russell
-----Original Messag

e-----
From: Mike Keitz +ADw-mkeitz+AEA-JUNO.COM+AD4-
To: PICLIST+AEA-MITVMA.MIT.EDU +ADw-PICLIST+AEA-MITVMA.MIT.EDU+AD4-
Date: Tuesday, July 13, 1999 12:52 PM
Subject: Re: Sigma Delta Converter


+AD4-On Tue, 13 Jul 1999 01:13:23 +120- Russell McMahon
+ADw-apptech+AEA-CLEAR.NET.NZ+AD4-
+AD4-writes:
+AD4-
+AD4APg-I have used both a standard microprocessor input pin as the sense
+AD4APg-element (PIC) and an internal  comparator (Z8) and an external
op-amp
+AD4APg-(LM324) as comparator. I am aware of the poor performance of the
+AD4APg-LM324 as a comparator
+AD4-
+AD4-The 324's comparator brother the 339 (also offered in a dual unit,
the
+AD4-393) should be about the same cost.  But then you need two chips
unless
+AD4-you can make the 339 work well as an amplifier.  There was a chip
with
+AD4-324 type amplifiers and 339 comparators on one chip.  It's likely
been
+AD4-discontinued.  Probably the major problem with the 324 is its low
+AD4-bandwidth.  If you slow the whole process down it may work better.
+AD4-
+AD4-Amplifying the signal before appling it to a MCU pin ought to
improve
+AD4-that method.  Metal-gate (4000 series) CMOS gates biased into linear
+AD4-operation make decent rock-cheap amplifiers.  The 4069UB is the best
+AD4-choice, but other gates will work if you happen to also need a logic
gate
+AD4-in the circuit.  Also, amplifiers made from 2-input gates can be
placed
+AD4-in low-power shutdown easily by a logic signal applied to the other
+AD4-input.  These amplifiers are maybe not so good for DC precision but
work
+AD4-well with small high-impedance AC signals.
+AD4-
+AD4-
+AD4-
+AD4AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwB
fAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwBfAF8AXwB
fAF8AXwBfAF8AXwBfAF8AXw-
+AD4-Get the Internet just the way you want it.
+AD4-Free software, free e-mail, and free Internet access for a month+ACE-
+AD4-Try Juno Web: http://dl.www.juno.com/dynoget/tagj.
+AD4-