[sdiy] new Texas ic log102 ? log ratio amp

[Sowa Roman]

Is there a need for low drift opamp in expo stage?

My understanding is this - expo pair current is determined
by resistor between some power point, say +5V, and opamp
summing node at 0V. Now, offset voltage can change with
temperature and time by aprox 1-2mV, so it's 200-400 ppm
total change. Kinda small...

I was considering using 'zero-drift' chopper opamp there
(AD8571, 2.30 USD), but then I thought, "why bother"

Roman

""-----Original Message-----
""From: Tim Ressel [mailto:madhun2001 at yahoo.com]
""Sent: Thursday, April 04, 2002 8:12 AM
""To: patchell; Peter M Blackett
""Cc: synth Diy list
""Subject: Re: [sdiy] new Texas ic log102 ? log ratio amp
""
""
""I've had really good luck with the heater servo design
""a la AN299 from National Semi. I changed the design to
""use a MAT04; it has good performance. I even ran it in
""an environmental chamber. It held together over a wide
""range. One can get low tempco resistors from Digi-key
""or Mouser. Burr-brown, TI, Linear Tech and others make
""really high accuracy, low drift opamps. One could even
""use chopper amps for the expo converter, if one has
""found a money tree   $$$$$$$$$$$$$$$$$$$$$$
""
""--TR


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