On Mon, 23 Jun 2003 16:45:46 -0500, Picdude wrote:
>=A0BTW, I'm using a super-duper high-class Radio-Shack mutltimeter, =
so=20
>=A0the 0.12mA figure may have a noticeable tolerance. =A0I expect th=
is to=20
>=A0be significant at these lower mA values, but hopefully the error =
is=20
>=A0not in the hundreds of percent! =A0=3D8-0

Back in my former life as a consumer electronics designer, I routinel=
y
had to measure the operating current of devices that ran on a single
button cell for years (i.e. maximum supply current was ~20uA or less)=
.
I found out quickly that almost any general purpose test instrument
introduced significant errors. In some cases it was 100's of percent!
Scope inputs, meter inputs, etc. all caused problems.

We solved the problem by making an active buffer circuit for the 'sco=
pe
using a battery powered high input impedance, ultra-low input bias
current op-amp. We also made an active current to voltage converter
using a similar technique. Accurately measuring micropower circuits i=
s
tricky business.=20

Long ago I read an excellent application note by Jim Williams of Line=
ar
Technology that dealt with the practical aspects of measurements in
micropower circuits. I think Maxim also has a similar application not=
e.

Matt Pobursky
Maximum Performance Systems

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