[Randy Yates]

What's the difference?
--
Randy Yates % "Remember the good old 1980's, when
Digital Signal Labs % things were so uncomplicated?"
% 'Ticket To The Moon'
http://www.digitalsignallabs.com % *Time*, Electric Light Orchestra

[Randy Yates]

Randy Yates writes:

What's the difference?

I should clarify my question to be, "For CMOS technology, what is the
difference?".

Wikipedia states:

An instrumentation (or instrumentational) amplifier is a type of
differential amplifier that has been outfitted with input buffers,
which eliminate the need for input impedance matching...

(http://en.wikipedia.org/wiki/Instrumentation_amplifier)

However, in CMOS technology, the exact same device type (PMOS or NMOS)
is used for both inputs, and the input circuitry is mirror-image. So I
don't see the need for "input impedance matching" in order to account
for topology differences since there IS no topology difference for CMOS.

There are imbalances between the "+" and "-" inputs due to process
variance, load differences, etc., but I would think that this imbalance
would be aggravated by using different amplifiers for each input.
--
Randy Yates % "How's life on earth?
Digital Signal Labs % ... What is it worth?"
% 'Mission (A World Record)',
http://www.digitalsignallabs.com % *A New World Record*, ELO

[Randy Yates]

Randy Yates writes:

Randy Yates writes:

What's the difference?

I should clarify my question to be, "For CMOS technology, what is the
difference?".

Wikipedia states:

An instrumentation (or instrumentational) amplifier is a type of
differential amplifier that has been outfitted with input buffers,
which eliminate the need for input impedance matching...

(http://en.wikipedia.org/wiki/Instrumentation_amplifier)

However, in CMOS technology, the exact same device type (PMOS or NMOS)
is used for both inputs, and the input circuitry is mirror-image. So I
don't see the need for "input impedance matching" in order to account
for topology differences since there IS no topology difference for CMOS.

There are imbalances between the "+" and "-" inputs due to process
variance, load differences, etc., but I would think that this imbalance
would be aggravated by using different amplifiers for each input.

OK, I see my problem now. Interpretation. When W states "which eliminate
the need for impedance matching" they don't mean impedance matching
the opamp itself but the "amplifier," i.e., the opamp plus the
resistors used to configure the opamp into a closed-loop
(non-infinite-gain) differential configuration.

This great article by Ron Mancini cleared it up for me:

http://www.edn.com/article/CA207121.html
--
Randy Yates % "Midnight, on the water...
Digital Signal Labs % I saw... the ocean's daughter."
% 'Can't Get It Out Of My Head'
http://www.digitalsignallabs.com % *El Dorado*, Electric Light Orchestra