Thanks for explaining, Rich - that makes sense. I think that the
stability which Olin was talking about was in the sense of control
loop stability, not stability of offset voltage. I have never designed
an amplifier which works with 50 fA current input (!) before, but I
can appreciate how lots of things we normally take for granted must be
questioned at those signal levels!

Sean


On Tue, May 11, 2010 at 11:12 PM, Rich <rgrazia1@rochester.rr.com> wrote:
> Sean, I may have been a bit misleading. =A0Thank you for catching it. =A0=
I meant
> to say near zero volts output, as you pointed out. =A0Most op amps do not=
 have
> the very high open loop gain, which unity gain buffers do have. =A0The bu=
ffer
> example I gave was for the LM310. The only spec I could find on open loop
> gain was was super OLG. =A0 Op amps can be configured as unity gain easil=
y by
> connecting the output directly to the inverting input. =A0Sometimes resis=
tors
> are placed in the feedback loop and in the non-inverting input to balance
> the currents. =A0That configuration is generally stable for relatively la=
rge
> signal, and the higher the open loop gain of the device the better. =A0At
> small signals I believe the jth valley currents can contribute to drift.
> When one is operating in the low millivolts (near zero) there can develop=
 a
> small drift band which is okay if your measurement is not small enough to=
 be
> compromised. =A0Of course, we are talking about a DC configuration. The O=
P37
> is a precision op amp which is fairly reliable as a unity gain buffer. =
=A0It
> has a 1.8 million open loop gain. Drift is 0.2 uV/C where the LM310 is 10
> uV/C so it is really stable. =A0 Even the long term drift spec is good. =
=A0I
> have designed amplifiers that operate in the microvolts and even the board
> leakage can cause drift and affect the performance so the offset currents
> need to be balanced. =A0I designed a 50 femtoamp amplifier once and I cou=
ld
> not even mount the parts in a PCB. =A0So I have become keenly aware of the
> problems of small signal and drift and configuration.
>
>
> ----- Original Message -----
> From: "Sean Breheny" <shb7@cornell.edu>
> To: "Microcontroller discussion list - Public." <piclist@mit.edu>
> Sent: Tuesday, May 11, 2010 7:48 PM
> Subject: Re: [EE] Opamp design doubt
>
>
> Hi Rich,
>
> Not stable near zero what? I don't understand your comment. Near zero
> volts output?
>
> Sean
>
>
> On Tue, May 11, 2010 at 3:56 PM, Rich <rgrazia1@rochester.rr.com> wrote:
>> That is true, Russell. In fact most op amps configured as unity gain are
>> not stable near zero. That is why there are special very high open loop
>> gain devices called buffers or followers, like the LM 310, for example.
>>
>>
>
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>

-- =

http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist