On Jun 21, 8:59 am, Paul <energymo...@[EMAIL PROTECTED]
> wrote:
> On Jun 21, 8:41 am, John Larkin
>
>
>
> <jjlar...@[EMAIL PROTECTED]
> wrote:
> > On Sat, 21 Jun 2008 08:25:59 -0700 (PDT), Paul <energymo...@[EMAIL PROTECTED]
>
> > wrote:
>
> > >Hi,
>
> > >As you know, the *input* offset voltage is the voltage required
across
> > >the op-amp's input terminals to drive the output voltage to zero.
> > >Although it has been my experience that for most op-amps the input
> > >offset voltage is due to the "-" input pin for the *most* part. For
> > >example, according to Spice the input offset voltage on the "+" input
> > >pin on a LMC660A op-amp for a non-inverting amp circuit is a few
> > >nanovolts, disregarding thermoelectric effects mind you, but a few
> > >millivolts on the "-" input pin. Although as you know the input
signal
> > >is not applied to the "-" input pin for a non-inverting amp circuit,
> > >which means there's just a few nanovolts on the input of such a
> > >circuit if we disregard thermoelectric effects.
>
> > The offset voltage is *differential*. You can blame it on either pin,
> > or both pins... it doesn't matter who you blame, the result is the
> > same: offset voltage becomes measurement error.
>
> > >I have a INA116PA Instrumentation op-amp where Ib typ = 3fA, Ib max =
> > >25fA, and Vos typ = 0.5mV. Now it seems to me in order for there to
be
> > >0.5mV on the input of this Instrumentation op-amp circuit with 3fA
> > >bias current that the DUT input impedance would have to be 0.50mV /
> > >3.0fA = 170 Gohms. On the other hand, if the DUT input impedance is
> > >say 200 Kohms then would the input offset voltage be 3.0fA * 200Kohms
> > >= 0.6nV, disregarding thermoelectric effects?
>
> > The offset voltage error is a different thing from the input bias
> > current. They are unrelated [1]. You can of course generate a real,
> > external-to-the-opamp error voltage by dumping the bias current into
> > real external resistance, but that's a different matter entirely.
>
> > John
>
> > [1] Some opamps have low offsets and high bias currents, and some vice
> > versa. Chopper amps are low on both; cheap bipolars are high on both.
>
> Here's my main concern. If I build the INA116PA for DC application,
> which is an internal Instrumentation op-amp chip (3 op-amps), and the
> impedance of my DUT is 200 Kohms, then what bias currents could a good
> EE such as yourself expect? I mean, for a 200K ohm DUT input source we
> cannot have both 0.5mV offset and 3fA bias on the DUT. I think V=I*R
> applies, so if the bias current is 3fA then V = 3fA * 200Kohms = 0.6
> nV.
>
> Thanks,
> Paul
My apologies if this has been covered in the other branch of this
thread, which I don't have time to fully read...
As others have pointed out, the input bias current and input offset
voltage of the part are characteristics of the part. HOWEVER, the
external circuit strongly influences how well you can take advantage
of those characteristics. That is, the external circuit can
completely wipe out the potential benefits of either a low bias
current or a low offset voltage or both, even. For example, I used a
chopper-stabilized op amp to amplify the output of a diode RF
detector. The op amp has typically a pA of input bias current and
about a uV of input offset voltage. First, I had to be very careful
to guard the detector traces against currents leaking in from outside,
and then I had to be careful that the resistance between the guard
trace and the detector output trace (feeding the op amp input) was
high enough that a 1uV offset would not result in a current as large
or larger than the pA op amp bias current. For this part, that's only
a megohm or so, fairly easy to do, but in an earlier design using a
non-chopper amp where the offset voltage was up to a millivolt or so,
it was a killer. The RF detector diode is shunt between the guard and
the amplifier input, with RF fed in through a capacitor, and the zero
bias RF detector diode shows considerable current at a millivolt.
Seems like there should be some good references on applying low
offset, low bias amplifiers. I know that Bob Pease has had some good
articles on the trials and tribulations of testing amplifiers down in
the fA region--not trivial! His articles can be found with a search
on the web...
Cheers,
Tom
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