On Jun 22, 3:23 am, "Helmut Sennewald" <helmutsennew...@[EMAIL PROTECTED]
>
wrote:
> "Paul" <energymo...@[EMAIL PROTECTED]
> schrieb im
Newsbeitragnews:c8301dfa-b8fd-47e0-8aac-7f028584b21d@[EMAIL PROTECTED]
>
>
>
>
>
> > 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.
>
> > The LMC660A has a typical voltage offset of 1mV and bias current of
> > 2fA, but that depends what type of op-amp circuit. According to Spice
> > the input voltage offset for an inverting or differential circuit is
> > about what the Vos spec says, but for a non-inverting circuit it's a
> > few nanovolts on the "+" input pin. I'm wondering if the Vos in
> > datasheets is referring to a certain type of op-amp circuit such as
> > the inverting type (http://hyperphysics.phy-astr.gsu.edu/Hbase/
> > Electronic/opampvar.html#c2).
>
> > Regards,
> > Paul
>
> Hello Paul,
> Maybe it helps if you think about the transistor circuit
> of an opamp.
>
> The first stage of an opamp consists of a differential
> amplifier made by a pair of two well matched transistors.
> The difference of the Vgs(Mosfet opamp) or Vbe(bipolar opamp)
> of these two transistors in the input stage is the main
> contributor for the offset voltage.
>
> Offset voltage is always measured between the + and - input.
> What you have measured at the +input is the bias(leakage)
> current multiplied by the value of the resistor connected
> to the +pin.
>
> Best regards,
> Helmut- Hide quoted text -
I appreciate all of the replies! All of these years I've had this
false idea about the datasheets Vos burnt into my head. I've always
assumed that if the datasheet said the op-amps Vos was say 50uV then
that's the lowest input voltage (by my def: the voltage applied on the
input device due to the op-amp) one can expect with a typical op-amp
circuit such as an inverter or non-inverter.
So it's true that one could achieve input voltages in the nanovolt
region on a 200K ohm DUT from an Instrumentation op-amp chip such as
INA116PA even though the datasheet Vos spec is 2mV?
Thanks,
Paul
INA116PA datasheet:
http://focus.ti.com/lit/ds/symlink/ina116.pdf
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