On Jun 22, 9:15 am, "Helmut Sennewald" <helmutsennew...@[EMAIL PROTECTED]
>
wrote:
> "Paul" <energymo...@[EMAIL PROTECTED]
> schrieb im
Newsbeitragnews:d46baf44-9ad4-4d95-a262-a427a3bbc7f4@[EMAIL PROTECTED]
>
>
>
>
>
> > 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
>
> Hello Paul,
>
> Yes you can apply voltages as small as you like.
> they will be still amplified by the gain G, set with
> the feedback resistors. The drawback of any Vos
> is that you will have an output voltage of (Vos+Vin)*G .
> This menas you have to either adjust the offset voltage
> already at the input or you have to subtract Vos*G at
> the output.
>
> Best regards,
> Helmut- Hide quoted text -
Thanks! As you said the output offset can always be corrected, but
it's great to know that a 2mV op-amp chip such as the INA116PA can
apply DC voltages as low as a few nanovolts on the input device
without adding shunt resistors. Of course one can always add a shunt
resistor to lower the input voltage across the DUT, something I knew
about, but of course that has obvious effects of decreasing the DUT's
effective input voltage to the op-amp.
I'm wondering if there are any op-amps or perhaps a BiFET amp circuit
that could achieve a few nanovolts across say a 200K ohm device while
consuming no more than a few microwatts. The idea is that such a
microwatt amp would have considerably less input thermoelectric
effects. Thermoelectric effects can generate a half dozen or more
microvolts on the DUT unless carefully balanced with dummy resistors.
I believe Linear Tech has some microwatt op-amps, but nothing near
25fA bias current.
Thanks,
Paul
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