op-amp nV input offset voltage [Design]

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From: Helmut Sennewald on 22 Jun 2008 12:15

"Paul" <energymover(a)gmail.com> schrieb im Newsbeitrag
news:d46baf44-9ad4-4d95-a262-a427a3bbc7f4(a)u12g2000prd.googlegroups.com...
> On Jun 22, 3:23 am, "Helmut Sennewald" <helmutsennew...(a)t-online.de>
> wrote:
>> "Paul" <energymo...(a)gmail.com> schrieb im
>> Newsbeitragnews:c8301dfa-b8fd-47e0-8aac-7f028584b21d(a)j1g2000prb.googlegroups.com...
>>
>>
>>
>>
>>
>> > On Jun 21, 8:41 am, John Larkin
>> > <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
>> >> On Sat, 21 Jun 2008 08:25:59 -0700 (PDT), Paul <energymo...(a)gmail.com>
>> >> 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

From: Paul on 22 Jun 2008 12:43

On Jun 22, 9:15 am, "Helmut Sennewald" <helmutsennew...(a)t-online.de>
wrote:
> "Paul" <energymo...(a)gmail.com> schrieb im Newsbeitragnews:d46baf44-9ad4-4d95-a262-a427a3bbc7f4(a)u12g2000prd.googlegroups.com...
>
>
>
>
>
> > On Jun 22, 3:23 am, "Helmut Sennewald" <helmutsennew...(a)t-online.de>
> > wrote:
> >> "Paul" <energymo...(a)gmail.com> schrieb im
> >> Newsbeitragnews:c8301dfa-b8fd-47e0-8aac-7f028584b21d(a)j1g2000prb.googlegroups.com...
>
> >> > On Jun 21, 8:41 am, John Larkin
> >> > <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> >> >> On Sat, 21 Jun 2008 08:25:59 -0700 (PDT), Paul <energymo...(a)gmail.com>
> >> >> 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

From: Rene Tschaggelar on 26 Jun 2008 04:53

Paul wrote:
>
> 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.

Paul,
a thermoelectric effect means you get a voltage
from a temperature difference in case different
metal combinations are involved. They act as
input offset voltage, independent on the bias
current.
These thermoelectric effects are in the microvolt
per Kelvin region. and thus are only to be
considered in high DC-gain applications.

While FET Input opamps have far lower bias currents,
they don't achieve the low input offset voltage
common to bipolar input OpAmps.

There are Fet input opAmps that get rid of the
input offset voltage by trading bandwidth against
the chopper feature.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

From: Paul on 29 Jun 2008 00:51

On Jun 26, 1:53 am, Rene Tschaggelar <n...(a)none.net> wrote:
> Paul wrote:
>
> > 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.
>
> Paul,
> a thermoelectric effect means you get a voltage
> from a temperature difference in case different
> metal combinations are involved. They act as
> input offset voltage, independent on the bias
> current.
> These thermoelectric effects are in the microvolt
> per Kelvin region. and thus are only to be
> considered in high DC-gain applications.
>
> While FET Input opamps have far lower bias currents,
> they don't achieve the low input offset voltage
> common to bipolar input OpAmps.
>
> There are Fet input opAmps that get rid of the
> input offset voltage by trading bandwidth against
> the chopper feature.
>
> Rene

Hi,

I'll try to clarify:

I am referring to the input voltage on the *DUT* caused by the op-amp,
and therefore if the bias current through the DUT is decreased then
the offset voltage on the DUT will be less-- ohms law.

The op-amps I am working with have offsets around 0.5uV to a few uV.
Therefore thermoelectric effects should be considered. As far as I
know instrumentation op-amp appear to have to least thermoelectric
effects since both input pins go to the same polarity on both op-amps,
the + pin, but there are still thermoelectric effects since both op-
amps are not 100% identical. Other circuits such as the inverter
require dummy resistors and such to help reduce the thermoelectric
voltages on the DUT.

My interest in BiFET's is to design a low power amp circuit with low
bias current.

Thanks,
Paul

From: Paul on 29 Jun 2008 00:52

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