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From: John Larkin on 21 Jun 2008 15:58
On Sat, 21 Jun 2008 10:01:51 -0700 (PDT), Paul <energymover(a)gmail.com>
wrote:

>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.

No. Those are characteristics of the chip.


The actual error voltage that a *circuit* generates is made up of
several contributors. One is the input offset voltage of the chip
itself. An additional error is any voltage drops created in external
resistors by the opamp bias currents. The errors are generally assumed
to add, because we can maybe know the polarity of the bias current
from the datasheet (but often we don't even know that) and we never
know the polarity of the offset, unless we measure it on one real
opamp.



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 don't think that's right. The Spice model shouldn't be that stupid.
Sounds like an interpretation issue.




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).

No, it's a property of the chip, not the circuit it's used in.

John

From: Jamie on 21 Jun 2008 18:39
Paul 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.
>
> 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?
>
> INA116PA datasheet:
> http://focus.ti.com/lit/ds/symlink/ina116.pdf
>
> Regards,
> Paul
The last time I checked, the offset voltage would be the difference
between the (-) and (+) input with the Op-amp in (-) loop back mode.

So, if you were to put an op-amp in (-) loop back and lets say 5 volts
on the (+) input, the (-) input should be offset no more than what the
spec's state.

Or, I guess if you were using a +/- to common supply, you can simply
tie the (+) to common with op-amp in (-) loop back. You should be seeing
that offset factor at the (-)/output..

Maybe things have changed but that is what I go by..

http://webpages.charter.net/jamie_5"

From: Robert Baer on 22 Jun 2008 04:48
Paul wrote:

> 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.
>
>
>
>
> 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
Incorrect, one *can* have eve 3mV offset and 3fA bias.
Learn what an ideal op-amp is, then learn about each of the various
real-life error components.
From: Helmut Sennewald on 22 Jun 2008 06:23

"Paul" <energymover(a)gmail.com> schrieb im Newsbeitrag
news: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





From: Paul on 22 Jun 2008 11:32
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
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