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From: Pieter on 2 Jul 2008 04:42
On Sat, 28 Jun 2008 21:52:29 -0700 (PDT), Paul <energymover(a)gmail.com>
wrote:

>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

To prevent thermoelecric voltages, keep all pins at the same
temparature. But also all surrounding resistors etc. A cooling airflow
gives temperature differences. And resistors and opamps that get warm
may give some effects.

Pieter
From: Phil Hobbs on 2 Jul 2008 08:21
Pieter wrote:

> To prevent thermoelecric voltages, keep all pins at the same
> temparature. But also all surrounding resistors etc. A cooling airflow
> gives temperature differences. And resistors and opamps that get warm
> may give some effects.
>
> Pieter

This is especially true of temperature gradients across devices that
aren't metallic throughout, e.g. metal oxide resistors and
semiconductors. The thermocouple constants between silicon and any
metal are up in the 700-1500 mV/K range, depending on doping---which
dwarfs the slope of any metal/metal thermocouple.

Cheers,

Phil Hobbs
From: Tom Bruhns on 2 Jul 2008 12:45
On Jun 21, 8:59 am, Paul <energymo...(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.
>
> 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
From: Phil Hobbs on 2 Jul 2008 17:20
Tom Bruhns wrote:

> 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...
>
One good set is Jim Williams' 2001 trio of articles on his 1 ppm
accuracy DAC. They're on the EDN web site someplace--search on "1 ppm"
and "20 bit DAC", and they'll come right up. He even goes into details
like the thermocouple constant of two different vendors' copper wire:
+-0.2 uV/K, believe it or not--as big as Cu:Au or Cu:Ag.

Cheers,

Phil Hobbs
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