offset error adjustment circuit

[[email protected]]

The system I'm studying has
integrator made of op amp(PA85),and they made additional
circuit for offset error adjustment with OPA277.
However,I've never seen the mechanism behind the circuit before.
Reference I searched say that we can adjust offset error with
potentiometer,but I still
don't know how we can do such work with op-amp.(automatic adjustment)

It will ve very helpful if I can attatch schematics,but I don't know
how I can post bmp file.

 

[John Popelish]

The system I'm studying has
integrator made of op amp(PA85),and they made additional
circuit for offset error adjustment with OPA277.
However,I've never seen the mechanism behind the circuit before.
Reference I searched say that we can adjust offset error with
potentiometer,but I still
don't know how we can do such work with op-amp.(automatic adjustment)

It will ve very helpful if I can attatch schematics,but I don't know
how I can post bmp file.

A group that allows graphic attachments is:
alt.binaries.schematics.electronic
Unfortunately, this (and all groups with graphic content) is
not archived nor is it accessible through Google Groups, but
only through direct access to a usenet server.

If you contact your Internet service provider, they can
probably tell you how to post directly to and read from
their usenet server.

Till you work that out, you may email me a schematic and I
will post it to alt.binaries.schematics.electronic for you,
under this thread title, with instructions to comment back
here for you.

Or you can locate one of the many free photo posting
services on the web and put the schematic up, there, and
bring us the URL for that location.

 

[John Larkin]

The system I'm studying has
integrator made of op amp(PA85),and they made additional
circuit for offset error adjustment with OPA277.
However,I've never seen the mechanism behind the circuit before.
Reference I searched say that we can adjust offset error with
potentiometer,but I still
don't know how we can do such work with op-amp.(automatic adjustment)

It will ve very helpful if I can attatch schematics,but I don't know
how I can post bmp file.

The PA85 is a power amplifier with mediocre input offset voltage and
offset drift specs. A better, precision amplifier like the OPA can be
used to measure and correct for the input offset of the PA85. The
result is a "composite" amplifier, combining the best features of
both.

You can post a pic to one of the free image hosting sites, like
Supload, and give us the link here.

John

 

[Winfield]

The system I'm studying has an integrator made with an opamp
(PA85), and they made an additional circuit for offset error
adjustment with an OPA277. However, I've never seen the
mechanism behind this circuit before. I searched references
saying we can adjust the offset error with a potentiometer,
but I don't know how we can do such work with an op-amp
(automatic adjustment).

It will be very helpful if I can attach schematics, but I
don't know how I can post a bmp file.

Why not try making an "ASCII schematic"? First, your
browser or usenet program must be set to display in a
fixed-spacing (non-proportional) font, such as Courier
or Courier New.

Then use - + | and similar symbols to draw your circuit.

The advantages are substantial: Everyone can instantly
see your circuit and think about it, and the circuit
and discussion surrounding it are archived on Google.

 

[Kingcosmos]

The system I'm studying has
integrator made of op amp(PA85),and they made additional
circuit for offset error adjustment with OPA277.
However,I've never seen the mechanism behind the circuit before.
Reference I searched say that we can adjust offset error with
potentiometer,but I still
don't know how we can do such work with op-amp.(automatic adjustment)

It will ve very helpful if I can attatch schematics,but I don't know
how I can post bmp file.

I have seen circuits that use an inverting integrator, A2, from the
output of A1 that feeds back to the non-inverting input of A1 for
'auto-zeroing' DC offset.

 

[[email protected]]

The PA85 is a power amplifier with mediocre input offset voltage and
offset drift specs. A better, precision amplifier like the OPA can be
used to measure and correct for the input offset of the PA85. The
result is a "composite" amplifier, combining the best features of
both.

You can post a pic to one of the free image hosting sites, like
Supload, and give us the link here.

John

Here is part of the schematic
http://img.villagephotos.com/p/2007-8/1272856/schem.JPG

 

[John Popelish]

Here is part of the schematic
http://img.villagephotos.com/p/2007-8/1272856/schem.JPG

I think this circuit is pretty simple to describe. Lets
assume the PA85 amplifier has a 10 mV offset. If its +
input were grounded, that offset would be forced to appear
at the - input, and when the input voltage at the left were
exactly zero, there would be 10 mV across the input voltage
to current converting input resistance (made up of the total
of R32, 35 and 37). That voltage would produce a current
through those resistors that would ramp the integrating
capacitor around the PA85 at a steady rate.

The additional opamp (with an assumed offset that is a tiny
fraction of that of the PA85) which according to this data
sheet:
http://focus.ti.com/lit/ds/symlink/opa277.pdf
has a maximum of about 0.165 mV.

Any offset voltage appearing at the - input of the PA85 is
integrated (and inverted) by the OPA277 and that integrated
output is connected as the reference for the PA85
integrator, in stead of ground. This shifts the - input
voltage toward zero and the OPA277 integration slows to a
crawl as the voltage approaches the much lower offset of the
OPA277. Equilibrium is reached when the voltage at the -
input of the PA85 is the offset voltage of the OPA277, and
the voltage of the + input of the PA85 is the negative of
its offset plus the offset of the OPA277. The gain of the
PA85 keeps its - input voltage very stable, in spite of any
input voltages from the HV boost input, so the OPA277
essentially ignores the actual input signal and responds
only to offset.

 

[John Larkin]

I think this circuit is pretty simple to describe. Lets
assume the PA85 amplifier has a 10 mV offset. If its +
input were grounded, that offset would be forced to appear
at the - input, and when the input voltage at the left were
exactly zero, there would be 10 mV across the input voltage
to current converting input resistance (made up of the total
of R32, 35 and 37). That voltage would produce a current
through those resistors that would ramp the integrating
capacitor around the PA85 at a steady rate.

The additional opamp (with an assumed offset that is a tiny
fraction of that of the PA85) which according to this data
sheet:
http://focus.ti.com/lit/ds/symlink/opa277.pdf
has a maximum of about 0.165 mV.

Any offset voltage appearing at the - input of the PA85 is
integrated (and inverted) by the OPA277 and that integrated
output is connected as the reference for the PA85
integrator, in stead of ground. This shifts the - input
voltage toward zero and the OPA277 integration slows to a
crawl as the voltage approaches the much lower offset of the
OPA277. Equilibrium is reached when the voltage at the -
input of the PA85 is the offset voltage of the OPA277, and
the voltage of the + input of the PA85 is the negative of
its offset plus the offset of the OPA277. The gain of the
PA85 keeps its - input voltage very stable, in spite of any
input voltages from the HV boost input, so the OPA277
essentially ignores the actual input signal and responds
only to offset.

The jfets-as-diodes trick looks a bit extreme, given that this is a
pretty low-impedance circuit.

I've seen a variant, where the precision opamp is in the forward gain
loop, not just off to the side. There was an Apex appnote for
improving both the DC and the noise behavior of one of their all-CMOS
opamps that was very bad in both respects.

John

 

[John Popelish]

(snip)
The jfets-as-diodes trick looks a bit extreme, given that this is a
pretty low-impedance circuit.

I've seen a variant, where the precision opamp is in the forward gain
loop, not just off to the side. There was an Apex appnote for
improving both the DC and the noise behavior of one of their all-CMOS
opamps that was very bad in both respects.

After looking at the data sheet for the PA85:
http://www.apexmicrotech.com/mainsite/pdf/pa85u.pdf
with something like 2 mV of offset, unless you pick a high
priced version of the OPA277, the improvement with this
circuit isn't that dramatic. If I was going to go to all
this trouble, I think I might use an auto zero opamp.
Perhaps something like an LTC1150 with worst case offset
voltage of 10 uV:
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1027,P1310,D3092