Offset calibration of Op-amp - is that really ok?
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Offset calibration of Op-amp - is that really ok?
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Klaus Kragelund
Guest
Posted: Sun Dec 04, 2005 5:35 pm    Post subject: Re: Offset calibration of Op-amp - is that really ok? Reply with quote
Winfield Hill skrev:

Quote:
Klaus Kragelund wrote...

Why not just use a better op-amp?

Well because of the costs. The application is up to 200-300k/year
- so every last nickle counts

If there's a uP involved, how about switching to ground, measure
the offset and stores its value.


Well, that was the thing I was talking about in the original post -
calibration....

Regards

Klaus

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Fred Bloggs
Guest
Posted: Sun Dec 04, 2005 5:35 pm    Post subject: Re: Offset calibration of Op-amp - is that really ok? Reply with quote
Klaus Kragelund wrote:
Quote:
Winfield Hill skrev:


Klaus Kragelund wrote...

Why not just use a better op-amp?

Well because of the costs. The application is up to 200-300k/year
- so every last nickle counts

If there's a uP involved, how about switching to ground, measure
the offset and stores its value.



Well, that was the thing I was talking about in the original post -
calibration....

Regards

Klaus


Have you compared the price of one good OA to the cheap OA + all the
accessory circuits required for this otherwise unnecessary calibration?
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John Larkin
Guest
Posted: Mon Dec 05, 2005 1:06 am    Post subject: Re: Offset calibration of Op-amp - is that really ok? Reply with quote
On Fri, 02 Dec 2005 16:48:56 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@My-Web-Site.com> wrote:

Quote:
On 2 Dec 2005 01:02:28 -0800, "Klaus Kragelund"
klauskvik@hotmail.com> wrote:

Hi

I have an application in which I need to amplify a signal say 100times.
The opamp is a LM324 with ~5mV offset voltage. If I do nothing the
output might be 500mV off target

I plan to calibrate the op-amp continously by using a switch to set the
input to 0mV (instead of the signal) and then sample the resulting
voltage with the microcontroller to store a calibration number.

That might be fine a that cures any drift the Vos has over time and
temperature.

But what about non-linearities of the offset versus the input signal
range? Will the Vos be the same for say a 0V input and a 5V input?

Thanks

Klaus

Signal Frequency?

If DC or in a limited time frame, just store VOS in a capacitor...
chopper-style.

I built a uV meter that way 25 years ago to find shorts on PCB's by
measuring trace drops.

...Jim Thompson


But now, in the 21st century, it's more common to use a uP to switch
in zero, digitize the result, and subtract that from subsequent live
readings.

(Not with a single-supply LM324, of course.)

Excuse me, it's time for my mid-morning nap.

John

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Jim Thompson
Guest
Posted: Mon Dec 05, 2005 1:13 am    Post subject: Re: Offset calibration of Op-amp - is that really ok? Reply with quote
On Sun, 04 Dec 2005 11:06:12 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

Quote:
On Fri, 02 Dec 2005 16:48:56 -0700, Jim Thompson
To-Email-Use-The-Envelope-Icon@My-Web-Site.com> wrote:

On 2 Dec 2005 01:02:28 -0800, "Klaus Kragelund"
klauskvik@hotmail.com> wrote:

Hi

I have an application in which I need to amplify a signal say 100times.
The opamp is a LM324 with ~5mV offset voltage. If I do nothing the
output might be 500mV off target

I plan to calibrate the op-amp continously by using a switch to set the
input to 0mV (instead of the signal) and then sample the resulting
voltage with the microcontroller to store a calibration number.

That might be fine a that cures any drift the Vos has over time and
temperature.

But what about non-linearities of the offset versus the input signal
range? Will the Vos be the same for say a 0V input and a 5V input?

Thanks

Klaus

Signal Frequency?

If DC or in a limited time frame, just store VOS in a capacitor...
chopper-style.

I built a uV meter that way 25 years ago to find shorts on PCB's by
measuring trace drops.

...Jim Thompson


But now, in the 21st century, it's more common to use a uP to switch
in zero, digitize the result, and subtract that from subsequent live
readings.

(Not with a single-supply LM324, of course.)

Excuse me, it's time for my mid-morning nap.

John

But, if you're taking a lot of gain, the output of the amplifier will
saturate on the signal, rendering the subtraction invalid.

So much for the 21st Century ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
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John Larkin
Guest
Posted: Mon Dec 05, 2005 1:35 am    Post subject: Re: Offset calibration of Op-amp - is that really ok? Reply with quote
On Sun, 04 Dec 2005 12:13:28 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@My-Web-Site.com> wrote:

Quote:
On Sun, 04 Dec 2005 11:06:12 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Fri, 02 Dec 2005 16:48:56 -0700, Jim Thompson
To-Email-Use-The-Envelope-Icon@My-Web-Site.com> wrote:

On 2 Dec 2005 01:02:28 -0800, "Klaus Kragelund"
klauskvik@hotmail.com> wrote:

Hi

I have an application in which I need to amplify a signal say 100times.
The opamp is a LM324 with ~5mV offset voltage. If I do nothing the
output might be 500mV off target

I plan to calibrate the op-amp continously by using a switch to set the
input to 0mV (instead of the signal) and then sample the resulting
voltage with the microcontroller to store a calibration number.

That might be fine a that cures any drift the Vos has over time and
temperature.

But what about non-linearities of the offset versus the input signal
range? Will the Vos be the same for say a 0V input and a 5V input?

Thanks

Klaus

Signal Frequency?

If DC or in a limited time frame, just store VOS in a capacitor...
chopper-style.

I built a uV meter that way 25 years ago to find shorts on PCB's by
measuring trace drops.

...Jim Thompson


But now, in the 21st century, it's more common to use a uP to switch
in zero, digitize the result, and subtract that from subsequent live
readings.

(Not with a single-supply LM324, of course.)

Excuse me, it's time for my mid-morning nap.

John

But, if you're taking a lot of gain, the output of the amplifier will
saturate on the signal, rendering the subtraction invalid.

So much for the 21st Century ;-)

...Jim Thompson


So cut the analog gain to allow a little headroom, do the subtraction,
then multiply by a modest gain cal factor, 1.2 ballpark, to get the
range back. All these modern ADCs have more codes than anybody really
needs, anyhow; nobody will ever now.

John
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Jim Thompson
Guest
Posted: Mon Dec 05, 2005 1:35 am    Post subject: Re: Offset calibration of Op-amp - is that really ok? Reply with quote
On Sun, 04 Dec 2005 11:41:58 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

Quote:
On Sun, 04 Dec 2005 12:13:28 -0700, Jim Thompson
To-Email-Use-The-Envelope-Icon@My-Web-Site.com> wrote:

On Sun, 04 Dec 2005 11:06:12 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Fri, 02 Dec 2005 16:48:56 -0700, Jim Thompson
To-Email-Use-The-Envelope-Icon@My-Web-Site.com> wrote:

On 2 Dec 2005 01:02:28 -0800, "Klaus Kragelund"
klauskvik@hotmail.com> wrote:

Hi

I have an application in which I need to amplify a signal say 100times.
The opamp is a LM324 with ~5mV offset voltage. If I do nothing the
output might be 500mV off target

I plan to calibrate the op-amp continously by using a switch to set the
input to 0mV (instead of the signal) and then sample the resulting
voltage with the microcontroller to store a calibration number.

That might be fine a that cures any drift the Vos has over time and
temperature.

But what about non-linearities of the offset versus the input signal
range? Will the Vos be the same for say a 0V input and a 5V input?

Thanks

Klaus

Signal Frequency?

If DC or in a limited time frame, just store VOS in a capacitor...
chopper-style.

I built a uV meter that way 25 years ago to find shorts on PCB's by
measuring trace drops.

...Jim Thompson


But now, in the 21st century, it's more common to use a uP to switch
in zero, digitize the result, and subtract that from subsequent live
readings.

(Not with a single-supply LM324, of course.)

Excuse me, it's time for my mid-morning nap.

John

But, if you're taking a lot of gain, the output of the amplifier will
saturate on the signal, rendering the subtraction invalid.

So much for the 21st Century ;-)

...Jim Thompson


So cut the analog gain to allow a little headroom, do the subtraction,
then multiply by a modest gain cal factor, 1.2 ballpark, to get the
range back. All these modern ADCs have more codes than anybody really
needs, anyhow; nobody will ever now.

John

uP's are for people who can't do analog ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
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John Larkin
Guest
Posted: Mon Dec 05, 2005 1:35 am    Post subject: Re: Offset calibration of Op-amp - is that really ok? Reply with quote
Once you have a uP in the signal path, all sorts of stuff becomes free
(once you write the code!): background autozero/autocal, adaptive
lowpass filtering, smart dithering, synchronous detection,
linearization, AC coupling, temperature compensation, crosstalk
nulling, slew limiting, you name it. The problem is knowing when to
stop, how deep into 2nd and 3rd-order effects you want to go before
you get into real trouble.

This gadget is a 16-channel, 16-bit DAC with programmable output
ranges from +-25 mV to +-12.5 volts. It uses a 16-bit bipolar dac
followed by a 10-bit mdac that acts as a switched attenuator to define
the ranges.

http://www.highlandtechnology.com/DSS/V470DS.html


So on the lowest range, the mdac is set to 2 lsb's out of 1024. Dac
error specs are in terms of lsb error relative to fullscale. So what's
the mdac error *relative to output* when you set it to 2? Technically,
it's +-0.5 LSB, which is up to +-25% gain error. If you look at the
inverted r-2r ladder architecture, you could guess that it's actually
a lot better. And it is. But the cmos switches aren't perfect, and the
switches have a bad tc, so at low codes the gain is higher than
theoretical and the excess gain has a nasty positive tc that's code
dependent.

But we have a temperature sensor on the board, and a uP...

John
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Pooh Bear
Guest
Posted: Mon Dec 05, 2005 9:07 am    Post subject: Re: Offset calibration of Op-amp - is that really ok? Reply with quote
Jim Thompson wrote:

Quote:
uP's are for people who can't do analog ;-)

Hey !

Some can do both. ;-)

Graham
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Tony Williams
Guest
Posted: Mon Dec 05, 2005 9:58 am    Post subject: Re: Offset calibration of Op-amp - is that really ok? Reply with quote
Quote:
Moreover the temperature drift of the offset voltage is pretty
low so an autozero at startup might be sufficient (or perhaps
even a productionline calibration)

No! The input bias current of the opamp continually
charges [C], so the (Vos+8mV) across it must be
renewed immediately before each (quick) ADC reading
of Vin.

--
Tony Williams.
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CC
Guest
Posted: Sat Dec 10, 2005 8:06 am    Post subject: Re: Offset calibration of Op-amp - is that really ok? Reply with quote
Jim Thompson wrote:
Quote:
uP's are for people who can't do analog ;-)

...Jim Thompson


So tell me the model of your analog PC then. ;-)


Good day!


--
_____________________
Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
SuSE 9.1 Linux 2.6.5
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Sjouke Burry
Guest
Posted: Sat Dec 10, 2005 8:32 am    Post subject: Re: Offset calibration of Op-amp - is that really ok? Reply with quote
CC wrote:
Quote:
Jim Thompson wrote:

uP's are for people who can't do analog ;-)

...Jim Thompson



So tell me the model of your analog PC then. ;-)


Good day!


The basic processor type is the 741 -)-)-)

Of course we also use more advanced types,
but that is propriatary info.......
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Jim Thompson
Guest
Posted: Sat Dec 10, 2005 8:36 am    Post subject: Re: Offset calibration of Op-amp - is that really ok? Reply with quote
On Fri, 09 Dec 2005 18:06:20 -0800, CC <somewhere@overthe.rainbow.net>
wrote:

Quote:
Jim Thompson wrote:
uP's are for people who can't do analog ;-)

...Jim Thompson


So tell me the model of your analog PC then. ;-)


Good day!

Vannevar Bush, 1945 Model...

http://www-eecs.mit.edu/AY95-96/events/bush/photos.html

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
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neon



Joined: 25 Feb 2006
Posts: 586
Posted: Mon Jun 26, 2006 11:30 am    Post subject: Reply with quote
lot of advice to digest what for? An op amp designer has 2 things to consider voltage offset and current ofset.And they are there as +- it depends on the amps. that can be calibrated out. Now witha gain of 100 or more current offset can betroublesome basicaly big resistor f/b means big current offset means big output with shorted input.
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neon



Joined: 25 Feb 2006
Posts: 586
Posted: Thu Jul 06, 2006 10:42 am    Post subject: Re: Offset calibration of Op-amp - is that really ok? Reply with quote
Tony Williams wrote:
In article <1133514148.212070.222180@z14g2000cwz.googlegroups.com>,
Klaus Kragelund <klauskvik@hotmail.com> wrote:

[snip]
Quote:
But what about non-linearities of the offset versus the input
signal range? Will the Vos be the same for say a 0V input and a
5V input?

But the gain is 100. So MaxVin cannot exceed MaxVout/100,
which limits Vin to 100mV or so.

BTW: If you are on a single rail supply Auto-zero can be
done, but requires two ADC readings of two artificial
offsets with a known ratio. Then a few sums in the uP.
the signal of 5 mv is there whether you have single rail or -+ rail it is the input differential ofset and yes you may adjust it out by shorting the input and adj. for that gain there is also current ofset to consider too.
--
Tony Williams.
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neon



Joined: 25 Feb 2006
Posts: 586
Posted: Thu Jul 06, 2006 10:45 am    Post subject: Re: Offset calibration of Op-amp - is that really ok? Reply with quote
Anonymous wrote:
Klaus Kragelund wrote:
Quote:
Hi

I have an application in which I need to amplify a signal say 100times.
The opamp is a LM324 with ~5mV offset voltage. If I do nothing the
output might be 500mV off target

I plan to calibrate the op-amp continously by using a switch to set the
input to 0mV (instead of the signal) and then sample the resulting
voltage with the microcontroller to store a calibration number.

That might be fine a that cures any drift the Vos has over time and
temperature.

But what about non-linearities of the offset versus the input signal
range? Will the Vos be the same for say a 0V input and a 5V input?

The sensitivity of Vos to the common mode voltage can be deduced from
the common mode rejection ratio (CMRR) which is listed on page 3 of the
LM324 data sheet as 85dB typical, 65dB worst case. For a 5V common mode
shift, this is 160uV typical, 2.8mV worst case.

On psge 7 of the data sheet, there is a plot of typical common mode
rejection ratio versus frequency, where you can see that it starts
getting worse at frequencies above 10kHz.

There are amplifiers around offering better common moder rejection
ratios - the Linear Technology LT1014 which was intended to be an
upgrade from the LM324, has a CMRR of 97dB (worst case) and 114dB
typical, which pushes your worst case common mode shift down to 70uV.

Linear Technology also offer a range of auto-zeroing "zsero drift"
amplifiers, which include your auto-zeroing scheme. Analog Devices
offer similar parts.
the offset have nothing to do with db power rejection is the input differential and the spec say min. too.
-------------
Bill Sloman, Nijmegen
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