Offset calibration of Op-amp - is that really ok?

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.

------------- Bill Sloman, Nijmegen

...and there are a number of op-amps that have VOS in the tens of microvolts region, not to mention chopper stabilized op-amps.

Why not just use a better op-amp?

"Klaus Kragelund" a écrit dans le message de news: snipped-for-privacy@z14g2000cwz.googlegroups.com...

Check CMRR, or use an inverter stage if you can.

I don't think the LM324 is too bad in that department, assuming the power supply is something like 10V. Some of the "better" R-R input op-amps have a horrible shift of several hundred uV in the Vos as you transition from one input stage to the other. Bill S says 160uV typical from the specs for a 5V change, but what configuration do you have that would change the CM voltage by anything like 5V? Full scale is going to be less than 10mV, right? Watch effects due to the resistance of your switches and the relatively hefty Ib of the LM324.

Best regards, Spehro Pefhany

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.

Mike Harrison skrev:

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

Thanks

Klaus

snipped-for-privacy@ieee.org skrev:

Great - thanks, but I can't spend much more on the part so the LT1014 is out of the question. Good point about the CMRR though

Thanks

Klaus

Tony Williams skrev:

I am on a single rail. Can you elaborate on that - why not just one reading when as you say the CMRR isn't important for high gains due to the limited input range?

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

Amplifier Vout = G(Vin + Vos). A single rail amplifier cannot deliver a negative Vout. If Vos is negative then the amplifier will fail to give the correct output for all values of Vin less than Vos. This applies to signal readings as well as trying to auto-zero to a 0V reference. The 324 max Vos is +/- 7mV, so it cannot be guaranteed to work for Vin less than 7mV.

I realise now (with that gain of x100) that you probably do need to operate correctly for Vin less than 7mV, and the two-stage Vos correction is not appropriate.

Time for Plan B.... which involves putting a small voltage in series with Vin, slightly larger than +7mV. _ |- \\ Sw1 C + |Op >-->ADC Vin----+----+----+---+/+--+--||----+----|+_/ | | | | 1uF? | \\ _|_ _|_ + + / /_\\ \\_/ Sw2/ Sw3/ \\ | | + + | | | | | 0v----+----+----+--------+--/\\/\\--+---/\\/\\--+2.5V 10R /|\\ 3k | 8mV approx.

To Auto-zero. Open Sw1, Close Sw2 and Sw3. Wait 50uS (or so), for C to stabilise. Read ADC, store effective (Vos+8mV).

To read Vin. Open Sw2 and Sw3, Close Sw1. Read ADC. Subtract effective (Vos+8mV).

Note the relatively high value of C. The LM324 has a maximum input bias current of 250nA, which causes the voltage on C to drift upwards slightly when measuring Vin. 1uF is a drift of 250uV/mS, so the ADC must be read as quickly as possible.

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

Winfield Hill skrev:

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

Regards

Klaus

Tony Williams skrev:

Nice solution, Tony :-)

And this circuit can be used for all the inputs for all the non-ideal amplifiers in the circuit (switching the circuit via an analog switch)

- and with not that high a penalty cost-wise

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)

Thanks

Klaus

Found my notes. Posted at....

Newsgroups: alt.binaries.schematics.electronic Subject: Re: Offset calibration of Op-amp - is that really ok? - AutoZeroVOS.pdf Message-ID:

...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

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

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

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

...Jim Thompson