what is AutoZero?

[fonograph]

I was reading the thread on the dmm noise comparsion and I found this term"auto zero" there alot.What does it do? It seems like all the high end bench top multimeters have it,I have read that it have to do something with the flicker noise in Keithley 7510 when set to 10 NLPC,is it some kind of anti flicker noise thing?

[tautech]

Of course if you leave meter leads unattached values shown will be from the RFI from surroundings, we all accept that.
In the Siglent DMM's not called an Auto zero as such but means the same thing.
Relative is the term used and it can be defined to zero or another value and can be selected as ON or OFF.

It's handy for nulling lead resistance out.

[danadak]

"Auto-Zero" can mean different things, not a "standardized" term. That
being said "normally" refers to a signal chain that makes connections
internally to measure its own offsets, and use that to correct the DUT
(Device Under Test) readings.


https://www.maximintegrated.com/en/app-notes/index.mvp/id/1041


http://www.electronics-tutorial.net/electronic-instruments/digital-voltmeters/dual-slope-dvm/index.html


Regards, Dana.

[fonograph]

Still dont understand.

So it only works for dual slope adc multimeters? If it cancels internal "offset voltage" then why not run it all the time?
What are these internal offset voltages and offset currents? Why not just cancel them in DSP once and be done with it for the rest of the life of the instrument?

[David Hess]

Automatic zero may apply to different things.

danadak's link shows how it is connected with ADCs.  Integrating slope converters usually include a high impedance input buffer which because it is built on a CMOS process, has compromised offset voltage and offset voltage drift so an input multiplexer is used to alternately measure the input offset voltage and signal.  This is how an integrating slope converter using a cheap CMOS process can maintain an input offset voltage below 10 microvolts over time and temperature.

Chopper stabilized amplifiers effectively do the same thing but at a much higher rate allowing them to also reduce flicker noise.  Delta-sigma converters also gain the same advantage from chopping their inputs.

In multimeters, automatic zero might be disabled to gain a higher sample rate or because is produces charge injection into the input interfering with certain circuits and measurements.  I have a couple of old 4.5 digit multimeters which use a low drift JFET input stage for sanity checking if I suspect this is a problem.

[Marco]

What are these internal offset voltages and offset currents? Why not just cancel them in DSP once and be done with it for the rest of the life of the instrument?

It changes with temperature, humidity, pressure, age, cosmic rays, whatever ... basically the only thing you know about offsets is that they don't change very fast, most of the time.

So multimeters often auto-calibrate. You short the input to ground, measure the offset and digitally subtract it from a value measured from the signal. Since you want a steady stream of measurements you simply do this once every measurement. This is closely related to correlated double sampling, although that term is mostly used in the context of image sensors.

Auto-zero is most commonly used in the context of amplifiers for more sophisticated and complex schemes than the above mentioned auto-calibration. The advantage being that you don't have to waste an entire measurement cycle every time while removing some of the "higher" frequency flicker noise as well (offset is basically "low" frequency flicker noise, auto-calibration as above can only remove this "low" frequency flicker noise and relies on simple low pass filtering for the "higher" frequency flicker noise). The downside being that it creates a small AC current on the input (in theory auto-calibration does so too, but it's so low frequency it's unlikely to matter).

[David Hess]

So it only works for dual slope adc multimeters? If it cancels internal "offset voltage" then why not run it all the time?

No, it applies to many different ADC and instrument types.  Usually automatic zeroing in one form or another runs continuously.  Sometimes it is synchronized or deliberately not synchronized to prevent intermodulation with other noise sources.

What are these internal offset voltages and offset currents?  Why not just cancel them in DSP once and be done with it for the rest of the life of the instrument?

Some of the offsets are fixed, some vary with temperature or time, and some are random like different types of noise.  Flicker or 1/f noise is particularly troublesome because it increases at lower frequencies making it difficult to filter out.  Luckily it can be handled by chopping and related techniques.

[fonograph]

Marco,what do you mean "create AC current"? Does this AutoZero inject AC current into the device under test? Or does it create AC current reading error? Isnt AutoZero supposed to get rid of acuracy degrading voltages and currents,not create them?

[Marco]

It injects AC current and a small bit of DC/bias current into the device under test.

The AC current is simply charge injection from the switches, they are made from FET's and some of the gate signal leaks through.

As for the bias current, if you watch what is happening in the autozero demystified link at Analog you see that the signal is switched between two opamp inputs. These inputs have an input capacitance and slightly different offset voltages, so they are at ever so slightly differing DC voltages. The device under test has to feed charge into the capacitor every phase change to make up the difference and that creates a current with a DC component.

PS. input protection creates some bias current too, but this can be made almost arbitrarily small.