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By Kevin Tretter 

Product Marketing Manager  Analog & Interace Products Division

Microchip Technology Inc.

At rst glance, the term “auto-zero” op amp may appear to be

something new, but in reality this

architectural concept has been

around or decades. This article

explores the history behind auto-

zero op amps and provides a

high-level overview o the archi-

tecture. Additionally, the articleexplores the inherent benets o 

this architecture or signal-con-

ditioning applications. Finally, an

example application is analyzedto urther compare the auto-zero

architecture to that o traditional

op amps.

Brief history

Chopper ampliers have been

around or decades, dating back 

close to 60 years. The chopper

amplier was invented to addressthe need or an ultralow-oset,

low-drit op amp—something

that was superior to the bipolarop amps available at the time. In

the original chopper amplier, the

amplier’s input and output are

switched (or chopped), causing

the input signal to be modulated,

corrected or oset error and then

unmodulated at the output. This

technique allowed or low oset

voltage and low drit, but alsohad limitations. Since the input

to the amplier is being sampled,

the input-signal requency had tobe limited to less than hal o the

chopping requency to prevent

aliasing. In addition to the band-

width limitation, the act o chop-

ping causes signicant glitches

to appear, requiring ltering on

the output to smooth out the

resulting ripples.

  The next generation o sel-

correcting ampliers improved

on the chopper amplier by

creating a chopper-stabilized

op amp. This architecture usestwo ampliers: a “main” amplier

and a “null” amplier, as shown

in Figure 1. The null amplier

corrects its own oset error by

shorting the inputs and applying

a correction actor to its own null

pin, ater which it monitors and

corrects the oset o the main

amplier. This architecture hasa big advantage over the older

chopper ampliers, as the main

amplier is always connected tothe input and output o the IC.

 Thus, the bandwidth o the main

amplier determines the input-

signal bandwidth. Thereore, the

input bandwidth is no longer

dependent on the chopping

requency. Charge injection rom

the switching action is still an is-

sue, which can cause transients

and can couple with the input

signal, causing intermodulation

distortion.

  The auto-zero architectureis similar in concept to that o a

chopper-stabilized amplier in

that there is a nulling amplier

and a main amplier. However,

signicant improvements have

been made over the years to

minimize noise, charge injection

and other perormance issues as-

sociated with chopper-stabilizedop amps. Various manuacturers

use dierent terms to dene this

architecture, such as “auto-zero,”“autocorrelating zeroing” and

“zero-drit.” Regardless o the ter-

minology, the basic underlying

architecture is the same.

Advantages

As described, the auto-zero archi-

tecture continually sel corrects

or the oset-voltage error o the

amplier. This results in several

distinct advantages over tradi-

tional op amps.

Low ofset voltage—The nullingamplier continually cancels its

own oset voltage and then ap-

plies a correction actor to the

main amplier. The requency

o this correction varies depend-

ing upon the actual design, but

typically occurs thousands o 

times per second. For example,

the MCP6V01 auto-zero amplierrom Microchip Technology cor-

rects the main amplier every

100µs, or 10,000 times each sec-ond. This continual correction al-

lows or ultra-low oset voltages

that are much lower than tradi-

tional op amps. Additionally, the

process o correcting the oset

voltage also corrects other DC

specications, such as power-

supply rejection and common-

mode rejection. Thereore,

auto-zero ampliers are able to

achieve superior rejection to

that o traditional ampliers.

Figure 1: Shown is a simplied chopper-stabilized unctional diagram.

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Using auto-zero op ampsin signal-conditioning apps

SIGNALS

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Low drit over temperature,

time—All ampliers, regardless o process technology and architec-

ture, have an oset voltage that

changes over temperature and

time. Most op amps speciy this

oset drit over temperature in

terms o volts per degree Celsius.

  This drit can vary substantiallyrom amplier to amplier, but

or a traditional amplier it is

typically on the order o several

micro-volts to tens o micro-voltsper degree Celsius. This oset

drit can be very problematic in

high-precision applications; un-

like initial oset errors, this drit

cannot be accounted or with a

one-time system calibration.

In addition to driting over

temperature, an amplier’s osetvoltage tends to change over time

as well. For traditional op amps,

this drit over time (sometimes

called aging) typically isn’t speci-ed in the datasheet, but it can

create signicant errors over the

lie o the device.

  The auto-zero architecture

inherently minimizes both the

drit over temperature and time

by continually sel correcting

the oset voltage. In this way, an

auto-zero amplier can achievesignicantly better drit peror-

mance over traditional op amps.

For example, the MCP6V01 opamp mentioned previously has

a maximum temperature drit o 

only 50nV/°C.

Eliminates 1/ noise—Flicker noise,

or 1/ noise, is a low-requency

phenomenon caused by irregu-

larities in the conduction path

and noise due to the bias currentswithin the transistors. At higher

requencies, 1/ noise is negligi-

ble as the white noise rom othersources begins to dominate. This

low-requency noise can be very

problematic i the input signal

is near DC, such as the outputs

rom strain gauges, pressure sen-

sors, thermocouples etc.

In an auto-zero based amplier,

the 1/ noise is removed as part

o the oset-correction process.

  This noise source appears at the

input and is relatively slow mov-

ing. Hence, it appears to be a part

o the amplier’s oset and gets

compensated accordingly.

Low bias current —Bias currentis the amount o current ow

into the inputs o the amplier

to bias the input transistors. The

magnitude o this current can

vary rom microamperes down to

picoamperes and is strongly de-

pendent upon the architecture o 

the amplier-input circuitry. This

parameter becomes extremelyimportant when connecting a

high-impedance sensor to the

input o an amplier. As thebias current ows through this

high impedance, a voltage drop

occurs across the impedance,

resulting in a voltage error. For

these applications, a low bias cur-

rent is required.

Virtually all auto-zero ampliers

on the market today implement a

CMOS input stage, which resultsin very low bias currents. However,

the charge injection rom the

internal switching can result inslightly higher bias currents then

that o a more traditional, CMOS-

input op amp.

Quiescent current —For battery-

powered applications, quiescent

current is a critical parameter.

Because o the nulling ampliers

and other circuitry required to

support the sel-correcting auto-

zero architecture, auto-zero am-

pliers typically consume more

quiescent current or a given

bandwidth and slew rate, relative

to traditional ampliers. However,

signicant improvements havebeen made to increase the ef-

ciency o this architecture. Some

op amps, such as Microchip

  Technology’s MCP6V03, oer a

Chip Select or shutdown pin to

minimize quiescent current when

the device is not active.

Application example  The previous section identied

several parameters in which the

auto-zero architecture helps toincrease amplier perormance.

 This section explores an example

application using a strain gauge,

which highlights some o the

advantages o an auto-zero

amplier.

Portable weigh scales are pop-

ular devices or weighing small

items such as precious metals,  jewelry and medications. These

devices are battery-powered and

typically require accuracy downto a tenth o a gram, i not bet-

ter. Thus, this application requires

high-precision, low-power signal

conditioning or the strain gauges

used to measure the weight.

A strain gauge uses electrical

resistance to quantiy the amount

o strain caused by an external

orce. There are several dierent

types o strain gauges, the most

common o which is a metallic

strain gauge. This type o strain

gauge is composed o a wire or

small piece o metal oil. When

a orce is applied, the strain on

the gauge is altered (either posi-tively or negatively), resulting in a

change in the strain gauge’s elec-

trical resistance. This change in

resistance can then be measured

and the magnitude o the applied

orce quantied. Typically, one or

more strain gauges are arranged

in a Wheatstone-bridge congu-

ration, due to the excellent sen-sitivity that this circuit oers. The

change in the resistance value is

small, so the overall voltage out-put o such a Wheatstone-bridge

circuit is small. For this example,

we will assume a 10mV ull-scale

output.

Figure 2 is a simplied circuit

analyzed or this application.

Please note that this circuitry is

not intended to be a complete

representation, but is simplied toshow the benets o the auto-zero

architecture. For example, the out-

puts o the Wheatstone-bridgecircuit should be buered to

provide a high-impedance input,

which is not shown in the circuit

diagram. In this circuit, the ampli-

er is congured or a dierential

gain o 500, so a ull-scale output

rom the Wheatstone bridge will

ideally produce a 5V output rom

the amplier.

Due to the high amount o 

gain required in this application,

the oset voltage o the ampli-

Figure 2: Here’s a simplied application circuit diagram.

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