Demystifying Auto- current is not too large, a fixed-frequency auto-zero amplifier can

be used with excellent results.

Zero Amplifiers—2 It is generally desirable to limit the signal bandwidth to the lowest
value needed, since this minimizes the effect of the chopping clock
noise and also minimizes total noise. Remember that total voltage
They essentially eliminate offset, drift, and noise for an auto-zero amplifier is proportional to the square root
1/f noise. Here are some design ideas. of the signal bandwidth (EN = eN × √BW). A simple low-pass filter
can be created by adding optional capacitors (C) in parallel with
by Eric Nolan and Reza Moghimi the feedback resistors. Additional gain and filtering at amplified
Part 1 of this two-part article explains how auto-zero amplifiers voltage levels can be provided by an additional stage using ordinary
work and identifies their important characteristics. As promised, amplifiers. With their high open-loop gain, auto-zero amplifiers
this installment will discuss a few application ideas that are can easily provide closed-loop gains of 100× to 1000×, permitting
particularly well suited to auto-zero amplifiers inexpensive CMOS amplifiers with several millivolts of offset and
fairly high voltage noise to be used in the following stage without
Applying auto-zero amplifiers really is not much different from
sacrificing system accuracy. Using a high stage gain for the auto-
applying any operational amplifier. Most new designs have the
zero amplifier can also add an extra pole to the filter roll-off if the
same pinout and functionality as any other amplifier. DC closed-
amplifier’s gain-bandwidth divided by the stage gain is less than
loop gain is set by resistors in the same manner; functions such as
half the chopping clock frequency. However, filter performance
filtering, integration, and the like can be done in the same way. In
will be affected by variations in GBW from amplifier to amplifier.
most applications, the principal accommodation will be limiting
the bandwidth to exclude the chopping noise and IMD artifacts If the signal frequency exceeds about half the chopping clock
from the passband. For auto-zero amplifiers with fixed-chopper frequency, an auto-zero amplifier type with a pseudorandom clock
frequency, this generally limits their application to dc or low- rate, such as the AD8571, can be used. In this case, the maximum
frequency signals below 1 or 2 kHz. overall accuracy will be slightly degraded due to the slightly higher
wideband noise and the higher bandwidth; but the chopping clock
PRECISION CURRENT SHUNTS will not produce a large error term at the clock frequency, and the
A precision shunt current sensor benefits by some of the unique effects of IMD will be minimized.
attributes of auto-zero amplifiers used in a differencing
configuration (Figure 1). Shunt current sensors are used in STRAIN BRIDGES
precision current sources for feedback control systems. They are Another common application, where the low offset and related
also used in a variety of other applications, including battery fuel low-frequency performance help to achieve wide dynamic range,
gauging, torque feedback controls in electric power steering and is with strain bridges. Used in force and pressure sensors and in
precision power metering. weigh scales, these bridges usually produce a relatively small output
voltage level, even at full scale. In this example, three of the four
amplifiers in a quad AD8554 are used for excitation and differential
SUPPLY RS
RL
amplification (see Figure 2). The full-scale output may be a few
I 0.1⍀
tens of millivolts. In this case, the very low offset voltage of the
100k⍀ 100⍀ auto-zero amplifier contributes minimal error to the measured
e = 1000 RS I signal. The long sample times of weigh scales benefit from the lack
100mV/mA C
5V
of 1/f noise. The low long-term drift of the amplifiers also minimizes
or eliminates recalibration requirements.
AD8551 5V
100k⍀ 100⍀ 2

REF 6 2.5V AD8554A

C 192 2N2222
1k⍀
4 3
3.5V
Figure 1. Current shunt amplifier. BRIDGE EXCITATION

In such applications, it is desirable to use a shunt with very low 20k⍀ 8k⍀

resistance to minimize the series voltage drop; this minimizes the

waste of power and allows the measurement of high currents
5V
without significant voltage drop. A typical shunt might be 0.1 Ω. 0mV–35mV F.S.
AD8554C
At measured current values of 1+ amperes, the shunt’s output 0–3.5V
signal is hundreds of millivolts, or even volts, and amplifier error 5V

sources are not critical. However, at low measured current values AD8554B
in the 1-mA range, the 100-µV output voltage of the shunt demands 10.1k⍀ 1M⍀
a very low offset voltage and drift to maintain absolute accuracy.
Low input bias currents are also needed, so that “injected” bias 10.1k⍀
1M⍀
current does not become a significant percentage of the measured
current. High open-loop gain, CMR, and PSR all help to maintain
the overall circuit accuracy. As long as the rate of change of the Figure 2. Single-supply strain-gauge bridge amplifier.

Analog Dialogue 34-3 (2000) 1

Pressure sensor systems, which usually require linearization to feedback, auto-zero amplifiers teamed up with precision references
produce accurate output values, benefit from the low offset and can provide effective solutions to these problems.
drift. A well-characterized sensor can be scaled and linearized
4.7V
without concern for interaction with the amplifier, since the added TO 5V
amplifier error terms are negligible. The low input bias currents
2 500⍀
enable the use of higher bridge impedances, too; this can
2N2907A
significantly improve system power consumption in portable or AD780 6 AD8551
2.5V
loop-powered applications, because the bridge excitation current
2.5k⍀
can be much lower for the same output range. The lower bridge 4 VOUT = 4.5V

excitation current also minimizes errors due to bridge self-heating. 10␮F 0.1␮F
0.1␮F
Most strain-gauge bridge applications are low-frequency by nature,
5k⍀ 4k⍀
so the limited useful bandwidth of fixed-frequency auto-zero 0.01% 3.9⍀
0.01%
amplifiers is not an issue. The use of bridges with higher frequency
outputs or with ac excitation can be accommodated through the a. Regulated 4.5 V from a supply as low as 4.7 V.
use of randomly clocked auto-zero amplifiers.
5V
INFRARED (IR) SENSORS
Infrared (IR) sensors, particularly thermopiles, are increasingly
(1)
being used in temperature measurement for applications as wide- 2.5V AD8551
REFERENCE
ranging as automotive climate controls, human ear thermometers, 2.5V

home-insulation analysis and automotive-repair diagnostics. The (2)

relatively small output signal of the sensor demands high gain with
(1) USE ATTENUATOR FOR OUTPUT <2.5V
very low offset voltage and drift to avoid dc errors. If interstage ac (2) USE ATTENUATOR FOR OUTPUT >2.5V
coupling is used (Figure 3), the low offset and drift prevents the
input amplifier’s output from drifting close to saturation. The low b. Simple buffering.
input bias currents generate minimal errors from the sensor’s +2.5V
output impedance. As with pressure sensors, the very low amplifier 2
drift with time and temperature eliminates additional errors once 6 +2.048V

the temperature measurement has been calibrated. The low 1/f ADR290 10k⍀
10k⍀
noise improves SNR for dc measurements taken over periods often
4 +2.5V
exceeding 1/5 second. Figure 3 shows an amplifier that can bring
–2.048V
ac signals from 100 to 300 microvolts up to the 1 to 3-V level.
AD8551
10k⍀ 100k⍀ –2.5V
100⍀ 100k⍀
5V TIGHT-TOLERANCE—MATCHED
5V RESISTORS NEEDED

100␮V–300␮V 10␮F
c. ±2.048-V reference from ±2.5-V supply.
IR Figure 4. Precision amplifiers enhance flexibility of preci-
DETECTOR 10k⍀ sion references.
ƒC 1.6Hz

TO BIAS
The above are just a few ideas to suggest the wide applicability of
VOLTAGE auto-zero amplifiers. Almost any application that deals with small
Figure 3. High-input-impedance ac-coupled preamplifier input signals and wide dynamic range over a moderate signal
for thermopile. bandwidth is worthy of consideration for performance
improvement using an auto-zero amplifier. Systems that are
PRECISION REFERENCES (FIGURE 4) calibrated and must maintain performance over extended periods
Precision voltage reference ICs in low-voltage systems may lack without maintenance will also benefit. Any application requiring
the flexibility to handle all the jobs they are called upon to do. For tight channel-to-channel matching of dc performance is also a
example, (a) they may require low dropout voltage, or (b) they candidate. The dc error contributions of high-gain auto-zero
may have to handle sourcing and sinking load currents, or (c) the amplifiers are so small that using multichannel, or even single-
system in which they are used may simply need a negative reference channel, devices will not substantially degrade the matching of
in addition to the positive reference.With their extremely low offsets multiple input channels. Multiple devices (duals and quads) will
and drift, their output drive capability, and the use of active also be helpful with matched low-frequency ac inputs.

2 Analog Dialogue 34-3 (2000)