Data Sheet AD8571/AD8572/AD8574
Rev. F | Page 15 of 28
FUNCTIONAL DESCRIPTION
The AD8571/AD8572/AD8574 are CMOS amplifiers that
achieve their high degree of precision through random frequency
auto-zero stabilization. The autocorrection topology allows the
AD8571/AD8572/AD8574 to maintain its low offset voltage over
a wide temperature range, and the randomized auto-zero clock
eliminates any inter-modulation distortion (IMD) errors at the
amplifier output.
The AD8571/AD8572/AD8574 can run from a single-supply
voltage as low as 2.7 V. The extremely low offset voltage of 1 µV
and no IMD products allow the amplifier to be easily configured
for high gains without risk of excessive output voltage errors, which
makes the AD8571/AD8572/AD8574 an ideal amplifier for
applications requiring both dc precision and low distortion for ac
signals. The extremely small temperature drift of 5 nV/°C
ensures a minimum of offset voltage error over its −40°C to
+125°C temperature range. These combined features make the
AD8571/AD8572/AD8574 an excellent choice for a variety of
sensitive measurement and automotive applications.
AMPLIFIER ARCHITECTURE
Each AD8571/AD8572/AD8574 op amp consists of two
amplifiers: a main amplifier and a secondary amplifier that is used
to correct the offset voltage of the main amplifier. Both consist of
a rail-to-rail input stage, allowing the input common-mode
voltage range to reach both supply rails. The input stage consists
of an NMOS differential pair operating concurrently with a
parallel PMOS differential pair. The outputs from the differential
input stages are combined in another gain stage whose output is
used to drive a rail-to-rail output stage.
The wide voltage swing of the amplifier is achieved by using two
output transistors in a common-source configuration. The output
voltage range is limited by the drain-to-source resistance of
these transistors. As the amplifier is required to source or sink
more output current, the voltage drop across these transistors
increases due to their on resistance (R
DS
). Simply put, the output
voltage does not swing as close to the rail under heavy output
current conditions as it does with light output current. This is a
characteristic of all rail-to-rail output amplifiers. Figure 12 and
Figure 13 show how close the output voltage can get to the rails
with a given output current. The output of the AD8571/
AD8572/AD8574 is short-circuit protected to approximately
50 mA of current.
The AD8571/AD8572/AD8574 amplifiers have exceptional
gain, yielding greater than 120 dB of open-loop gain with a load
of 2 kΩ. Because the output transistors are configured in a
common-source configuration, the gain of the output stage, and
thus the open-loop gain of the amplifier, is dependent on the
load resistance. Open-loop gain decreases with smaller load
resistances, which is another characteristic of rail-to-rail output
amplifiers.
BASIC AUTO-ZERO AMPLIFIER THEORY
Autocorrection amplifiers are not a new technology. Various IC
implementations have been available for more than 15 years,
and some improvements have been made over time. The
AD8571/AD8572/AD8574 design offers a number of significant
performance improvements over older versions while attaining a
very substantial reduction in device cost. This section offers a
simplified explanation of how the AD8571/AD8572/AD8574
are able to offer extremely low offset voltages and high open-
loop gains.
As noted in the Amplifier Architecture section, each
AD8571/AD8572/AD8574 op amp contains two internal
amplifiers. One is used as the primary amplifier, and the other as
an autocorrection, or nulling, amplifier. Each amplifier has an
associated input offset voltage that can be modeled as a dc
voltage source in series with the noninverting input. In Figure 50
and Figure 51, these are labeled as V
OSA
and V
OSB
, where A
denotes the nulling amplifier and B denotes the primary
amplifier. The open-loop gain for the +IN and −IN inputs of
each amplifier is given as A
X
. Both amplifiers also have a third
voltage input with an associated open-loop gain of B
X
.
V
IN+
V
IN–
V
OUT
A
B
A
A
ΦA
1
ΦB
V
OSA
+
V
OSB
+
B
B
C
M2
C
M1
ΦA
2
V
NB
V
NA
–B
A
V
OA
ΦB
01104-050
Figure 50. Auto-Zero Phase of the Amplifier
V
IN+
V
IN–
V
OUT
A
B
A
A
ΦA
ΦB
V
OSA
+
V
OSB
+
B
B
C
M2
C
M1
ΦA
V
NB
V
NA
–B
A
V
OA
ΦB
01104-051
Figure 51. Output Phase of the Amplifier
There are two modes of operation determined by the action of
two sets of switches in the amplifier: an auto-zero phase and an
amplification phase.
