AD8551/AD8552/AD8554
FUNCTIONAL DESCRIPTION
As noted in the previous section on amplifier architecture, each
AD855x op amp contains two internal amplifiers. One is used as
the primary amplifier, the other as an autocorrection, or nulling,
amplifier. Each amplifier has an associated input offset voltage,
which can be modeled as a dc voltage source in series with the
noninverting input. In Figures 44 and 45 these are labeled as
VOSX, where x denotes the amplifier associated with the offset; A
for the nulling amplifier, B for the primary amplifier. The open-
loop gain for the +IN and –IN inputs of each amplifier is given
as AX. Both amplifiers also have a third voltage input with an
associated open-loop gain of BX.
The AD855x family of amplifiers are high precision rail-to-rail
operational amplifiers that can be run from a single supply volt-
age. Their typical offset voltage of less than 1 µV allows these
amplifiers to be easily configured for high gains without risk of
excessive output voltage errors. The extremely small tempera-
ture drift of 5 nV/°C ensures a minimum of offset voltage error
over its entire temperature range of –40°C to +125°C, making
the AD855x amplifiers ideal for a variety of sensitive measure-
ment applications in harsh operating environments such as
under-hood and braking/suspension systems in automobiles.
The AD855x family are CMOS amplifiers and achieve their
high degree of precision through autozero stabilization. This
autocorrection topology allows the AD855x to maintain its low
offset voltage over a wide temperature range and over its operat-
ing lifetime.
There are two modes of operation determined by the action of
two sets of switches in the amplifier: An autozero phase and an
amplification phase.
Autozero Phase
In this phase, all φA switches are closed and all φB switches are
opened. Here, the nulling amplifier is taken out of the gain loop
by shorting its two inputs together. Of course, there is a degree of
offset voltage, shown as VOSA, inherent in the nulling amplifier
which maintains a potential difference between the +IN and –IN
inputs. The nulling amplifier feedback loop is closed through φA2
Amplifier Architecture
Each AD855x op amp consists of two amplifiers, a main amplifier
and a secondary amplifier, 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.
and VOSA appears at the output of the nulling amp and on CM1
,
an internal capacitor in the AD855x. Mathematically, we can ex-
press this in the time domain as:
VOA t = A V
t − B V
t
(1)
[ ]
OSA[ ] OA[ ]
A
A
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 rDS of these transistors increases, raising the
voltage drop across these transistors. Simply put, the output volt-
age will not swing as close to the rail under heavy output current
conditions as it will with light output current. This is a character-
istic of all rail-to-rail output amplifiers. Figures 6 and 7 show how
close the output voltage can get to the rails with a given output
current. The output of the AD855x is short circuit protected to
approximately 50 mA of current.
which can be expressed as,
AAVOSA
t
[ ]
VOA t =
[ ]
(2)
1+ BA
This shows us that the offset voltage of the nulling amplifier
times a gain factor appears at the output of the nulling amplifier
and thus on the CM1 capacitor.
V
V
IN+
A
V
OUT
B
IN؊
B
B
⌽B
V
OA
The AD855x 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 configu-
ration, the gain of the output stage, and thus the open-loop gain
of the amplifier, is dependent on the load resistance. Open-loop
gain will decrease with smaller load resistances. This is another
characteristic of rail-to-rail output amplifiers.
C
M2
⌽B
V
OSA
+
⌽A
A
V
A
NB
؊B
A
⌽A
C
M1
V
NA
Basic Autozero Amplifier Theory
Figure 44. Autozero Phase of the AD855x
Amplification Phase
Autocorrection amplifiers are not a new technology. Various IC
implementations have been available for over 15 years and some
improvements have been made over time. The AD855x design
offers a number of significant performance improvements over
older versions while attaining a very substantial reduction in de-
vice cost. This section offers a simplified explanation of how the
AD855x is able to offer extremely low offset voltages and high
open-loop gains.
When the φB switches close and the φA switches open for the
amplification phase, this offset voltage remains on CM1 and
essentially corrects any error from the nulling amplifier. The
voltage across CM1 is designated as VNA. Let us also designate
VIN as the potential difference between the two inputs to the
primary amplifier, or VIN = (VIN+ – VIN–). Now the output of the
nulling amplifier can be expressed as:
VOA t = A V t −V
t
− B V
t
(3)
[ ] IN [ ] OSA[ ]
(
)
NA[ ]
A
A
–10–
REV. 0
ADI [ ADI ]
