AD8551ARM Analog Devices, AD8551ARM Datasheet - Page 11

• Manufacturer Part Number: AD8551ARM
• Description: Zero-Drift/ Single-Supply/ Rail-to-Rail Input/Output Operational Amplifiers
• Manufacturer: Analog Devices

Because A is now open and there is no place for C
charge, the voltage V
voltage at the output of the nulling amp V
switching frequency T
phases every 0.5
And substituting Equation 4 and Equation 2 into Equation 3 yields:
For the sake of simplification, let us assume that the autocorrection
frequency is much faster than any potential change in V
V
a function of temperature variation or long-term wear time, both of
which are much slower than the auto-zero clock frequency of the
AD855x. This effectively makes V
arrange Equation 5 and rewrite it as:
or,
We can already get a feel for the autozeroing in action. Note the
V
nulling amplifier has greatly reduced its own offset voltage error
even before correcting the primary amplifier. Now the primary
amplifier output voltage is the voltage at the output of the
AD855x amplifier. It is equal to:
In the amplification phase, V
V
REV. 0
A was closed. If we call the period of the autocorrection
OSB
OS
OUT
V
V
V
V
OA
term is reduced by a 1 + B
. This is a good assumption since changes in offset voltage are
OA
IN+
IN
t
t
t
Figure 45. Output Phase of the Amplifier
V
A V
OUT
A V
B IN
A V
A IN
A IN
V
OA
t
V
t
T
t
NA
t
A
t
S
A V
NA
. Therefore, in the amplification phase:
A V
S
B
B
t
, then the amplifier switches between
A V
B OSB
A V
at the present time t is equal to the
A
A OSA
A
A
IN
V
V
OA
1
OSA
NA
t
IN
+
= V
B
A
t
OS
t
B
V
t
A
factor. This shows how the
B
NB
OSB
time invariant and we can re-
A
1
2
A
A B V
1
, so this can be rewritten as:
V
1
A V
V
T
V
A
OA
OSA
A
S
OSA
B
B
B
A
OA
A OSA
B V
A
V
A
IN
NA
B NB
1
at the time when
A B V
B
A
t
A
A
B
B
t
A
A OSA
B
1
V
C
C
B
M2
V
M1
1
2
OSA
M1
NB
T
B
V
S
OSA
A
OUT
to dis-
or
(4)
(8)
(5)
(6)
(7)
(9)
–11–
Combining terms,
The AD855x architecture is optimized in such a way that
A
A
simplified to:
Most obvious is the gain product of both the primary and nulling
amplifiers. This A
high open-loop gain. To understand how V
the overall effective input offset voltage of the complete amplifier,
we should set up the generic amplifier equation of:
Where k is the open-loop gain of an amplifier and V
effective offset voltage. Putting Equation 12 into the form of
Equation 11 gives us:
And from here, it is easy to see that:
Thus, the offset voltages of both the primary and nulling ampli-
fiers are reduced by the gain factor B
offset voltage from several millivolts down to an effective input
offset voltage of submicrovolts. This autocorrection scheme is
what makes the AD855x family of amplifiers among the most
precise amplifiers in the world.
High Gain, CMRR, PSRR
Common-mode and power supply rejection are indications of
the amount of offset voltage an amplifier has as a result of a
change in its input common-mode or power supply voltages. As
shown in the previous section, the autocorrection architecture of
the AD855x allows it to quite effectively minimize offset volt-
ages. The technique also corrects for offset errors caused by
common-mode voltage swings and power supply variations.
This results in superb CMRR and PSRR figures in excess of
130 dB. Because the autocorrection occurs continuously, these
figures can be maintained across the device’s entire temperature
range, from –40 C to +125 C.
Maximizing Performance Through Proper Layout
To achieve the maximum performance of the extremely high
input impedance and low offset voltage of the AD855x, care
should be taken in the circuit board layout. The PC board sur-
face must remain clean and free of moisture to avoid leakage
currents between adjacent traces. Surface coating of the circuit
board will reduce surface moisture and provide a humidity
barrier, reducing parasitic resistance on the board. The use of
guard rings around the amplifier inputs will further reduce leak-
age currents. Figure 46 shows how the guard ring should be
configured and Figure 47 shows the top view of how a surface
mount layout can be arranged. The guard ring does not need to
V
A
A
B
OUT
= A
B
is much greater than A
B
t
V
V
and B
OUT
OUT
V
IN
t
t
A
V
V
t A
A
= B
OUT
V
OS EFF
B
V
A
IN
IN
,
AD8551/AD8552/AD8554
B
B
term is what gives the AD855x its extremely
and B
t A B
t A B
k
A B
A
A
A
V
V
B
A
B
A
OSA
. These allow Equation 10 to be
IN
A
>> 1. Also, the gain product of
B
A V
V
V
A B V
A
A
OS EFF
V
OS EFF
A
1
A
OSB
,
. This takes a typical input
,
OSA
B OSA
B
OSA
A
A B
V
A
and V
OSB
A
A V
B OSB
OSB
OS, EFF
relate to
is its
(10)
(11)
(13)
(12)
(14)