AD8551 Analog Devices, AD8551 Datasheet - Page 15

• Manufacturer Part Number: AD8551
• Description: Zero-Drift, Single-Supply, RRIO Single Op Amp
• Manufacturer: Analog Devices

Specifications of AD8551

• -3db Bandwidth: 1.5MHz
• Slew Rate: 0.4V/µs
• Vos: 1µV
• Ib: 10pA
• # Opamps Per Pkg: 1
• Input Noise (nv/rthz): 42nV/rtHz
• Vcc-vee: 2.7V to 6V
• Isy Per Amplifier: 975µA
• Packages: SOIC,SOP

Amplification Phase
When the φB switches close and the φA switches open for the
amplification phase, this offset voltage remains on C
essentially, corrects any error from the nulling amplifier. The
voltage across C
designated as the potential difference between the two inputs to
the primary amplifier, or V
amplifier can be expressed as
Because φA is now open and there is no place for C
discharge, the voltage (V
the voltage at the output of the nulling amp (V
when φA was closed. If the period of the autocorrection switching
frequency is labeled t
phases every 0.5 × t
Substituting Equation 4 and Equation 2 into Equation 3 yields
For the sake of simplification, assume that the autocorrection
frequency is much faster than any potential change in V
V
voltage are 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 renders V
time invariant; therefore, Equation 5 can be rearranged and
rewritten as
OSB
V
V
V
V
IN–
IN–
IN+
IN+
. This is a valid assumption because changes in offset
V
V
V
NA
OA
OA
[
[ ]
[ ]
t
t
t
= ]
=
=
V
A
A
Figure 50. Auto-Zero Phase of the AD855x
Figure 51. Output Phase of the Amplifier
NA
A
A
M1
V
(
ФA
ФA
V
⎡ −
⎢ ⎣
IN
is designated as V
t
IN
S
. Therefore, in the amplification phase
[ ]
t
ФB
S
ФB
[ ]
, then the amplifier switches between
t
1
2
+
t
−
NA
S
A
V
⎤
⎥ ⎦
V
V
), at the present time (t), is equal to
A
IN
OSA
OSA
OSA
V
= (V
OSA
+
+
[ ]
t
[ ]
t
)
IN+
−
A
A
−
A
A
NA
B
− V
V
V
A
A
–B
–B
. Furthermore, V
OA
OA
V
A
A
A
IN−
B
NA
V
V
A
NA
). Thus, the nulling
NA
V
[ ]
t
ФB
ФA
1
ФB
ФA
OSA
+
A
OA
A
B
B
B
⎡ −
⎢ ⎣
) at the time
t
A
B
B
B
C
C
B
C
C
V
V
M1
M2
M1
M2
NB
M1
M1
NB
1
2
to
t
and,
V
V
S
IN
OSA
OUT
OUT
⎤
⎥ ⎦
is
OS
or
Rev. D | Page 15 of 24
(3)
(4)
(5)
or
From these equations, the auto-zeroing action becomes evident.
Note the V
the nulling amplifier has greatly reduced its own offset voltage
error even before correcting the primary amplifier. This results
in the primary amplifier output voltage becoming the voltage at
the output of the AD855x amplifier. It is equal to
In the amplification phase, V
Combining terms,
The AD855x architecture is optimized in such a way that
Also, the gain product of A
allow Equation 10 to be simplified to
Most obvious is the gain product of both the primary and
nulling amplifiers. This A
extremely high open-loop gain. To understand how V
V
complete amplifier, establish 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
Thus, it is evident that
The offset voltages of both the primary and nulling amplifiers
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
the outstanding feature of the AD855x series that continues to
V
OSB
OUT
A
relate to the overall effective input offset voltage of the
V
V
V
V
V
V
V
V
[ ]
t
A
OA
OA
OUT
OUT
OUT
OS
OUT
OUT
= A
=
,
[ ]
[ ]
EFF
t
t
[ ]
[ ]
[ ]
[ ]
A
t
t
=
t
OS
t
B
=
=
B
k
≈
=
=
and B
≈
≈
term is reduced by a 1 + B
V
A
A
×
V
IN
V
V
V
A
A
A
(
OSA
IN
IN
IN
V
B
V
⎛
⎜
⎜
⎝
[ ]
t
V
(
IN
IN
V
[ ]
[ ]
[ ]
A
t
t
t
B
IN
+
+
IN
[ ]
= B
A
t
(
A
+
A
[ ]
V
A
A
[ ]
t
A
A
t
V
+
OSB
B
B
B
B
B
+
OS
V
AD8551/AD8552/AD8554
+
+
A
and B
A
A
A
1
,
OSB
V
B
A
A
EFF
V
A
+
+
+
OSB
B
A
(
OSA
B
1
V
OA
A
B
B
B
term is what gives the AD855x its
)
+
+
OS
A
is much greater than A
B
A
A
)
= V
B
(
B
+
)
,
V
>> 1
EFF
A
⎞
⎟
⎟
⎠
+
A
B
. This takes a typical input
OSA
B
)
⎡
⎢
⎢
⎣
V
NB
1
A
B
A
A
V
OSA
+
, so this can be rewritten as
A
A
1
+
A
NB
B
B
+
B
A
⎛
⎜
⎜
⎝
V
A
A
V
−
A
factor. This shows how
V
B
OSB
IN
A
A
OSA
)
A
[ ]
t
B
+
A
+
V
A
1
OSA
V
B
+
V
OS, EFF
OSB
OSA
B
B
OSA
A
. These
⎞
⎟
⎟
⎠
is its
and
⎤
⎥
⎥
⎦
(10)
(11)
(12)
(13)
(14)
(9)
(6)
(7)
(8)