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AD8551ARM-R2 数据表(PDF) 15 Page - Analog Devices

部件名 AD8551ARM-R2
功能描述  Zero-Drift, Single-Supply, Rail-to-Rail Input/Output Operational Amplifiers
PDF  24 Pages
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制造商  AD [Analog Devices]
网页  http://www.analog.com
标志 AD - Analog Devices

AD8551ARM-R2 数据表(HTML) 15 Page - Analog Devices

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AD8551/AD8552/AD8554
Rev. C | Page 15 of 24
+
AB
BB
CM2
VIN+
VNB
CM1
VOA
–BA
VNA
ФB
ФA
AA
VOSA
ФB
ФA
VOUT
VIN–
Figure 50. Auto-Zero Phase of the AD855x
Amplification Phase
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. Furthermore, VIN is
designated as the potential difference between the two inputs to
the primary amplifier, or VIN = (VIN+ − VIN−). Thus, the nulling
amplifier can be expressed as
[]
[]
(
)
[]
t
V
B
t
V
t
V
A
t
V
NA
A
OSA
IN
A
OA
−
−
=
]
[
(3)
+
AB
BB
CM2
VIN+
VNB
CM1
VOA
–BA
VNA
ФB
ФA
AA
VOSA
ФB
ФA
VOUT
VIN–
Figure 51. Output Phase of the Amplifier
Because φA is now open and there is no place for CM1 to
discharge, the voltage (VNA), at the present time (t), is equal to
the voltage at the output of the nulling amp (VOA) at the time
when φA was closed. If the period of the autocorrection switching
frequency is labeled tS, then the amplifier switches between
phases every 0.5 × tS. Therefore, in the amplification phase
[]
⎥⎦
⎤
⎢⎣
⎡ −
=
S
NA
NA
t
t
V
t
V
2
1
(4)
Substituting Equation 4 and Equation 2 into Equation 3 yields
[]
[]
[]
A
S
OSA
A
A
OSA
A
IN
A
OA
B
t
t
V
B
A
t
V
A
t
V
A
t
V
+
⎥⎦
⎤
⎢⎣
⎡ −
−
+
=
1
2
1
(5)
For the sake of simplification, assume that the autocorrection
frequency is much faster than any potential change in VOSA or
VOSB. This is a valid assumption because changes in offset
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
[]
[ ]
(
)
A
OSA
A
A
OSA
A
A
IN
A
OA
B
V
B
A
V
B
A
t
V
A
t
V
+
−
+
+
=
1
1
(6)
or
[]
[]
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+
+
=
A
OSA
IN
A
OA
B
V
t
V
A
t
V
1
(7)
From these equations, the auto-zeroing action becomes evident.
Note the VOS term is reduced by a 1 + BA factor. This shows how
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
[ ]
[ ]
(
)
NB
B
OSB
IN
B
OUT
V
B
V
t
V
A
t
V
+
+
=
(8)
In the amplification phase, VOA = VNB, so this can be rewritten as
[]
[]
[]
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+
+
+
+
=
A
OSB
IN
A
B
OSB
B
IN
B
OUT
B
V
t
V
A
B
V
A
t
V
A
t
V
1
(9)
Combining terms,
[]
[]
()
OSA
B
A
OSA
A
A
B
B
B
IN
OUT
V
A
B
V
B
A
B
A
A
t
V
t
V
+
+
+
+
=
1
(10)
The AD855x architecture is optimized in such a way that
AA = AB and BA
B
= B
B
OS
time invariant; therefore, Equation 5 can be rearranged and
rewritten as
B
B
B
and BA
B
>> 1
Also, the gain product of AAB
B
B
is much greater than AB
B
. These
allow Equation 10 to be simplified to
[ ]
[ ]
(
)
OSB
OSA
A
A
A
IN
OUT
V
V
A
B
A
t
V
t
V
+
+
≈
(11)
Most obvious is the gain product of both the primary and
nulling amplifiers. This AAB
B
A
term is what gives the AD855x its
extremely high open-loop gain. To understand how VOSA and
VOSB
B
relate to the overall effective input offset voltage of the
complete amplifier, establish the generic amplifier equation of
(
)
EFF
OS
IN
OUT
V
V
k
V
,
+
×
=
(12)
where k is the open-loop gain of an amplifier and VOS, EFF is its
effective offset voltage.
Putting Equation 12 into the form of Equation 11 gives
[ ]
[ ]
A
A
EFF
OS
A
A
IN
OUT
B
A
V
B
A
t
V
t
V
,
+
≈
(13)
Thus, it is evident that
A
OSB
OSA
EFF
OS
B
V
V
V
+
≈
,
(14)
The offset voltages of both the primary and nulling amplifiers
are reduced by the Gain Factor BA. This takes a typical input
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



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