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| 型號： | AD8554ARZ |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 7/24頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC OPAMP CHOPPER R-R QUAD 14SOIC |
| 標(biāo)準(zhǔn)包裝： | 56 |
| 放大器類型： | 斷路器（零漂移） |
| 電路數(shù)： | 4 |
| 輸出類型： | 滿擺幅 |
| 轉(zhuǎn)換速率： | 0.4 V/µs |
| 增益帶寬積： | 1.5MHz |
| 電流 - 輸入偏壓： | 10pA |
| 電壓 - 輸入偏移： | 1000µV |
| 電流 - 電源： | 850µA |
| 電流 - 輸出 / 通道： | 30mA |
| 電壓 - 電源，單路/雙路(±)： | 2.7 V ~ 5.5 V |
| 工作溫度： | -40°C ~ 125°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 14-SOIC（0.154"，3.90mm 寬） |
| 供應(yīng)商設(shè)備封裝： | 14-SOIC |
| 包裝： | 管件 |
| 產(chǎn)品目錄頁面： | 766 (CN2011-ZH PDF) |
Data Sheet
AD8551/AD8552/AD8554
Rev. E | Page 15 of 24
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–
01101-
051
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
t
V
t
V
2
1
(4)
Substituting Equation 4 and Equation 2 into Equation 3 yields
[ ]
A
S
OSA
A
OSA
A
IN
A
OA
B
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 VOS time invariant;
therefore, Equation 5 can be rearranged and rewritten as
[ ]
(
)
A
OSA
A
OSA
A
IN
A
OA
B
V
B
A
V
B
A
t
V
A
t
V
+
+
=
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
B
IN
OUT
V
A
B
V
B
A
B
A
t
V
t
V
+
=
1
(10)
The AD855x architecture is optimized in such a way that
AA = AB and BA = BB and BA >> 1
Also, the gain product of AABB is much greater than AB. These
allow Equation 10 to be simplified to
[ ]
(
)
OSB
OSA
A
IN
OUT
V
A
B
A
t
V
t
V
+
≈
(11)
Most obvious is the gain product of both the primary and nulling
amplifiers. This AABA term is what gives the AD855x its extremely
high open-loop gain. To understand how VOSA and VOSB relate to
the overall effective input offset voltage of the complete amplifier,
establish the generic amplifier equation of
(
)
EFF
OS
IN
OUT
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
EFF
OS
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
+
≈
,
(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
earn the reputation of being among the most precise amplifiers
available on the market.
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