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AD8571ARM-R2 Datasheet(PDF) 15 Page - Analog Devices

Part # AD8571ARM-R2
Description  Zero-Drift, Single-Supply, Rail-to-Rail Input/Output Operational Amplifiers
PDF  24 Pages
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Manufacturer  AD [Analog Devices]
Direct Link  http://www.analog.com
Logo AD - Analog Devices

AD8571ARM-R2 Datasheet(HTML) 15 Page - Analog Devices

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AD8571/AD8572/AD8574
Rev. B | Page 15 of 24
VIN+
VIN–
VOUT
AB
AA
ΦA
ΦB
VOSA
+
VOSB
+
BB
CM2
CM1
ΦA
VNB
VNA
–BA
VOA
ΦB
Figure 50. Auto-Zero Phase of the Amplifier
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. The potential difference
between the two inputs to the primary amplifier is designated as
VIN, or VIN = (VIN+ − VIN–). The output of the nulling amplifier
can then be expressed as
[]
[]
[ ]
[ ]
t
V
B
t
V
t
V
A
t
V
NA
A
OSA
IN
A
OA
−
−
=
(
(3)
VIN+
VIN–
VOUT
AB
AA
ΦA
ΦB
VOSA
+
VOSB
+
BB
CM2
CM1
ΦA
VNB
VNA
–BA
VOA
ΦB
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 designated as 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)
and 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, it can be assumed that the
autocorrection frequency is much faster than any potential
change in VOSA or VOSB. This is a good assumption since 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 AD857x. This effectively
makes the VOS time invariant, and Equation 5 can be rewritten as
[]
[ ]
(
)
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)
Here, the auto-zeroing becomes apparent. Note that 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. Thus, the primary
amplifier output voltage is the voltage at the output of the
AD857x 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
OSA
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 yields
[]
[]
()
OSB
B
A
OSA
B
A
B
A
B
IN
OUT
V
A
B
V
B
A
B
A
A
t
V
t
V
+
+
+
+
=
1
(10)
The AD857x architecture is optimized in such a way that
AA = AB and BA = BB and BA >> 1. In addition, the gain product
to AABB is much greater than AB. Thus, Equation 10 can 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 AABA term is what gives the AD857x its
extremely high open-loop gain. To understand how VOSA and
VOSB relate to the overall effective input offset voltage of the
complete amplifier, set up 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)
Therefore
A
OSB
OSA
EFF
OS
B
V
V
V
+
≈
,
(14)



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