AD8551/AD8552/AD8554
V
V
IN+
AA
1+ BA
V
1+ BA
OSA − AABAVOSA
A
V
OUT
VOA
[
t
]
]
= AAVIN
[
t
]
+
(6)
B
IN–
B
B
ФB
V
OA
C
M2
or
V
ФB
OSA
+
ФA
A
⎛
⎞
⎟
⎟
⎠
VOSA
1+ BA
A
V
NB
⎜
VOA
[t
= AA VIN
[
t
]
+
(7)
⎜
–B
⎝
A
ФA
C
M1
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
V
NA
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
VOUT
[t
]
= AB
VIN
[t
]
+VOSB
+ BBVNB
(8)
In the amplification phase, VOA = VNB, so this can be rewritten as
⎡
⎢
⎤
⎥
⎛
⎜
⎜
⎝
⎞
⎟
⎟
⎠
VOSB
1 + BA
VOUT
[t
]
= ABVIN
[t
]
+ ABVOSB + BB
A
VIN
[t
]
+
(9)
A
⎢
⎣
⎥
⎦
VOA[t] = AA
VIN
[t
]
−VOSA
[t
]
− BAVNA
[t
]
(3)
Combining terms,
VOUT = VIN
V
IN+
A
V
OUT
B
AABAVOSA
1+ BA
V
IN–
[t
]
[t
]
(
AB + ABBB
)
+
+ ABVOSA
(10)
B
B
ФB
V
OA
C
M2
V
ФB
ФA
OSA
+
The AD855x architecture is optimized in such a way that
ФA
A
A
V
NB
AA = AB and BA = BB and BA >> 1
–B
A
C
M1
Also, the gain product of AABB is much greater than AB. These
allow Equation 10 to be simplified to
V
NA
VOUT
[t
]
≈ VIN
[t
]AABA + AA
VOSA +VOSB
(11)
Figure 51. Output Phase of the Amplifier
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
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
V
OSB relate to the overall effective input offset voltage of the
complete amplifier, establish the generic amplifier equation of
VOUT = k × VIN + VOS,EFF (12)
1
2
⎡
⎣
⎤
VNA
[t
]
= VNA t − tS
(4)
where k is the open-loop gain of an amplifier and VOS, EFF is its
effective offset voltage.
⎢
⎥
⎦
Substituting Equation 4 and Equation 2 into Equation 3 yields
Putting Equation 12 into the form of Equation 11 gives
1
2
⎡
⎣
⎤
VOUT
[t
]
≈ VIN
[t
]AABA +VOS,EFF AABA
(13)
(14)
AABAVOSA t − tS
⎢
⎥
⎦
VOA
[t
]
= AAVIN
[t
]
+ AAVOSA
[t
]
−
(5)
1 + BA
Thus, it is evident that
OSA +VOSB
V
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
VOS,EFF
≈
BA
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
Rev. C | Page 15 of 24
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