ADA4841-1YRZ Analog Devices, ADA4841-1YRZ Datasheet - Page 13

• Manufacturer Part Number: ADA4841-1YRZ
• Manufacturer: Analog Devices

THEORY OF OPERATION
AMPLIFIER DESCRIPTION
The ADA4841-1 is a low power, low noise, precision voltage-
feedback op amp for single or dual voltage supply operation.
The ADA4841-1 is fabricated on ADI’s second generation
XFCB process and features a trimmed supply current and an
offset voltage. The 2 nV/√Hz voltage noise (very low for a
1.1 mA supply current amplifier), 60 μV offset voltage, and sub
1 μV/°C offset drift is accomplished with an input stage made of
an undegenerated PNP input pair driving a symmetrical folded
cascode. A rail-to-rail output stage provides the maximum
linear signal range possible on low voltage supplies and has the
current drive capability needed for the relatively low resistance
feedback networks required for low noise operation. CMRR,
PSRR, and open-loop gain are all well above 100 dB, preserving
the precision performance in a variety of configurations. Gain
bandwidth is kept high for this power level to preserve the
outstanding linearity performance to frequencies up to
100 kHz. The ADA4841-1 has a power-down function to
further reduce power consumption. All this results in a low
noise, power efficient, precision amplifier that is well suited
for high resolution and precision applications.
DC PERFORMANCE CONSIDERATIONS
Figure 37 shows a typical connection diagram and the major dc
error sources. The ideal transfer function (all error sources set
to 0 and infinite dc gain) can be written as
This reduces to the familiar forms for inverting and
noninverting op amp gain expressions:
(Noninverting gain, V
(Inverting gain, V
V
V
V
OUT
OUT
OUT
Figure 37. Typical Connection Diagram and DC Error Sources
– V
– V
IN
IP
=
= 1
= 1
+
+
⎛ −
⎜
⎜
⎝
⎛
⎜
⎜
⎝
⎛
⎜
⎜
⎝
R
R
R
G
S
+
+
R
G
F
R
R
R
R
IP
⎞
× ⎟ ⎟
⎠
F
G
G
F
= 0 V)
⎞
× ⎟ ⎟
⎠
⎞
× ⎟ ⎟
⎠
V
IN
IN
I
I
B
V
B
V
+
–
= 0 V)
IP
IP
+ V
−
OS
⎛
⎜
⎜
⎝
R
R
R
F
–
G
F
⎞
× ⎟ ⎟
⎠
V
IN
+ V
OUT
–
Rev. A | Page 13 of 20
(1)
(2)
(3)
The total output voltage error is the sum of errors due to the
amplifier’s offset voltage and input currents. The output error
due to the offset voltage can be estimated as
where:
This is measured with the input and output at midsupply.
VCM is the common-mode voltage.
V
CMRR is the common-mode rejection ratio.
PSRR is the power supply rejection ratio.
A is the dc open-loop gain.
The output error due to the input currents can be estimated as
Note that setting R
error due to the input bias current.
NOISE CONSIDERATIONS
Figure 38 illustrates the primary noise contributors for the
typical gain configurations. The total rms output noise is
the root-mean-square of all the contributions.
vn _ R
V
V
V
V
vn _ R
⎛
⎜
⎝
V
P
OUT
OFFSET
OUT
p
NOM
OFFSET
is the power supply voltage.
ERROR
ERROR
G
S
=
=
is the specified power supply voltage.
NOM
NOM
4kT × R
4kT × R
=
=
is the offset voltage at the specified supply voltage.
(
+
R
Figure 38. Noise Sources in Typical Connection
S
CMRR
F
G
VCM
||
R
S
G
equal to R
)
R
R
×
+
G
S
⎛
⎜
⎜
⎝
V
1
P
+
PSRR
−
R
R
V
ien
ien
G
F
F
PNOM
⎞
⎟
⎟
⎠
||R
I
B
ven
G
−
R
compensates for the voltage
−
+
F
R
V
S
OUT
A
×
⎛
⎜
⎜
⎝
1
⎞
× ⎟
⎠
vn _ R
+
⎛
⎜
⎜
⎝
ADA4841-1
R
R
1
G
F
F
+
=
⎞
× ⎟ ⎟
⎠
R
R
+ vout_en –
4kT × R
G
F
I
B
⎞
⎟
⎟
⎠
+
F
(4)
(5)