Datasheet

Data Sheet ADA4084-1/ADA4084-2/ADA4084-4
Rev. I | Page 31 of 36
DESIGNING LOW NOISE CIRCUITS IN SINGLE-
SUPPLY APPLICATIONS
In single-supply applications, devices like the ADA4084-1/
ADA4084-2/ADA4084-4 extend the dynamic range of the
application through the use of rail-to-rail operation. Referring to
the op amp noise model circuit configuration illustrated in
Figure 109, the expression for the total equivalent input noise
voltage of an amplifier for a source resistance level, R
S
, is given by
[
]
22
2
)()()(2
nOA
SnOA
nR
nT
eee
Ri
+×+=
, units in
Hz
V
w
here:
(e
nR
)
2
is the source resistance thermal noise voltage power (4kTR).
k is the Boltzmann’s constant, 1.38 × 10
–23
J/K.
T is the ambient temperature in Kelvin of the circuit, 273.15 +
T
A
(°C).
(i
nOA
)
2
is the op amp equivalent input noise current spectral
power (1 Hz bandwidth).
R
S
= 2R, the effective, or equivalent, circuit source resistance.
(e
nOA
)
2
is the op amp equivalent input noise voltage spectral
power (1 Hz bandwidth).
e
nR
e
nR
e
nOA
i
nOA
i
nOA
R
NOISELESS
R
NOISELESS
08237-076
IDEAL
NOISELESS
OP AMP
R
S
= 2R
F
igure 109. Op Amp Noise Circuit Model Used to Determine Total Circuit
Equivalent Input Noise Voltage and Noise Figure
As a design aid, Figure 110 shows the equivalent thermal noise
of the ADA4084-1/ADA4084-2/ADA4084-4 vs. the total source
resistance. Note that for source resistance less than 1 kΩ, the
equivalent input noise voltage of the ADA4084-1/ADA4084-2/
ADA4084-4 is dominant.
08237-077
TOTAL SOURCE RESISTANCE, R
S
(Ω)
100
1
EQUIVALENT THERMAL NOISE (nV/ Hz)
10
10k
ADA4084-1/ADA4084-2/ADA4084-4
TOTAL EQUIVALENT NOISE
RESISTOR THERMAL
NOISE ONLY
100 1k 100k
FREQUENCY = 1kHz
T
A
= 25°C
F
igure 110. Equivalent Thermal Noise vs. Total Source Resistance
Because circuit SNR is the critical parameter in the final analysis,
the noise behavior of a circuit is sometimes expressed in terms
of its noise figure (NF). The noise figure is defined as the ratio
of the signal-to-noise output of a circuit to its signal-to-noise input.
Noise figure is generally used for RF and microwave circuit analysis
in a 50 Ω system. This is not very useful for op amp circuits where
the input and output impedances can vary greatly. For a more
complete description of noise figure, see the MT-052 Tutorial,
Op Amp Noise Figure: Don’t be Misled.
Signal levels in the application invariably increase to maximize
circuit SNR, which is not an option in low voltage, single-supply
applications.
Therefore, to achieve optimum circuit SNR in single-supply
applications, choose an operational amplifier with the lowest
equivalent input noise voltage, along with source resistance
levels that are consistent with maintaining low total circuit noise.
COMPARATOR OPERATION
Although op amps are quite different from comparators,
occasionally an unused section of a dual or a quad op amp can
be used as a comparator; however, this is not recommended for
any rail-to-rail output op amps. For rail-to-rail output op amps,
the output stage is generally a ratioed current mirror with bipolar
or MOSFET transistors. With the device operating open-loop,
the second stage increases the current drive to the ratioed mirror
to close the loop. However, the loop cannot close, which results in
an increase in supply current. With the op amp configured as a
comparator, the supply current can be significantly higher (see
Figure 111). Configure an unused section as a voltage follower
with the noninverting input connected to a voltage within the
input voltage range. The ADA4084-1/ADA4084-2/ADA4084-4
have unique second stage and output stage designs that greatly
reduce the excess supply current when the op amp is operating
open-loop.
800
0
0 36
SUPPLY CURRENT (µA)
V
SY
(V)
08237-078
100
200
300
400
500
600
700
4 8 12 16 20 24 28 32
T
A
= 25°C
R
L
= ∞
COMPARATOR
OUTPUT LOW
COMPARATOR
OUTPUT HIGH
BUFFER
F
igure 111. Supply Current vs. Supply Voltage (V
SY
)
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