ADA4084-2_13 Datasheet, PDF (24/28 Pages) Analog Devices – 30 V, Low Noise, Rail-to-Rail I/O, Low Power Operational Amplifier

ADA4084-2
DESIGNING LOW NOISE CIRCUITS IN SINGLE-
SUPPLY APPLICATIONS
In single-supply applications, devices like the ADA4084-2 extend
the dynamic range of the application through the use of rail-to-rail
operation. Referring to the op amp noise model circuit configu-
ration illustrated in Figure 82, the expression for an amplifier’s
total equivalent input noise voltage for a source resistance level,
RS, is given by
  e_nT  2 (e_nR)²  (_inOA _RS)² (e_nOA)² , units in
V
Hz
where:
RS = 2R, the effective, or equivalent, circuit source resistance.
(enR)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 +
TA (°C).
(inOA)² is the op amp equivalent input noise current spectral
power (1 Hz bandwidth).
(enOA)² is the op amp equivalent input noise voltage spectral
power (1 Hz bandwidth).
R
e_nR
e_nOA
NOISELESS
i_nOA
R
e_nR
NOISELESS
i_nOA
IDEAL
NOISELESS
OP AMP
R_S = 2R
Figure 82. Op Amp Noise Circuit Model Used to Determine Total Circuit
Equivalent Input Noise Voltage and Noise Figure
As a design aid, Figure 83 shows the total equivalent input noise
of the ADA4084-2 and the total thermal noise of a resistor for
comparison. Note that for source resistance less than 1 kΩ, the
equivalent input noise voltage of the ADA4084-2 is dominant.
100
FREQUENCY = 1kHz
T_A = 25°C
ADA4084-2 TOTAL
EQUIVALENT NOISE
10
RESISTOR THERMAL
NOISE ONLY
1
100
1k
10k
100k
TOTAL SOURCE RESISTANCE, R_S (Ω)
Figure 83. ADA4084-2 Equivalent Thermal Noise vs. Total Source Resistance
Data Sheet
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
a circuit’s output signal-to-noise to its input signal-to-noise.
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 Mislead, available at www.analog.com.
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, it is recommended that an operational amplifier
with the lowest equivalent input noise voltage be chosen, 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 part operating open
loop, the second stage increases the current drive to the ratioed
mirror to close the loop. However, it cannot, 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 84). An unused section should be configured as a voltage
follower with the noninverting input connected to a voltage within
the input voltage range. The ADA4084-2 has unique second
stage and output stage designs that greatly reduce the excess
supply current when the op amp is operating open loop.
800
COMPARATOR
700
OUTPUT LOW
600
BUFFER
500
COMPARATOR
OUTPUT HIGH
400
300
200
100
0
0
ADA4084-2
T_A = 25°C
R_L = ∞
4
8
12 16 20 24 28 32 36
V_SY (V)
Figure 84. Supply Current vs. Supply Voltage
Rev. C | Page 24 of 28