ADA4084-2ARMZ AD [Analog Devices], ADA4084-2ARMZ Datasheet - Page 22

• Manufacturer Part Number: ADA4084-2ARMZ
• Description: 30 V, Low Noise, Rail-to-Rail
• Manufacturer: AD [Analog Devices]

ADA4084-2
As a design aid, Figure 77 shows the total equivalent input noise
of the
comparison. Note that for source resistance less than 1 kΩ, the
equivalent input noise voltage of the
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
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
Figure 77.
ADA4084-2
100
10
Tutorial, Op Amp Noise Figure: Don’t be Mislead,
1
ADA4084-2
100
FREQUENCY = 1kHz
T
A
= 25°C
and the total thermal noise of a resistor for
Equivalent Thermal Noise vs. Total Source Resistance
TOTAL SOURCE RESISTANCE, R
EQUIVALENT NOISE
ADA4084-2 TOTAL
1k
RESISTOR THERMAL
NOISE ONLY
ADA4084-2
10k
S
(Ω)
is dominant.
100k
Rev. A | Page 22 of 24
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 76). An unused section should be configured as a voltage
follower with the noninverting input connected to a voltage
within the input voltage range. The
second stage and output stage designs that greatly reduce the
excess supply current when the op amp is operating open loop.
800
700
600
500
400
300
200
100
0
0
Figure 78. Supply Current vs. Supply Voltage
COMPARATOR
OUTPUT LOW
4
8
COMPARATOR
OUTPUT HIGH
12
16
V
SY
(V)
20
ADA4084-2
24
BUFFER
28
Data Sheet
has unique
ADA4084-2
T
R
A
L
= 25°C
= ∞
32
36