ADA4084-2ARMZ AD [Analog Devices], ADA4084-2ARMZ Datasheet - Page 20

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

ADA4084-2
APPLICATIONS INFORMATION
FUNCTIONAL DESCRIPTION
The
tional amplifier. Intended for portable instrumentation, the
ADA4084-2
width, and low noise to make it an ideal choice in single-supply
applications that require both ac and precision dc performance.
Other low supply voltage applications for which the
is well suited are active filters, audio microphone preamplifiers,
power supply control, and telecommunications. To combine all
of these attributes with rail-to-rail input/output operation, novel
circuit design techniques are used.
For example, Figure 73 illustrates a simplified equivalent circuit
for the input stage of the ADA4084-2. It comprises a PNP
differential pair, Q1 and Q2, and an NPN differential pair, Q3
and Q4, operating concurrently. Diode D100 and Diode D101
serve to clamp the applied differential input voltage to the
ADA4084-2, thereby protecting the input transistors against
Zener breakdown of the emitter-base junctions. Input stage
voltage gains are kept low for input rail-to-rail operation. The
two pairs of differential output voltages are connected to the
second stage of the ADA4084-2, which is a modified compound
folded cascade gain stage. It is also in the second gain stage,
where the two pairs of differential output voltages are combined
into a single-ended output signal voltage used to drive the
output stage.
A key issue in the input stage is the behavior of the input bias
currents over the input common-mode voltage range. Input bias
currents in the
currents in Q1 and Q4 and in Q2 and Q3. As a result of this
design approach, the input bias currents in the
only exhibit different amplitudes; they also exhibit different
polarities. This effect is best illustrated by Figure 7, Figure 8,
Figure 30, Figure 31, Figure 53, and Figure 54. It is therefore
ADA4084-2
combines the attributes of precision, wide band-
Figure 73.
D2
D5
ADA4084-2
is a precision single-supply, rail-to-rail opera-
R4
Q4
ADA4084-2
R1
Q1
D100
D101
are the arithmetic sum of the base
R2
Equivalent Input Circuit
Q2
R3
Q3
D1
D4
ADA4084-2
ADA4084-2
not
Rev. A | Page 20 of 24
important that the effective source impedances connected to
the
performance.
To achieve rail-to-rail output, the
design employs a unique topology for both sourcing and sinking
current. This circuit topology is illustrated in Figure 74. The
output stage is voltage-driven from the second gain stage. The
signal path through the output stage is inverting; that is, for
positive input signals, Q13 provides the base current drive to Q19
so that it conducts (sinks) current. For negative input signals, the
signal path via Q18 → mirror → Q24 provides the base current
drive for Q23 to conduct (source) current. Both transistors
provide output current until they are forced into saturation.
Thus, the saturation voltage of the output transistors sets the
limit on the
short-circuit current limiting is determined by the maximum
signal current into the base of Q13 from the second gain stage.
The output stage also exhibits voltage gain. This is accomplished
by the use of common-emitter amplifiers, and, as a result, the
voltage gain of the output stage (thus, the open-loop gain of the
device) exhibits a dependence on the total load resistance at the
output of the ADA4084-2.
ADA4084-2
R5
ADA4084-2
Figure 74. ADA4084-2 Equivalent Output Circuit
Q13
inputs be balanced for optimum dc and ac
C1
V
BIAS
Q21
maximum output voltage swing. Output
MIRROR
Q18
R6
D20
R7 C2
Q24
ADA4084-2
Q23
Q19
output stage
Data Sheet
V
V
V
OUT
CC
EE