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器件3D模型￥ 198.726
AD844SQ 开发手册 - ADI（亚德诺）
制造商:
ADI（亚德诺）
分类:
放大器、缓冲器
封装:
CDIP-8
描述:
60兆赫， 2000 V / us的单片运算放大器 60 MHz, 2000 V/us Monolithic Op Amp
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AD844SQ数据手册
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–6–
REV. 0
AN-581
OP AMP BIASING USING A LINEAR VOLTAGE REGULATOR
For op amp circuits operating from the new 3.3 V standard,
a 1.65 V biasing voltage is needed. Zener diodes are
commonly available only down to 2.4 V. The easiest way
to provide this biasing voltage is to use a linear voltage
regulator, such as the ADM663A or ADM666A devices.
This is shown in Figure 6.
0.1F1F
*
*
V
S
*STAR GROUND
C
OUT
V
OUT
R
LOAD
V
S
R2
C1
V
IN
V
S
/2
REF
R1
V
S
/2
SENSE
V
OUT
(2)
V
SET
ADM663A
ADM666A
GND
V
IN
1.3V TO 16V
ADJUSTABLE
OUTPUT
R
A
R
B
*
*
Figure 6. An Op Amp Single Supply Biasing Circuit Using
A Linear Voltage Regulator
Although a Zener diode is usually the cheapest voltage
regulator available, a linear voltage regulator has lower
drift over temperature than a Zener and far less noise.
Resistors R
A
and R
B
are selected to provide the desired
V
S
/2 voltage reference; consult AD663A datasheet.
DC-COUPLED BATTERY-POWERED CIRCUITS
So far, only ac-coupled op amp circuits have been
discussed. Although with the use of suitably large input
and output coupling capacitors, an ac-coupled circuit can
operate at frequencies well below 1 Hz, some applications
require a true dc response.
Battery-powered applications permit the use of a “phan-
tom ground” circuit as shown in Figure 7. This provides
dual supply voltages, both positive and negative with re-
spect to ground, from a single battery. An op amp is
used to buffer the output of a V
S
/2 voltage divider. If a
low voltage battery such as 3.3 V is used, the op amp
should be a “rail-to-rail” device and able to operate
effectively from this supply voltage. The op amp also
needs to be able to supply an output current large
enough to power the load circuit. Capacitor C2 bypasses
the voltage divider output enough to prevent any resistor
noise from feeding into the op amp. This capacitor does
not need to provide power supply rejection because the
load current flows directly to ground and so any signal
currents flow equally from both sides of the battery.
Resistors R
A
and R
B
are selected to provide the desired
V
S
/2 voltage reference; consult AD663A datasheet.
0.1F
1F
*
*
*STAR GROUND
V
S
/2
+V
S
–V
S
V
S
/2
*
*
R
A
220k
C2
0.1F
R
B
220k
+
110k
0.1
F
1F
*
0.1F
Figure 7. Using an Op Amp to Provide a “Phantom
Ground” for Battery-Powered DC-Coupled Applications
NOISE ISSUES
Some op amp applications need a low noise amplifier
and low noise amplifier circuits require low resistance
values in the signal path. Johnson (resistor) noise
equals 4 nV times the square root of the resistance
value in kΩ. While the Johnson noise of a 1 kΩ resistor
is only 4 nV/√Hz, this increases to 18 nV/√Hz for a 20 kΩ
resistor and 40 nV/√Hz for a 100 kΩ resistor. Even though
the R
A
/R
B
resistor divider is bypassed to ground with a
capacitor (C2), these resistors set a limit on the mini-
mum value that can be used for the op amp’s feedback
resistor and, the larger this is, the greater the Johnson
noise. So low noise applications need to use much
smaller op amp biasing resistor values than the 100 kΩ
specified here. However, lower value resistors in the
divider mean higher power supply current and reduced
battery life.
Fortunately, the Zener diode biasing method supplies
V
S
/2 without the need for large resistors. As long as the
Zener is bypassed to keep its noise out of the circuit,
both noise and supply current can be kept low. The use
of a linear voltage regulator is even better, as its noise
and output impedance are both very low.
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