参数资料
型号: 5962-9755701HPA
厂商: AGILENT TECHNOLOGIES INC
元件分类: 隔离放大器
英文描述: ISOLATION AMPLIFIER, 100 kHz BAND WIDTH, CDIP8
封装: HERMETIC SEALED, DIP-8
文件页数: 7/16页
文件大小: 251K
代理商: 5962-9755701HPA
15
effective, the damping resistor
should be at least 39
.
PC Board Layout
In addition to affecting offset, the
layout of the PC board can also
affect the common mode
rejection (CMR) performance of
the isolation amplifier, due
primarily to stray capacitive
coupling between the input and
the output circuits. To obtain
optimal CMR performance, the
layout of the printed circuit board
(PCB) should minimize any stray
coupling by maintaining the
maximum possible distance
between the input and output
sides of the circuit and ensuring
that any ground plane on the PCB
does not pass directly below the
HCPL-7850. Using surface mount
components can help achieve
many of the PCB objectives
discussed in the preceding
paragraphs. An example through-
hole PCB layout illustrating some
of the more important layout
recommendations is shown in
Figures 26 and 27. See
Applications Note 1078,
Designing with Agilent
Technologies Isolation
Amplifiers
, for more information
on PCB layout consideration.
Post-Amplifier Circuit
The recommended application
circuit (Figure 24) includes a
post-amplifier circuit that serves
three functions: to reference the
output signal to the desired level
(usually ground), to amplify the
signal to appropriate levels, and
to help filter output noise. The
particular op-amp used in the
post-amp is not critical; however,
it should have low enough offset
and high enough bandwidth and
slew rate so that it does not
adversely affect circuit
performance. The offset of the
op-amp should be low relative to
the output offset of the HCPL-
7850, or less than about 5 mV.
To maintain overall circuit
bandwidth, the post-amplifier
circuit should have a bandwidth
at least twice the minimum
bandwidth of the isolation
amplifier, or about 200 kHz. To
obtain a bandwidth of 200 kHz
with a gain of 5, the op-amp
should have a gain-bandwidth
greater than 1 mHz. The post-
amplifier circuit includes a pair of
capacitors (C5 and C6) that form
a single-pole low-pass filter.
These capacitors allow the
bandwidth of the post-amp to be
adjusted independently of the
gain and are useful for reducing
the output noise from the
isolation amplifier (doubling the
capacitor values halves the circuit
bandwidth). The component
values shown in Figure 24 form a
differential amplifier with a gain
of 5 and a cutoff frequency of
approximately 100 kHz, and were
chosen as a compromise between
low noise and fast response
times. The overall recommended
application circuit has a
bandwidth of 66 kHz, a rise time
of 5.2
s and a delay to 90% of
8.5
s.
The gain-setting resistors in the
post-amp should have a tolerance
of 1% or better to ensure
adequate CMRR and gain
tolerance for the overall circuit.
Resistor networks with even
better ratio tolerances can be
used which offer better
performance, as well as reducing
the total component count and
board space.
The post-amplifier circuit can be
easily modified to allow for
single-supply operation. Figure
25 shows a schematic for a post
amplifier for use in 5 V single
supply applications. One
additional resistor is needed and
the gain is decreased to 1 to
allow circuit operation over the
full input voltage range. See
Application Note 1078,
Designing with Agilent
Technologies Isolation
Amplifiers
, for more information
on the post-amplifier circuit.
Other Information
As mentioned above, reducing the
bandwidth of the post amplifier
circuit reduces the amount of
output noise. Figure 22 shows
how the output noise changes as
a function of the post-amplifier
bandwidth. The post-amplifier
circuit exhibits a first-order low-
pass filter characteristic. For the
same filter bandwidth, a higher-
order filter can achieve even
better attenuation of modulation
noise due to the second-order
noise shaping of the sigma-delta
modulator. For more information
on the noise characteristics of the
HCPL-7850, see Application Note
1078, Designing with Agilent
Technologies Isolation
Amplifiers
.
The HCPL-7850 can also be used
to isolate signals with amplitudes
larger than its recommended
input range through the use of a
resistive voltage divider at its
input. The only restrictions are
that the impedance of the divider
be relatively small (less than 1 K
so that the input resistance (480
K
) and input bias current (0.6
A) do not affect the accuracy of
the measurement. An input
bypass capacitor is still required,
although the 68
series damping
resistor is not. (The resistance of
the voltage divider provides the
same function.) The low pass
filter formed by the divider
resistance and the input bypass
capacitor may limit the
achievable bandwidth.
相关PDF资料
PDF描述
5962-9755701HPC ISOLATION AMPLIFIER, 100 kHz BAND WIDTH, CDIP8
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