hcpl-7851-300 Avago Technologies, hcpl-7851-300 Datasheet - Page 15

• Manufacturer Part Number: hcpl-7851-300
• Description: Hermetically Sealed Analog Isolation Amplifier
• Manufacturer: Avago Technologies

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
Avago 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.
15
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 Avago
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
Avago 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
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
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.
so that the input
series