conflicting considerations,
therefore, must be weighed
against each other in selecting an
appropriate sense resistor for a
particular application. To
maintain circuit accuracy, it is
recommended that the sense
resistor and the isolation amplifier
circuit be located as close as
possible to one another. Although
it is possible to buy current-
sensing resistors from established
vendors (e.g., the LVR-1, -3 and
-5 resistors from Dale), it is also
possible to make a sense resistor
using a short piece of wire or
even a trace on a PC board.
Figures 30 and 31 illustrate the
response of the overall isolation
amplifier circuit shown in Figure
26. Figure 30 shows the response
of the circuit to a ± 200 mV 20
kHz sine wave input and Figure
31 the response of the circuit to a
± 200 mV 20 kHz square wave
input. Both figures demonstrate
the fast, well-behaved response of
the HCPL-7800.
the time scale is different from
the previous figures). The first
wave form is the output of the
application circuit with the filter
capacitors removed to show the
actual response of the isolation
amplifier. The second wave form
is the response of the same circuit
with the capacitors installed. The
recovery time and overshoot are
relatively independent of the
amplitude and polarity of the
overdrive signal, as well as its
duration.
Figure 32 shows how quickly the
isolation amplifier recovers from
an overdrive condition generated
by a 2 kHz square wave swinging
between 0 and 500 mV (note that
For more information, refer to
Application Note 1059.
VOLTAGE
CLOCK
VOLTAGE
REGULATOR
GENERATOR
REGULATOR
ISOLATION
BOUNDARY
Σ∆
LED DRIVE
CIRCUIT
DETECTOR
CIRCUIT
DECODER
AND D/A
ISO-AMP
OUTPUT
ISO-AMP
INPUT
ENCODER
FILTER
MODULATOR
Figure 28. HCPL-7800 Block Diagram.
Figure 29. PC Board Trace Pattern and Loading Diagram Example.
1-231
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
