HCPL-7860 Datasheet, PDF (24/28 Pages) Agilent(Hewlett-Packard) – Isolated 15-bit A/D Converter

Over-Range Detection
The over-range detection circuit
allows fast detection of when the
magnitude of the input signal on
channel 1 is near or beyond full
scale, causing the OVR1 output to
go high. This circuit can be very
useful in current-sensing applica-
tions for quickly detecting when a
short-circuit occurs. The over-
range detection circuit works by
detecting when the modulator
output data has not changed state
for at least 25 clock cycles in a
row, indicating that the input
signal is near or beyond full-
scale, positive or negative.
Typical response time to over-
range signals is less than 3 µs.
The over-range circuit actually
begins to indicate an over-range
condition when the magnitude of
the input signal exceeds approxi-
mately 250 mV; it starts to
generate periodic short pulses on
OVR1 which get longer and more
frequent as the input signal
approaches full scale. The OVR1
output stays high continuously
when the input is beyond full
scale.
The over-range detection circuit
continuously monitors channel 1
independent of which channel is
selected with the CHAN signal.
This allows continuous monitor-
ing of channel 1 for faults while
converting an input signal on
channel 2.
Adjustable Threshold
Detection
The adjustable threshold detector
causes the THR1 output to go
high when the magnitude of the
input signal on channel 1 exceeds
a user-defined threshold level.
The threshold level can be set to
one of 16 different values
between approximately 160 mV
and 310 mV. The adjustable
threshold detector uses a smaller
version of the main conversion
circuit in combination with a
digital comparator to detect when
the magnitude of the input signal
on channel 1 is beyond the
defined threshold level. As with
the main conversion circuit, there
is a trade-off between speed and
resolution with the threshold
detector; selecting faster detec-
tion times exhibit more noise as
the signal passes through the
threshold, while slower detection
times offer lower noise. Both the
detection time and threshold level
are programmable using bits 2
through 7 of configuration
register 2, as shown in Tables 7
and 8 below.
As with the over-range detector,
the adjustable threshold detector
continuously monitors channel 1
independent of which channel is
selected with the CHAN signal.
This allows continuous monitor-
ing of channel 1 for faults while
converting Channel 2.
Table 7. Threshold
Detection Configuration.
Threshold
Detection
Time
2 - 6 µs
3 - 10 µs
5 - 20 µs
10 - 35 µs
Configuration
Data Bits
Bit 7 Bit 6
Low Low
Low High
High Low
High High
Note: Bold italic type indicates default
values.
Table 8. Threshold Level Configuration.
Threshold Level
± 160 mV
± 170 mV
± 180 mV
± 190 mV
± 200 mV
± 210 mV
± 220 mV
± 230 mV
± 240 mV
± 250 mV
± 260 mV
± 270 mV
± 280 mV
± 290 mV
± 300 mV
± 310 mV
Configuration Data Bits
Bit 5
Bit 4
Bit 3
Bit 2
Low
Low
Low
Low
Low
Low
Low
High
High
Low
High
High
Low
Low
High
High
Low
High
High
Low
Low
Low
High
High
Low
High
High
Low
Low
High
High
Low
High
Note: Bold italic type indicates default values.
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