■ FIGURE 3
easier to just run through all the tests.
In the case where the input to
the line is shorted, there will be no
signal and the comparator will not
respond. I will have to use some logic
to determine if that happened.
The comparator has an enable
line which has this characteristic: If
the enable line is pulled high when
the signal is present, the output will
be held as long as the enable is high.
If the enable line is pulled high
before (or at the same time) as the
signal, the signal will not be detected
as long as the enable line is high. I
use this feature to prevent the
comparator from seeing the initial
pulse when looking for the reflected
pulse.
The controller, IC2, initiates the
pulse by pulling the trigger of IC1
low. The output of IC3 is fed back to
Q1 to cut off the pulse so it is only
as long as the response time of IC3;
about 10 nS. All four lines are driven
at the same time. Figure 2 is only
complete for one line. There will be
three more of IC4 and 12 more SSRs
and associated circuitry. All the
comparators can share the bias and
enable lines but it will not be
possible to series all the SSRs to
make them switch at the same time;
more outputs from the controller will
have to be used. The controller will
set up the SSRs for the test. Initiate
the pulse and wait for any anticipated
reflection; then, set up the next test.
There are four tests:
1. Check for an open at the
source. Set the bias at 3V,
24 February 2011
hold the enable line low (RB0
high), and feed the
comparator signal to latch W.
2. Check for a short at the
source. Set the bias at 1.0V,
hold the enable line low, and
feed the signal to latch X. If
the line is not shorted, latch X
will be set.
3. Check for an open at the load.
Set the bias at 0.5V, pulse the
enable line (RB0 low); and
feed the signal to latch Y. If
latch Y is set, there is an open
at the load. If latch Y is not
set, then either the line is
good or there is a short at the
input. If latch X is set and latch
Y is not, then the line is good.
4. Check for a short at the load.
Set the bias at a low negative
voltage and feed the inverted
(notQ) signal to latch Z. If
latch Z is set, there is a short
at the load and the line is not
good.
ANALOG ISOLATION
TECHNIQUES
QI am trying to make an ECG circuit. I am using a two p-amp instrumentation amplifier topology to
amplify the small differential voltage
from the body (p. 98, Linear Circuit
Design Handbook, H.Zumbahlen,
Newnes 2008). For this, I am using
the dual IC op-amp OP297. I have a
gain of 400. I am
using a three-port
isolator, the
AD210 (p.110,
same book), to
isolate the patient
from accidental
electric shock. My
problem is this:
How do I connect
the output signal
from the two op-amps in — amp to
the AD210? I am
a student of
biomedical
engineering, and I
AYour instrumentation amp has one output which should be connected to pin 19 of the AD210 as shown
in Figure 3 of the datasheet. If you
want unity gain, then Rf = 0 and
Rg = open. Figure 3 is the complete
schematic, although your preamp
may differ.
400 HZ GYRO DRIVE
QJust received my Nuts & Volts magazine. In regards to the question (High Voltages using Sine Waves)
on page 25 of the October issue, can
this idea be modified to put out say
115 VAC about one amp? I have
seen inverters for camping using a
car or battery to produce near sine
waves at 60 Hz. Can this be hacked
to put out 400 Hz to drive a small
aircraft gyro? I need at least 100
watts. I can convert the frequency of
your circuit to 400 Hz, but what is
the turns ratio of the output
transformer at 100 watts minimum?
— Craig Kendrick Sellen
AThe modification would be to change the oscillator frequency to 400 Hz and use a transformer rated
