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V7779
Span and zero
To set up the span and zero controls, examine the output that will finally
be required , i.e. the voltage or current output, to avoid errors in the
current drivers. If the (A±B)/2 facility is being used, then it is possible to
calibrate on (for instance) the A + B output, again for maximum
accuracy.
Note: It is necessary in this case to zero the unwanted transducer, or
remove the secondary and short the demodulator input so that
one transducer is examined at a time.
With transducers such as load cells that have an obvious point (i.e. no
load) then it is merely necessary to set the card span and zero as
described below. However, for LVDTs and half bridges it is necessary
to find the mechanical zero (i.e. centre of linear stroke) first. To
accomplish this:
a) Remove transducer connections from card input to demodulator.
b) Short the demodulator input, to simulate a transducer at centre of
stroke .
c) Read output from card.
d) Remove the short and reconnect transducer.
e) Adjust transducer to give same output reading as at step (c). The
transducer is now set to the middle of its linear stroke.
To set the card span and zero it is necessary to set some links and then
use the fine span and zero controls for final adjustment. There are nine
coarse span ranges, in two overlapping ranges of six each:
Range Transducer Sensitivity
Minimum
Maximum
1
250mV/V
750mV/V
2
100
3
50
4
25
5
10
6
5
4'
25
5'
10
6'
5
7
2.5
8
1
9
0.5
Selecting the x10 link increases the gain of the amplifier and so
reduces the necessary sensitivity of the transducer. The span control is
used to set the span in the range between minimum and maximum.
The above sensitivity ranges are for a standard ±5V or ±10mA output
(10V or 20mA total range).
If a different output range is required (say v volts) then the necessary
transducer sensitivities shown should be multiplied by v/10. For
example, if an output of ±3V is required (total range 6V) then range 1
becomes 250 x 0.6 to 750 x 0.6 which is 150 to 450mV/V.
Eight coarse zero ranges are provided and selected by linking up to
three off pins pairs Z1 to Z3. Again a fine control is used to set the zero
anywhere required. On minimum gain, the amount of zero offset
provided by the link is:
Z1
-1.5V
Z2
-3V
Z3
5.5V
Potentiometer
-2V to 0V
4
Select pin pair
250 - 750
300
100 - 300
150
50 - 150
75
25 - 75
30
10 - 30
15
5 - 15
75
250 - 750
30
100 - 300
15
50 - 150
7.5
25 - 75
3
10 - 30
1.5
5 - 15
=
-30% of 5V
=
-60% of 5V
=
110% of 5V
=
-40% to 0% of 5V
This means that on minimum gain, the ranges provided are:
Figure 12
As the gain is increased this opens out, so that at maximum gain it
becomes three times wider:
Figure 13
Note: Normal mode of operation is with all three links on, to provide
fine adjust about zero.
First select the appropriate coarse gain range by reference to the
Select x10
transducer data and the required output voltage. Set the transducer to
Link
the point at which zero links and controls for 0V output. Set transducer
No
to position at which full output is required and adjust span control for full
No
scale output. Re-check zero.
If an offset zero is required such as on a 4-20mA system) then set the
No
transducer to the minimum position and adjust zero controls for 4mA.
No
Set transducer to maximum position and adjust span for 20mA. Re-
No
check 4 and 20mA positions until fully set up, as, because the zero
No
position is offset, it is affected by the span controls.
Yes
Circuit operation
Yes
Most of the operating circuitry on the card is contained in the oscillator
Yes
and demodulator hybrids. A block diagram of the card is given below.
Yes
Power supply protection is provided to protect against reversed power
Yes
rails, the circuitry is also tolerant of the disconnection of one supply. A
pair of zener diodes offer basic protection against voltage spikes on the
Yes
supply rails.
The oscillator hybrid drives the transducer, at one of two frequencies,
and the signal from the transducer is fed into the demodulator hybrid.
This takes care of all the span and zero corrections.
The d.c. output from the demodulator is fed into another filter to reduce
output ripple even further and hence to the output.
The (A±B)/2 circuit is a set of simple amplifiers based on precision
resistors and provides all four outputs simultaneously. The current
drivers are based on those in the C30 range and are wired separately
so that they can be wired to any chosen output.
-5V -4V -3V -2V -1V 0V 1V 2V 3V 4V 5V
Z2
None
Z3 & Z2
Z1 & Z2
Z1
Z3, Z2, Z1 Z3, &, Z1
-5V -4V -3V -2V -1V 0V 1V 2V 3V 4V 5V
Z3, Z2 and Z1
Z1
None
Z3
Z3 and Z2
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