Sensoray 518 - Manual (Page 53)

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standards are switched through dedicated analog switches so as to not rob input channels from
the external sensor channels.
Next, the selected channel is passed through a programmable gain amplifier. Amplifier gain is
under control of the coprocessor’s onboard CPU. The onboard CPU sets amplifier gain level
based on the declared sensor type in the case of external sensor channels, or the reference type in
the case of internal reference standard channels.
Finally, the amplified signal is applied to the input of an integrating A/D converter.
Channel selection, amplifier gain, and A/D functions are all under control of the coprocessor’s
CPU. None of there circuits are directly accessible to the host.
Excitation Section
The excitation section supplies one of three pulsed DC signals to resistive sensors, depending on
the declared sensor type. The pulse duration is the sum of the sensor settling time plus the A/D
conversion time -- approximately 22 milliseconds total, or 13 milliseconds in the High Speed
Mode.”
In the case of strain and pressure gages, a fixed 10 volt excitation is applied. This voltage is
current limited so that shorting the excitation signals together or to ground will not damage
coprocessor circuitry.
In the case of RTD’s and the 400 ohm resistance range, a fixed current of approximately 1.2
milliamps is forced through the sensor.
Finally, a fixed 5 volts is applied in series with a reference resistor to sensors measured on all
other resistance ranges.
Timing
Three timing parameters are important from a system integration viewpoint: channel scan rate,
communication latency, and host processor speed. These timing elements are considered below.
Scan Rate
Channel scan rate is defined as the number of conversions per channel per second. The scan rate
is influenced primarily by the number of active channels in the scan loop. A secondary influence
is the frequency of communication between host and 518 coprocessor.
The basic channel processing time -- the scan time -- is approximately 22 milliseconds at the
default scan rate, or 13 milliseconds at the High Speed Mode scan rate. This is the amount of
time required for the 518 to configure its analog section, allow the front end to settle and digitize
the input signal. Note that the software linearization and filtering functions don’t require any
additional time as they execute concurrently with digitization.