Sensoray 417 - Manual (Page 35)

Instruction Manual 36
Hardware
Although the coprocessor’s analog circuitry is proprietary to Sensoray, an overview of the cir-
cuitry is provided here to provide some insight into the inner-workings of the board. Analog
circuitry is logically partitioned into three sections: reference, conditioning and measurement.
Reference Standards
The reference section consists of a set of standards that are used to enhance the stability and
accuracy of all sensor measurements. These standards are very stable over time and tempera-
ture. The exact values of the standards are stored in non-volatile memory when the board is
calibrated.
Reference standards are measured periodically by the onboard CPU. To make standards mea-
surements appear as transparent as possible, these measurements are interleaved with external
sensor measurements. The values of the digitized standards are stored in onboard RAM and
used to null offset and gain errors in the external sensor measurements.
A phenomenon known as warm-up noise occurs when the 417 is warming up from cold start or
when subjected to a thermal transient. When the coprocessor is exposed to changing ambient
temperatures, sensor data will appear somewhat noisy. This is caused by circuit gain and offset
characteristics changing faster than reference standards are measured. Warm-up noise subsides
when the 417 circuit board reaches thermal stability.
Signal Conditioning
Each channel is provided with a signal conditioning circuit. This circuit serves to (1) protect input
circuitry from high common-mode voltages, and (2) detect thermocouple open-circuit failures, and
(3) condition millivolt-level thermocouple signals for measurement.
Measurement Section
The measurement section is responsible for selecting and digitizing the external sensor input sig-
nals, as well as the internal reference standards.
The measurement section starts a digitization by selecting a channel to be measured. The
selected channel is switched through a differential analog multiplexer. Reference standards are
routed through dedicated switches so as to not rob inputs from external sensors.
Next, the selected channel is passed through a programmable gain amplifier. Gain level is con-
trolled by the coprocessor’s onboard CPU. The onboard CPU sets amplifier gain based on the
declared sensor type in the case of external sensor channels, or based on 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. The con-
verter circuit, based on a voltage-to-frequency converter, directly produces a floating-point
output value. Converter resolution increases with decreasing signal amplitude so that the best
resolution is provided for the lowest-level signals. Note: the specified converter resolution is
the worst-case resolution across the entire input range.