SIGNAL CONDITIONING, DATA ACQUISITION, AND SPECTRAL ANALYSIS - Design and Development of Medical Electronic Instrumentation - page 206

TABLE 5.1 Some of the Sensors with Onboard Electronics Widely Available to Measure a Diversity of Physical Variables

Sensor Manufacturer Representative Part Excitation Range Output

Temperature Analog Devices AD590 4 to 30 V dc 55 to 150°C 1 mA/ ° K

Peripheral pressure UFI 1010 None (piezoelectric) Typical human pulse 20-40 mV

pulse pressure from fi nger into 1 M Ω

Respiration UFI Pneumotrace II None (piezoelectric) Typical human chest 20-100 mV

expansion into 1 M Ω

Acceleration Analog Devices ADXL05 5 V dc Selectable 1 g Selectable 200 mV/g

or 5 g or 1 V/g

Fluid pressure MSI MPS3102 10 to 32 V dc 0 to 1000 psi 4 mV/psi

Barometric pressure Sensortechnics 144SC0811-BARO 7 to 24 V dc 800 to 1100 mbar 16 mV/mbar

Magnetic fi eld Honeywell HMC2003 6 to 15 V dc 2 G 1 V/G

Gas concentration Capteur Various with driver 5 V dc Sensor dependent 0 to 5 V range,

(CO, combustible circuit sensor dependent

gases, etc.)
Ac or dc current Lem Heme LTA50P 15 V dc 0 to 50 A 100 mV/A

(no contact)
Air/gas fl ow Honeywell AWM3300V 10 V dc 0 to 1 L /min 4 V/L /min

Humidity Thermometrics RHU-217 5 V dc 30 to 90% RH 33 mV/%RH

UNIVERSAL SENSOR INTERFACE 1

The device of Figure 5.1 is a simple sensor interface that plugs into the printer port of the
PC. With it, it is possible to excite, control, and read a wide variety of sensors directly from
a personal computer. This universal sensor interface features an eight-channel 12-bit A/D
converter with user-selectable input ranges, two 12-bit D/A converters, two 100-

A cur-
rent sources for direct sensor excitation, and an uncommitted current mirror and multiple
digital I/O lines.
A block diagram of the universal sensor interface is shown in Figure 5.2. Note that the
circuit can accept and read analog signals from up to eight sensors. The sensor-derived sig-
nals are fed through individual prescalers and applied to an eight-channel multiplexer, per-
mitting the user to select a particular sensor-generated signal. At any given time, one of the
eight scaled input signals is presented at the output of the multiplexer to a 12-bit A/D con-
verter. The voltage reference for the A/D is provided by a precision 2.5-V reference IC.
Analog outputs are generated by the universal sensor interface through dual voltage-
output D/A converters. The resolution of the D/A converters is also 12 bits. To simplify the
reading of resistive sensors, the universal sensor interface also includes a precision current-
reference IC which incorporates two 100-

µ

µ A current sources and an uncommitted current

mirror. These can be con fi gured to provide constant-current excitation in the range 100 to

400 A.

Power for the A/D section is obtained directly from the printer port. The D/A convert-
ers and current sources are powered from an internal

µ

5-V linear regulator. Whenever the
D/A converters or the current sources are needed, an external 12-V supply must

be provided. This power supply is not required if only the A/D converter section is used.
Finally, the digital inputs and outputs of the printer port which are not used by the circuitry
within the universal sensor interface are made available to the user as general-purpose I/O
lines. These can be used to read switches, provide power to sensors with integrated elec-
tronics, or e

9-V to

ff ect real-time control of devices based on information gathered from sensors.

1 The material in this section appeared originally in Popular Electronics [Prutchi, 1999].