DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

UNIVERSAL SENSOR INTERFACE 213dat dat (2 ^ clocknum) * bit ' accumulate from bit 11 to bit 0Next clocknum' next bitOut prinop, power notcs mux bit7 ' deassert *CS without upsetting bit 7Acquire dat' return 12-bit A/D countEnd FunctionSignal ConditioningInput signals rarely fit exactly within the universal sensor interface’s input range of 0 to2.5 V. Signals of smaller amplitude than the full range waste resolution, while signals outsidethe full range will end up being clipped to the limits of the range. Please note thatinput signals presented to the A/D multiplexer exceeding the range 0 to 2.5 V may causepermanent damage to the circuitry of the interface. Measuring a signal that spans 0 V anda value larger than 2.5 V is easily accomplished. A resistive voltage divider such as that ofFigure 5.5a can scale a large unipolar signal to the desired range. For ease of use, the universalsensor interface has onboard locations reserved for resistive voltage dividers. Thevoltage-divider resistor packs are marked R4 and R5. To use them, first select the appropriate10-pin single-in-line bussed resistor array for R5. Then select either a DIP resistorpack or individual 1 4 -W precision resistors to be placed on R4.Logging data from the sensors on a hyperbaric chamber (a pressurized vessel used tostudy the therapeutic use of high atmospheric pressures) provides a good example of howto select components for R4 and R5. Table 5.2 shows typical ranges for sensors that monitora small hyperbaric chamber. For this application, the range for analog channels 1 to 4of the universal sensor interface should be 0 to 5 V, the range for channel 5 should be 0 to10 V, and the range of channels 6 to 8 should remain at 2.5 V. Assume that an impedanceof 10 kΩ is appropriate for all channels.For this example, R5 can be selected to be a 10-pin bused 10 -kΩ resistor pack. A suitabledevice is a CTS 770-series 10-kΩ, single-in-line conformal 10-pin bused resistor network(Digi-Key part 770-101-10K-ND) or similar (e.g., Jameco 24643). 1% 1 4-W resistorsfor R4 are selected using the following formula:10kΩ2.5 V V max R4 10kΩAs such, 10-kΩ resistors should be placed between pads pairs [1,16], [2,15], [3,14], and[4,13] of R4 to yield a range of 0 to 5 V for channels 1 to 4. For channel 5, the formularequires a 30-kΩ resistor for a range of 0 to 10 V. Since 30 kΩ is not a standard value, a30.1-kΩ 1% resistor should be selected and soldered between pads 5 and 12 of R4. Channels6 to 8 do not require scaling, and thus pad pairs [6,11], [7,10], and [8,9] should be jumpered.As shown in Figure 5.5b, a small unipolar signal requires just an op-amp-based amplifierto take advantage of the full resolution of the A/D. Signals riding on a median different thanthe A/D’s midpoint (1.25 V) can be offset appropriately by using the circuit of Figure 5.5c.Alternatively, a “quick and dirty” way of introducing offset that can sometimes be used toenable measurement of bipolar signals is to place a 1.5-V battery in series with the signalsource.Current measurements can be obtained by using a suitable shunt. For example, asshown in Figure 5.5d, the popular 4- to 20-mA current loop used to convey informationfrom many industrial instruments and sensors can be converted to a voltage by using ametal-film 124 Ω 1% resistor shunt across the input terminals of the universal sensorinterface. Since the 4- to 20-mA current will be translated into the range 0.496 to 2.48 V,some measurement resolution will end up being wasted. If the full 12-bit resolution isdesired, you may use a 154 Ω 1% resistor instead and use an op-amp to introduce a0.616-V offset to the measurement.