Robert F. Coughlin Electronic Design Adding “intelligence” to measurement systems has become commonplace because 8bit microcontrollers are inexpensive and widely available, and they can be programmed in many of today’s popular higherlevel languages (e.g., C and Basic). Often, the main challenge is signalconditioning the sensor’s output into a signalended voltage that can fully exploit the input span of the microcontroller’s analogtodigital converter (ADC). By using basic math and a systematic approach, you can easily identify and design the needed hardware. This design technique is general enough to apply to all linear sensors.
Consider a design that must convert temperature ranging from 0 °C to 50 °C into a 0 to 5V signal – a common input span for 8bit ADCs. The equation to describe this linear sensor system is:
If we select a lowcost 1N914A silicon diode as the temperature sensor, we can characterize its linear performance with a sensor equation that’s typical of this type of semiconductorbased temperature detector:
where V_{T} is the diode’s temperaturederived signal. By solving Equation 2 for the temperature, T, and substituting into the system equation, Equation 1, we get the signalconditioning circuit’s design equation, which describes the electronics needed to properly interface the sensor to the microcontroller’s ADC input:
Equation 3 indicates that the signalconditioning circuit must amplify V_{T} by a gain of –50 and offset this voltage by 33.5 V. The circuit depicted in the Figure 1 can implement this design equation because its performance equation is:
Comparing the design equation (Equation 3) and performance equation (Equation 4) terms simplifies component selection. The signal path of the inverting adder circuit is set by making the gain terms’ resistance ratio, and the offset term, set to the needed 33.5 V. Making R_{NULL} = R_{I }enables you to calibrate the zero point of the circuit at any temperature. First, measure V_{NULL} and adjust the 50kW R_{OFF} potentiometer for a voltage of –670 mV, which is the sensor’s output at 0 °C. Then, with the sensor stable at a known temperature (for example, 24 °C), adjust the –50kW R_{F} potentiometer to make the output voltage of the operational amplifier conform to the system equation: V_{O} (@ 24 °C) = (100 mV/°C)×(24 °C) = 2.40 V. Materials on the topic
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