Adding “intelligence” to measurement systems has
become commonplace because 8-bit microcontrollers
are inexpensive and widely available, and they can be
programmed in many of today’s popular higher-level languages
(e.g., C and Basic). Often, the main challenge is signal-
conditioning the sensor’s output into a signal-ended voltage
that can fully exploit the input span of the microcontroller’s
analog-to-digital 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 5-V signal—a common input span
for 8-bit ADCs. The equation to describe this linear sensor
system is:
If we select a low-cost 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 VT is the diode’s temperature-derived signal. By solving
Equation 2 for the temperature, T, and substituting into the
system equation, Equation 1, we get the signal-conditioning
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 signal-conditioning circuit must
amplify VT by a gain of –50 and offset this voltage by 33.5 V.
The circuit depicted in the figure 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, –RF/RI, equal to
–50 and the offset term, –(RF)VREF/ROFF, set to the needed
33.5 V.
Making RNULL = RI enables you to calibrate the zero point
of the circuit at any temperature. First, measure VNULL and
adjust the 50-kV ROFF 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
–50-kV RF potentiometer to make
the output voltage of the operational
amplifier conform to the system equation:
VO (@ 24°C) = (100 mV/°C) (24°C) = 2.40 V.
See associated figure
