The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Using Sensors to Detect and Measure
Listing 9-4. Measures and displays solar energy detected by solar-cell
circuits at Channel 8 of ADC0848.
10 REM set A to address of ADC
20 A=0C000H
30 REM set C to channel to read (1-8)
40 C=8
50 REM use single-ended mode, select channel, start convert
60 XBY(A)=8+C-1
70 REM FS=full-scale voltage (5V)/full-sun output (4.75V)
80 FS=1.05
90 B=XBY(A)
100 T=INT(FS*B*100/255+.5)
110 PRINT “Solar energy = ”,T," percent of full sun"
120 PRINT “press any key to take another measurement”
130 D=GET : IF D=0 THEN GOTO 130
140 GOTO 60
150 END
Level Translating
As you can see, not every sensor has an output that can connect directly to the ADC0848’s
inputs. A sensor’s output may vary from -2 to -1V, from -0.5 to +0.5V, or from -12 to +12V.
In all of these cases, you need to shift the signal levels and sometimes adjust the signal range
to be compatible with a converter that requires inputs between 0 and 5 volts.
Figure 9-9 shows a general-purpose circuit that can amplify or reduce input levels, and can
also raise or lower the entire signal by adding or subtracting a voltage. Separate, independent
adjustments control the gain and offset. The circuit is a series of three op amps: a buffer, a
level shifter, and an amplifier. The example circuit uses three of the devices in an LF347
quad JFET-input op amp. The LF347 has fast response and high input impedance. You may
use a different op amp if you prefer.
The first op amp is a noninverting amplifier whose output at pin 1 equals VIN. The op amp
presents a high-impedance input to VIN, to minimize loading effects.
The second op amp is an inverting summing amplifier that shifts pin 1’s voltage up or down
as R5 is adjusted. Adjusting R5 raises and lowers the voltage at pin 7, but the signal’s shape
and peak-to-peak amplitude remain constant.
The third op amp is an inverting amplifier whose gain is adjusted by R4. This amplifier
increases or decreases the peak-to-peak amplitude of its input.
The Microcontroller Idea Book 167