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Chapter 9

Figure 9-5. Measurement modes available with the ADC0848 are single-ended (A), differential (B), and pseudodifferential (C).

Using Sensors to Detect and Measure

Figure 9-7. The LM385 series of voltage references includes 1.2V, 2.5V, and an adjustable version.

Listing 9-3 assumes that an LM34 connects to CH2 on the ADC0848, and that VREF is 2.5V. On request, it displays the current temperature.

For a smaller range, create a 1.2V reference with an LM385-1.2 and change line 70 in the program to match. Another option is the LM385 adjustable reference, which contains a reference diode and feedback amplifier. With the addition of a voltage source and resistors in a voltage divider, you can set the LM385’s output to the reference voltage you need. Use the formula shown to vary the resistors for different outputs.

Solar Energy

Figure 9-8 shows another sensor application, a solar cell that generates a current proportional to the intensity of the light hitting the cell. The output of the solar cell in the example varies

Chapter 9

Figure 9-8. VOUT varies with the light intensity, and output current, of the solar cell.

from 0 in darkness to 300 milliamperes in full sun. The voltage across the cell is about 0.5 volt.

An LF353 dual op amp converts the solar cell’s current into a voltage that the ADC0848 can measure. Most of the solar cell’s current flows through the 1-ohm resistor to ground. Since the solar cell’s voltage is only about 0.5 volt, the power dissipated by the 1-ohm resistor is only about 0.15 watt in full sun.

About one percent of the solar cell’s output flows through the 100-ohm resistor. This same current flows through the 1.6K resistor, with the result that the voltage at pin 1 of the LF353 varies from 0 to about -4.75V. This voltage is proportional to the intensity of the light hitting the solar cell. The second op amp is an inverter that converts the voltage to positive levels that the ADC0848 can measure.

Listing 9-4 assumes that pin 7 of the LF353 connects to Channel 8 of the ADC0848. On request, the program converts the analog input and displays the result.

Using Sensors to Detect and Measure

Listing 9-4. Measures and displays solar energy detected by solar-cell circuits at Channel 8 of ADC0848.

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.

Chapter 9

Figure 9-9. With this circuit, you can adjust the level and amplitude of an analog signal so that it varies from 0 to +5V.

As an example of how to use the circuit, if VIN varies from +12V to -12V, adjust R4 for a ±2.5-volt swing at VOUT, then adjust R5 to raise VOUT to achieve the desired 0-to-+5V swing.

Resistor R4 can increase the gain as well as decrease it. If you need to shift the signal level down instead of up, connect R5 to +15V instead of -15V. If you don’t need level shifting, you can remove R5 and connect pin 6 only to R1 and R2.

Using Sensors to Detect and Measure

Choosing a Converter

The ADC0848 is a good, general-purpose chip, but you may want to look at other converters, depending on your application. Below are some things to consider when choosing an A/D converter. Example answers describe the ADC0848, using information from its data sheet:

•What is the analog input range? (0V to V+)

•How many analog channels are there? (8)

•What is the converter’s resolution? (8 bits)

•How fast is the conversion? (30 microseconds typical, 60 microseceonds maximum)

•How accurate is the conversion? (±.1 LSB (least significant bit), 1/2 LSB version available)

•What are the power-supply requirements and power consumption? (+4.5 to +6V, 15 milliwatts)

•What input modes are available? (single-ended, differential, pseudo-differential)

•How is the converter controlled and interfaced? (control signals are WE, OE, CS

•Are there any special features on-chip (sample-and-hold, voltage reference, etc.)? (an internal clock times the conversions)

•What package types are available? (24-pin 0.3" DIP, 28-lead chip carrier)

Sample and Hold Circuits

An additional component that you may need for rapidly changing analog inputs is a sample-and-hold circuit. To ensure correct conversions, the analog input must not change in value while the conversion is taking place.

A sample-and-hold circuit ensures that the analog signal is stable by sampling the signal at the desired measurement time and storing it, usually as a charge on a capacitor. The converter uses this stored signal as the input to be converted.

When do you need a sample-and-hold? The ADC0848 requires 60 microseconds or less to convert, so you should get good results with inputs that do not vary more than 1 bit in this amount of time. When a rapidly changing input does require one, sample-and-hold ICs like the LF398 are available, or you can use a converter like the ADC0820, which has the sample-and-hold on-chip.

Chapter 10

an essential timing reference, by itself it doesn’t keep track of real-world time measured in seconds, minutes, and hours. But if you know the crystal’s frequency, you can measure seconds by counting the oscillations of the crystal. This is what BASIC-52’s real-time clock does.

A CLOCK1 statement starts the real-time clock, which causes the TIME operator to increment every 5 milliseconds. Reading the TIME operator tells you the number of seconds that have passed since the clock was enabled. CLOCK0 stops the clock and freezes TIME at its current value. TIME resets to 0 when the count reaches 65536 seconds (18 hours, 12.3 minutes), or when the statement TIME=0 executes. If you stop the clock and then then restart it, TIME will continue counting from where it left off, unless you first reset it to 0.

The ONTIME instruction jumps to a subroutine whenever TIME reaches the value you specify. Because the ONTIME subroutine is an interrupt routine, you use RETI, not RETURN, to end it.

Listing 10-1 is a program that counts seconds, minutes, and hours, and displays the current reading once per second. For accurate timekeeping, the XTAL operator must match the value of your timing crystal.

You can also use ONTIME to trigger periodic operations. Listing 10-2 is a program that toggles bit 7 of Port 1 once per second and displays the logic state of the bit after each toggle.

Clock Accuracy

The more accurate your timing reference, the more accurate your clock will be. You can tune the frequency of a crystal slightly by varying the value of one of the capacitors that connects from the crystal to ground.

Temperature variations will cause a crystal’s frequency to drift. Crystal accuracy is rated in parts per million per degree Celsius (often shortened to ppm). Over time, a crystal rated at ±10 ppm should vary no more than 0.001 percent per degree Celsius, or 0.86 seconds per day, if the temperature varies no more than ±1 degree Celsius. If your clock must be super-accurate, choose the most stable crystal you can find and and avoid temperature fluctuations.

You might think that you can get a more accurate real-time clock by adjusting XTAL to match your crystal’s actual frequency, rather than its rated value. You could measure the crystal’s frequency with a frequency counter, or experiment by varying the value of XTAL and monitoring the real-time clock to find the best match. For example, if your 12Mhz crystal actually oscillates at 11.97 Mhz, you could set XTAL equal to 11970000.