Philips Semiconductors Linear Products Product specification
ADC0803/4-1CMOS 8-bit A/D converters
August 31, 1994
561
decoder. The RD
and WR signals are generated by reading from
and writing to a dummy address.
Digitizing a Transducer Interface Output
Circuit Description
Figure 10 shows an example of digitizing transducer interface output
voltage. In this case, the transducer interface is the NE5521, an
LVDT (Linear Variable Differential Transformer) Signal Conditioner.
The diode at the A/D input is used to insure that the input to the A/D
does not go excessively beyond the supply voltage of the A/D. See
the NE5521 data sheet for a complete description of the operation of
that part.
Circuit Adjustment
To adjust the full scale and zero scale of the A/D, determine the
range of voltages that the transducer interface output will take on.
Set the LVDT core for null and set the Zero Scale Scale Adjust
Potentiometer for a digital output from the A/D of 1000 000. Set the
LVDT core for maximum voltage from the interface and set the Full
Scale Adjust potentiometer so the A/D output is just barely 1111
1111.
A Digital Thermostat
Circuit Description
The schematic of a Digital Thermostat is shown in Figure 11. The
A/D digitizes the output of the LM35, a temperature transducer IC
with an output of 10mV per °C. With V
REF
/2 set for 2.56V, this 10mV
corresponds to 1/2 LSB and the circuit resolution is 2°C. Reducing
V
REF
/2 to 1.28 yields a resolution of 1°C. Of course, the lower
V
REF
/2 is, the more sensitive the A/D will be to noise.
The desired temperature is set by holding either of the set buttons
closed. The SCC80C451 programming could cause the desired
(set) temperature to be displayed while either button is depressed
and for a short time after it is released. At other times the ambient
temperature could be displayed.
The set temperature is stored in an SCN8051 internal register. The
A/D conversion is started by writing anything at all to the A/D with
port pin P10 set high. The desired temperature is compared with the
digitized actual temperature, and the heater is turned on or off by
clearing setting port pin P12. If desired, another port pin could be
used to turn on or off an air conditioner.
The display drivers are NE587s if common anode LED displays are
used. Of course, it is possible to interface to LCD displays as well.
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