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Analog to Digital (A/D) conversion is accomplished by performing a Voltage to
Frequency (V/F) conversion on the input signal at IC U17 and running the frequency
output to a counter comprised of IC's U20, U21, U22.
The full scale digital output of the counter section is 80,000 counts, giving an A/D
resolution of 1 part in 80,000.
The combination of V/F converter and counter inherently provides an integrating Analog
to Digital conversion. The time base for this integration is controlled by the
micro-processor using the clock oscillator, U36. Jumper B15 allows the selection of
either a 4.0 MHz or 4.8 MHz frequency to minimize electrical pick-up at the operating
line frequency. The time duration of integration is selectable over the range of 67 msec
to 2.067 seconds. In the Model 300 an integrate period of 1.033 records is used for
reading the photo-detector outputs, a 133 msec integrate period is used for all other
signals.
Pots R27 and R31 provide offset and gain adjust respectively to the analog input of the
V/F converter, allowing the A/D section to be adjusted to match an external voltage
standard.
10.6.3.2 Digital to Analog Converters
Four Independent Digital to Analog Converters (DAC's) are contained in IC's U10 and
U11 and are used to generate the instrument's analog outputs. These DAC's have 12 bit
resolution and are fully buffered by OpAmps at U8 and U9. The outputs of the DAC's
are jumper selectable for full scale range at jumpers B6, B7, B8 and B9. The Full Scale
Ranges supported are:
• 0-100 mV
• 0-1 V
• 0-5 V
• 0-10 V
In the M300 the use of these four DAC's is:
DAC
CHANNEL
SIGNAL
0 Strip Chart Analog Output
1 DAS Analog Output
2 Spare
3 Test Function Analog Output
The DAC's are operated in bipolar mode allowing a "live zero" on all output. In addition,
DAC's 0 and 1 (Strip Chart and DAS) physically provide a Full Scale of 120% of the
nominal selected value with the micro-processor providing pre-scaling to achieve the
nominal value. This combination provides 20% over-range capability. The
P/N 02163G1 Teledyne API Model 300 CO Analyzer Instruction Manual - Page 93