3.4.2 DA0

The op amp supporting DA0 translates a 12%–88% duty cycle to an analog voltage range of 0 V to 3 V. The software operates only within this duty cycle; a duty cycle less than 12% is rounded down to 0%, and any duty cycle above 88% is rounded up to 100%.

DA0 uses a voltage divider that consists of R21 and R27 and a gain-offset circuit that con- sists of R26 and R29 to achieve the output range of 0 V to 3 V within the software duty cycle.

The DA0 output can be less than 100 mV for a 0% duty cycle and above 3.0 V for a 100% duty cycle. The duty cycle is programmed as the high-time count of 1024 total counts of the Rabbit 2000’s timer B. Thus, 256 counts would be 25% of 1024 counts, and corre- sponds to a 25% duty cycle.

Table 3 lists typical DA0 voltages measured for various duty cycle values with a load larger than 1 MΩ.

Table 3. Typical DA0 Voltages for Various Duty Cycles

Duty Cycle

Voltage

Programmed Count

(%)

(V)

 

 

 

 

0

0.074

0–122

 

 

 

12

0.076

123

 

 

 

25

0.530

256

 

 

 

50

1.467

512

 

 

 

75

2.406

768

 

 

 

88

2.875

901

 

 

 

100

3.345

902–1024

 

 

 

It is important to remember that the DA0 output voltage will not be realized instantaneously after programming in a value. There is a settling time because of the RC time constant (R21 R27 × C20), which is 7.68 ms. For example, the voltage at any given time is

V = VP – (VP VDA0)e(-t/RC)

(EQ 2)

where V is the voltage at time t, VP is the programmed voltage, VDA0 is the last DA0 out- put voltage from the D/A converter, and RC is the time constant (7.68 ms). The settling will be within 99.326% (or within about 21 mV for a 3 V change in voltage) after five time constants, or 38 ms. Six time constants, 46 ms, will allow settling to within 99.75% (or to within about 8 mV for a 3 V change in voltage). Seven time constants, 54 ms, will allow settling to within 99.91% (or to within about 3 mV for a 3 V change in voltage).

The settling time is reduced somewhat by precharging capacitor C20 with pulse-width modulation from PD2.

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Jackrabbit (BL1800)

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Digi BL1800 user manual 2 DA0, Typical DA0 Voltages for Various Duty Cycles