either a 0% or 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 corresponds to a 25% duty cycle.

Table 2 lists typical DA1 voltages measured for various duty cycle values with a load larger than 1 MΩ.

Table 2. Typical DA1 Voltages for Various Duty Cycles

Duty Cycle

Voltage

Programmed Count

(%)

(V)

 

 

 

 

0

0.002

0–122

 

 

 

12

0.620

123

 

 

 

25

1.242

256

 

 

 

50

2.483

512

 

 

 

72

3.567

742

 

 

 

100

3.567

743–1024

 

 

 

It is important to remember that the DA1 output voltage will not be realized instanta- neously after programming in a value. There is a settling time because of the RC time con- stant (R24 × C22), which is 10 ms. For example, the voltage at any given time is

V = V

P

– (V

P

– V

DA1

)e(-t/RC)

(EQ 1)

 

 

 

 

 

where V is the voltage at time t, VP is the programmed voltage, VDA1 is the last DA1 out- put voltage from the D/A converter, and RC is the time constant (10 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 50 ms. Six time constants, 60 ms, will allow settling to within 99.75% (or to within about 8 mV for a 3 V change in voltage). Seven time constants, 70 ms, will allow settling to within 99.91% (or to within about 3 mV for a 3 V change in voltage).

An LM324 op amp, which can comfortably source 10 mA throughout the D/A converter range, drives the D/A converter output. If the output voltage is above 1 V, the D/A con- verter can comfortably sink 10 mA. Below 1 V, the D/A converter can only sink a maxi- mum of 100 µA.

To summarize, DA1 is provided uncalibrated, can be programmed with a resolution of

5 mV and a peak-to-peak ripple less than 20 mV over the range from 0.7 V to 3.5 V and 0 V. The settling time to within 21 mV is 50 ms.

User’s Manual

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Digi BL1800 user manual Typical DA1 Voltages for Various Duty Cycles