Digi BL1800 user manual Typical DA1 Voltages for Various Duty Cycles

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 25