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either a 0% or a 100% duty cycle. The duty cycle is programmed as the
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) | ||
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0 | 0.002 | ||
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12 | 0.620 | 123 | |
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25 | 1.242 | 256 | |
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50 | 2.483 | 512 | |
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72 | 3.567 | 742 | |
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100 | 3.567 | ||
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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 | (EQ 1) | |
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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
User’s Manual | 25 |