Digi JACKRABBIT BL1800 User Manual download pdf (Page 27)

User’s Manual 23

3.4 D/A Converters

Two digital-to-analog (D/A) converter outputs, DA0 and DA1, are supplied on the Jack-

rabbit. These are shown in Figure 10.

The D/A converters have no reference voltage. Although they may be fairly accurate from

one programmed voltage to the next, they do not have absolute accuracy. This is because

Vcc can change ±5%, the PWM outputs might not achieve the full 0 V and 5 V rail out of

the processor, and the gain resistors in the circuit have a 1% tolerance. The D/A converters

therefore need individual calibration, with the calibration constants held in software

before absolute accuracy can be relied on. The Jackrabbit is sold without such calibration.

Figure 10. Schematic Diagram of D/A Converters

Note that DA0 is used to provide a reference voltage for the A/D converter and is unavail-

able for D/A conversion when the A/D converter is being used.

Pulse-width modulation (PWM) is used for the D/A conversion. This means that the digi-

tal signal, which is either 0 V or 5 V, is a train of pulses. This means that if the signal is

taken to be usually at 0 V (or ground), there will be 5 V pulses. The voltage will be 0 V for

a given time, then jump to 5 V for a given time, then back to ground for a given time, then

back to 5 V, and so on. A hardware filter in the circuit consisting of a resistor and capacitor

averages the 5 V signal and the 0 V signal over time. Therefore, if the time that the signal

is at 5 V is equal to the time the signal is 0 V, the duty cycle will be 50%, and the average

signal will be 2.5 V. If the time at 5 V is only 25% of the time, then the average voltage

will be 1.25 V. Thus, the software needs to only vary the time the signal is at 5 V with

respect to the time the signal is at 0 V to achieve any desired voltage between 0 V and 5 V.

–

Vcc

LM324

R29

1 MW

PD2

PD1

R22

10 kW

R26

82.5 kW

DA0

R20

1.1 kW

R27

255 kW

C20

100 nF

R21

110 kW

–

LM324

PD4

DA1

1.1 kW

100 nF

R24

100 kW

C22

R25

R28

100 kW

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Digi JACKRABBIT BL1800 User Manual Page 27

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