Файл: Digital design with CPLD applications and VHDL (R. Dueck, 2000).pdf

Solution

a.Va (0/256) Vref 0 V

b.Va (128/256) Vref (1/2) Vref 5 V

c.Va (64/256) Vref (1/4) Vref 2.5 V

d.Va (192/256) Vref (3/4) Vref 7.5 V

In general, a DAC input code consisting of 1 followed by all 0s generates an output value of 1⁄2 full scale. A code of 01 followed by all 0s yields an output of 1⁄4 full scale. An

output of 11 followed by all 0s generates an output of 3⁄4 full scale.

SECTION 12.2C REVIEW PROBLEM

12.4Calculate Va for an 8-bit R-2R ladder DAC when the input code is 10100001. Assume that Vref is 10 V.

MC1408 Integrated Circuit D/A Converter

K E Y T E R M

Multiplying DAC A DAC whose output changes linearly with a change in DAC

reference voltage.

A common and inexpensive DAC is the MC1408 8-bit multiplying digital-to-analog converter. This device also goes by the designation DAC0808. A logic symbol for this DAC is shown in Figure 12.11.

FIGURE 12.11

MC1408 DAC

The output current, Io, flows into pin 4. Io is a binary fraction of the current flowing into pin 14, as specified by the states of the digital inputs. Other inputs select the range of output voltage and allow for phase compensation.

Figure 12.12 shows the MC1408 in a simple D/A configuration. R14 and R15 are approximately equal. Pin 14 is approximately at ground potential. This implies:

1.That the DAC reference current can be calculated using only Vref ( ) and R14 (Iref

Vref ( )/R14)

2.That R15 is not strictly necessary in the circuit. (It is used primarily to stabilize the circuit against temperature drift.)

The reference voltage must be set up so that current flows into pin 14 and out of pin 15. Thus, Vref ( ) must be positive with respect to Vref ( ). (It is permissible to ground pin 14 if pin 15 is at a negative voltage.)

Io is given by:

Since the output is proportional to Vref ( ), we refer to the MC1408 as a multiplying

DAC.

Io should not exceed 2 mA. We calculate the output voltage by Ohm’s law: VoIo RL. The output voltage is negative because current flows from ground into pin 4.

The open pin on the Range input allows the output voltage dropped across RL to range from 0.4 V to 5.0 V without damaging the output circuit of the DAC. If the Range input is grounded, the output can range from 0.4 to 0.55 V. The lower voltage range allows the output to switch about four times faster than it can in the higher range.

FIGURE 12.12

MC1408 Configured for

Unbuffered Analog Output