Книга: Introduction to Microprocessors and Microcontrollers

Digital to analog conversion (DAC)

Changing a group of digital bit values to an analog voltage is basically just the reverse process of the A–D conversion that we met in the previous section.

Most digital to analog converters operate by adding current together then converting the result into an analog voltage. The binary levels are used to switch currents on or off.

Let’s assume a 4-bit digital signal in which the most significant bit is made to generate a current of 8 mA and the others produce, in turn, 4, 2 and finally 1 mA. If the digital signal to be converted happened to be 10112, then the first, third and fourth current sources would be activated giving a total of 8+2+1=11 mA (Figure 17.4).

Figure 17.4 A DAC with a current or voltage output

In some DACs the final output is a changing current but in others it has been converted to a variable voltage. It just depends on which integrated circuit you choose to use. In the ones offering a voltage output, the total current is then passed through a resistor. If we choose a nice easy value like 1 k?, the voltage across the resistor would be 11 mA?1 k?=11 V.

In a similar way, we can see that all binary values between 0000 and 1111 would be converted into voltages between 0 V and 15 V. There are a couple of specifications that may need to be checked to decide on which one to use.

Resolution

This is the number of digital bits used to convert into an analog voltage. Typical values available are from 4 to 18 bits. As the digital input changes by a single bit, say from 1000 to 1001, the resultant voltage or current increases by a discrete step. The size of this step is determined by the number of bits used compared with the maximum value of the output current or voltage.

For example, if we used 4 bits then this would provide a total of 16 different steps and if the maximum happened to be 8 V, each step will represent a voltage change of 0.5 V. Thus, a steadily increasing digital signal will cause the analog voltage to increase in small discrete steps like a staircase. This is all very similar to the cause of quantization error.

Speed

The speed of operation is very dependent on the chip being considered. The conversion times available from an exceedingly fast 1 ns to a sluggish 5 ?s.

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