Page 227 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Digital Circuits and Logic Design

Notes Except for sign In Figure 12.12a, the R-2R ladder and OA are said to operate in a voltage mode,
while the connection in Figure 12.12b is said to operate in a current mode.
Be careful while the operational amplifier (OA) is connected as a unity-gain
non-inverting amplifier. It has very high input impedance, and the output
voltage is equal to the input voltage.

Draw the truth table of Binary ladder output voltages.

12.3 D/A Converters

Either the resistive divider or the ladder can be used as the basis for a digital-to-analog (D/A)
converter. It is in the resistive network that the actual translation from a digital signal to an analog
voltage takes place. There is, however, the need for additional circuitry to complete the design
of the D/A converter.
As an integral part of the D/A converter there must be a register that can be used to store the
digital information. The simplest register is formed by use of RS flip-flops, with one flip-flop per
bit. There must also be level amplifiers between the register and the resistive network to ensure
that the digital signals presented to the network are all of the same level and are constant. Finally,
there must be some form of gating on the input of the register such that the flip-flops can be set
with the proper information from the digital system. A complete D/A converter in block-diagram
form are shown in Figure 12.13a.

Let us expand on the block diagram shown in this Figure 12.13a by drawing the complete schematic
for a 4-bit D/A converter as shown in Figure 12.13b. You will recognize that the resistor network
used is of the ladder type.
The level amplifiers each have two inputs: one input is the + 10 V from the precision voltage
source, and the other is from a flip-flop. The amplifiers work in such a way that when the input
from a flip-flop is high, the output of the amplifier is at + 10 V. When the input from the flip-flop
is low, the output is 0 V.

The four flip-flops form the register necessary for storing the digital information. The flip-flop
on the right represents the MSB, and the flip-flop on the left represents the LSB. Each flip-flop
is a simple RS latch and requires a positive level at the R or S input to reset or set it. The gating
scheme for entering information into the register is straightforward and should be easy to
understand. With this particular gating scheme, the flip-flops need not be reset (or set) each time
new information is entered. When the READ IN line goes high, only one of the two gate outputs
connected to each flip-flop is high, and the flip-flop is set or reset accordingly. Thus data are
entered into the register each time the READ IN (strobe) pulse occurs. D flip-flops could be used
in place of the RS flip-flops.

12.3.1 Multiple Signals

Quite often it is necessary to decode more than one signal–for example, the X and Y coordinates
for a plotting board. In this event, there are two ways in which to decode the signals.

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