Page 232 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Unit 12: A/D and D/A Converters

The D/A converter output current I , is given as Notes
0
I = I (A1/2 + A2/4 + A3/8 + .......+A8/256) ...(12.5)
0 ref
where A1, A2, A3, ..., A8 are the digital input levels (1 or 0).
The OA is connected as a current-to-voltage converter, and the output voltage is given as
V = I * R ...(12.6)
0
0
Substituting Eqs. (12.4) and (12.5) into Eq. (12.6),
V = V /R Rref * (A1/2 + A2/4 + A3/8 + .......+A8/256) * R
ref
0
...(12.7)
If we set the OA feedback resistor R equal to R , then
ref
V = V * (A1/2 + A2/4 + A3/8 + .......+A8/256) ...(12.8)
0 ref
Let us try out Eq. (12.8). Suppose all digital inputs are 0s (all low). Then

V = V * (0/2 + 0/4 + 0/8 +.......+0/256)
0 ref
= V * 0 = 0.0Vdc
ref
Now, suppose all digital inputs are 1s (all high). Then

V = V * (1/2 + ¼ + 1/8 + ....... + 1/256)
ref
0
(V ) * (256/256) = 0.996 * V
ref ref
Since V in Figure. 12.18 is + 10 Vdc, the output voltage is seen to have a range between 0.0 and
ref
+9.96 Vdc. It does not quite reach + 10 Vdc, but this is characteristic of this type of circuit. This
circuit is essentially the current-mode operation discussed in the previous section and illustrated
in Figure 12.12b.
12.9: In Figure 12.18, A1 is high, A2 is high, A5 is high, A7 is high. The other

digital inputs are all low. What is the output voltage V ?
0
Solution:
V = 10 * (1/2 + ¼ + 1/32 + 1/128) = 10 * 0.789 = 7.89V
0
Self Assessment

True or False:
1. The basic advantage in converting a digital signal into an equivalent analog signal change
the n digital voltage levels into one equivalent analog voltage.
(a) True (b) False
2. Resistive divider has two serious drawbacks: one of the first is the fact that each resistor in
the network has a different value.

(a) True (b) False
3. Either the resistive divider or the ladder can be used as the basis for a digital-to-analog
(D/A) converter.
(a) True (b) False
4. D/A converter can be regarded as a logic block having numerous digital inputs and a single
analog output.

(a) True (b) False

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