Question 4.64: A 5-bit flash ADC in Figure P4-64 uses a reference voltage o......
A 5-bit flash ADC in Figure P4-64 uses a reference voltage of 6 V.
Find the output code for the analog inputs v_S = 3.5 V, 2.3 V, and 5.3 V. If the reference voltage is changed to 8 V, which of these codes would change?
For each OP AMP, if v_S is greater than v_{Ni}, then the output is a binary 1. Otherwise, the output is a binary 0. Using voltage division, with V_{REF} = 6 V, each resistor drops one volt, so v_{N1} = 5 V, v_{N2} = 4 V, v_{N3} = 3 V, v_{N4} = 2 V, and v_{N5} = 1 V. If the reference voltage is increased to 8 V, each resistor drops 1.333 V, so we have v_{N1} = 6.667 V, v_{N2} = 5.333 V, v_{N3} = 4 V, v_{N4} = 2.667 V, and v_{N5} = 1.333 V. The following table summarizes the digital output signals, (b_1, b_2, b_3, b_4, b_5), for each of the analog input signals.
Answer:
Presented above.
| v_S (V) | V_{REF} = 6 V | V_{REF} = 8 V |
| 3.5 | (0, 0, 1, 1, 1) | (0, 0, 0, 1, 1) |
| 2.3 | (0, 0, 0, 1, 1) | (0, 0, 0, 0, 1) |
| 5.3 | (1, 1, 1, 1, 1) | (0, 0, 1, 1, 1) |