Topic: Digital Electronics
Following waveform shows output of a 4 bit DAC with 5 V reference voltage. The 4 bit digital input of DAC is connected to 4 bit up counter, the one bit input of DAC is stuck at '0', which is this bit?
(The waveform shows a staircase signal starting from 0V, stepping up to 0.9375V, then dropping to 0V, stepping up to 0.9375V, then jumping to 2.5V, stepping up to 3.4375V, and dropping to 2.5V repeatedly.)
- A. Bit-0 (LSB)
- B. Bit-1
- C. Bit-2
- D. Bit-3 (MSB)
Answer: C. Bit-2
Explanation: For a 4-bit DAC with a 5V reference, the step size is $5/16 = 0.3125\text{ V}$.
The expected output sequence for a normal up-counter would increase by 0.3125V per step. The sequence shown goes 0V, 0.3125V, 0.625V, 0.9375V (inputs 0000 to 0011).
The next expected input is 0100 (which corresponds to $4 \times 0.3125 = 1.25\text{ V}$), but the output drops back to 0V, indicating the DAC receives 0000 instead.
Similarly, when the counter reaches 1000 to 1011, the output is 2.5V to 3.4375V, but drops to 2.5V when it should be 1100. This indicates that the 3rd bit from the right (Bit-2, where LSB is Bit-0) is stuck at 0.