Digital Circuits

Analog-to-Digital Conversion

18 questions By Tony R. Kuphaldt

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Question 1 of 18

The circuit shown here is a four-bit analog-to-digital converter (ADC). Specifically, it is a flash converter, so named because of its high speed:

Explain why we must use a priority encoder to encode the comparator outputs into a four-bit binary code, and not a regular encoder. What problem(s) would we have if we were to use a non-priority encoder in this ADC circuit?

Reveal answer

I won’t directly answer this question, but instead pose a “thought experiment.” Suppose the analog input voltage (V_in) were slowly increased from 0 volts to the reference voltage (V_ref). What do the outputs of the comparators do, one at a time, as the analog input voltage increases? What input conditions does the encoder see? How would a primitive “diode network” type of encoder (which we know does not encode based on priority) interpret the comparator outputs?

Notes:
Here, I show students a very practical application of a priority encoder, in which the necessity of priority encoding should be apparent after some analysis of the circuit.
Question 2 of 18

Predict how the operation of this “flash” analog-to-digital converter (ADC) circuit will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):

Resistor R₁₆ fails open:

Resistor R₁ fails open:

Comparator U₁₃ output fails low:

Solder bridge (short) across resistor R₁₄:

For each of these conditions, explain why the resulting effects will occur.

Reveal answer

Resistor R₁₆ fails open: Output code will be 15 (1111) all the time.

Resistor R₁ fails open: If V_in < V_ref, output will be 0 (0000); if V_in > V_ref, output will be 15 (1111).

Comparator U₁₃ output fails low: Output will assume the “13” state (1101) unless V_in exceeds that analog value, then the ADC will register properly.

Solder bridge (short) across resistor R₁₄: There will be no distinctive “13” state (1101), the analog values for all the other states adjusting slightly to fill the gap.

Notes:
The purpose of this question is to approach the domain of circuit troubleshooting from a perspective of knowing what the fault is, rather than only knowing what the symptoms are. Although this is not necessarily a realistic perspective, it helps students build the foundational knowledge necessary to diagnose a faulted circuit from empirical data. Questions such as this should be followed (eventually) by other questions asking students to identify likely faults based on measurements.
Question 3 of 18

This “flash” ADC circuit has a problem. The output code jumps from 0000 to 1111 with just the slightest amount of input voltage (V_in). In fact, the only time it outputs 0000 is when the input terminal is slightly negative with reference to ground:

Identify at least two possible component faults that could cause this problem, and explain your reasoning in how you made the identifications.

Reveal answer

One possible fault is that resistor R₁₆ has failed open, but this is not the only possibility.

Notes:
Have your students explain their reasoning in class to you, so that you may observe their diagnostic thought processes.