# What use would a D flip-flop being fed an analog signal have?

I'm contemplating a non-electronic system that is analogous to a D flip-flop being fed an analog signal and a fixed-frequency clock. The system has an analog input which changes over time. The output of the system is a single bit. The analog input is sampled at fixed intervals. The output goes high if the input exceeds one value, and goes low if the input deceeds a lower value.

I feel like this circuit has a name or common application, but I can't track down what it might be.

• Schmitt trigger , normally inverter output 1/3 to 2/3Vcc thresholds. 'HC14 Commented Feb 3, 2017 at 3:55
• but this not the same as a D FF, it is just a hysteretic input. Commented Feb 3, 2017 at 4:09
• i think a D flip-flop does have hysteresis. that positive feedback is how it remembers its state. so i think your first comment is correct. Commented Feb 3, 2017 at 4:46
• This is called a clocked comparator. It could be implemented with a Schmitt trigger on the D input, but most usually with a comparator. You would use it where you interface into a larger synchronous system with fixed timing domains. Commented Feb 3, 2017 at 5:32
• Another name is latched comparator. The hysteresis is not a standard feature, but easily added with a feedback resistor. Commented Feb 3, 2017 at 6:06

If the output bit stream is low-pass-filtered and fed back and subtracted from the analog input, you have a first-order delta-sigma tracking converter.

• Fascinating idea! Commented Feb 5, 2017 at 3:34

The output goes high if the input exceeds one value, and goes low if the input deceeds a lower value.

That sentence describes a Schmitt trigger: -

Top signal is the input. Middle single is a normal comparator output. Bottom signal is the Schmitt trigger output - notice how it switches high when the analogue input passes the high trigger threshold and only returns to a low output when the analogue input falls below the low trigger input value.

That implies a sample and hold circuit: -

It sounds like you are describing a schmitt trigger being fed from the output of a sample and hold circuit that, in turn, is fed from the analogue input.

The output of the system is a single bit.

Yes it is, agreed.

It's an A/D converter, of course. Whatever the D threshold is, at clock time the Q and /Q outputs very quickly settle on a digital representation of the one-bit digitization of the input value (0 for under-threshold, 1 for over-threshold).

A high-resolution digitization, it isn't. But, the captured value is sampled at a very precise time.

• it's a Schmitt trigger. Commented Feb 3, 2017 at 4:47
• If there are two thresholds, that's not a D input to a flip/flop. And, if there's a master clock, it's not a Schmitt trigger. Maybe a Schmitt trigger with a latch? Commented Feb 3, 2017 at 4:52
• any gate has a different OFF threshold than ON threshold. and the difference between the two thresholds is called the "noise margin". (oops. i got that terminology wrong, the noise margin is the difference of the output voltage range and input range for each logic state.) i guess you can call it a clocked Schmitt trigger. Commented Feb 3, 2017 at 5:03
• If the signal were DC coupled, it might have margins, but AC coupling happens, too. The size of the Schmitt deadband might mean that the A/D conversion just rounds off toward a no-change. Commented Feb 3, 2017 at 5:10