# Relating frequency to speed

1. Apr 1, 2012

### Femme_physics

I have this system,

http://img696.imageshack.us/img696/488/vau2.jpg [Broken]

And I'm asked to explain the meaning of increasing the frequency of the system to conversation time. Of course I would imagine the more frequency, the less conversion time.

I presume that is what I found to be "sampling rate". http://en.wikipedia.org/wiki/Sampling_frequency

But I haven't found any formulation to relate frequency of the system to conversion time....is there such one?

Last edited by a moderator: May 5, 2017
2. Apr 1, 2012

### Femme_physics

and...apparently I'm Evo now...

3. Apr 1, 2012

### M Quack

can you describe in words what the circuit does? Presumably, the counter is at zero when you start, and Vx=0...

Oh, and I'm Evo, too now.

4. Apr 1, 2012

### Femme_physics

Kinda weird. According o the question it's a system that converts analog voltage to digital. Though, according to the sketch, it's a D to A system.

I believe that it is a digital to analog converter according to the questions I see.

So going from this standpoint.

we have a counter that counts the clock pulses and present the result usually in binary form. Then you got the converter who converts the binary signal to analog. First a resolution is calculated through the resolution formula, then the binary signal is converted to decimal and using the formula Va = Resolution x The Decimal Value of the Binary signal

We can find out Va.

I think the role of the op-amp is kind of like a switch (not entirely sure). Vcc is the supply voltage. U2 is an AND gate that takes together the inputs from VCC, and the clock pulse to form a combined signal. I think. What does all of that mean.... I think I lost myself. Any guidance, please?

5. Apr 1, 2012

### M Quack

The key to the circuit is when the counter stops counting. Can you find out when that happens?

6. Apr 1, 2012

### I like Serena

D0-D7 together form the digital output signal.

The counter counts up on a positive pulse, or down on a negative pulse.

Let's assume the counter starts at zero, meaning Vx starts at 0 as mister Quack suggested.
What would then be the output of the op-amp?

7. Apr 2, 2012

### Femme_physics

Are you telling me that anything other than the D to A is basically a mechanism to convert analog to digital?

It appears to me that the counter is in fact the converter.

Alright.

That's kind of a trick question, because I'm used that in comparators I have the supply voltage-- then I know. But I really don't know what to make of this Vcc that's NOT connected to the comparator, and the fact this weird comparator does not appear to have any supply voltages.

When V- = V=

or when Vx = Va

Based on the comparator formula.

8. Apr 2, 2012

### M Quack

You are right, the counter provides the digital outputs D0-D7

Not sure if the counter can count up and down. Given that U2 seems to be a simple logic gate, I'd say it only counts up when it gets a pulse. To make the circuit useful in practice one would have to add a reset switch.

U1 works as a comparator. Vcc and ground connections are just not shown.

Exactly.

Now, knowing that, can you again describe in words what the circuit does and how it works?

9. Apr 2, 2012

### Femme_physics

So a converter and counter are the same thing?

So how do you explain the Vcc in the drawing?

The clockpulse starts the count, and together with Va the AND gate assures the signal goes out only if both legs receive input. The counter converts the analog signal to binary digits (based on the resolution...number of legs...etc)...and the D to A converter re-converts the digital signal to analog which gets to the minus leg of the comparator. When Vx reaches a high enough value to equal Va the system restarts.

10. Apr 2, 2012

### M Quack

The whole circuit acts as a converter. A counter is just a counter, but in this circuit it provides the output signals.

Don't really know. For a standard OpAmp this should work without the pull up. For an open collector you need the pull up.

Perfect so far.
??? The counter only receives digital pulses from the AND gate U2...
Close, but no cigar. When Vx reaches Va, the comparator switches to 0 and the AND gate U2 blocks any further pulses from the clock. The counter stops.

11. Apr 2, 2012

### Femme_physics

I would say the show stop before the AND gate, in the Op-Amp, but using the AND gate as the show-stopper always is sensible to me.

The counter RECIEVES digital pulses? Are you sure? The way I see it, it's:

From D to A, to the counter - ANALOG
From the counter to D to A - DIGITAL

I don't see how it changes along the way. Picture this, at Vx you agree that the signal is analog, yes? So....keeps going....reaches the op-amp...op-amp doesn't change it.... keeps going.... reaches the AND gate with the pulse...still analog....reaches the counter....turns binary...then hits the D to A again and returns to analog!

Voila!

But you would agree that there are two converters, the counter which acts as a A to D converter, and the D to A converter in the sketch?

12. Apr 2, 2012

### Staff: Mentor

Hi Femme_physics. The counter really is just a binary counter. As input it accepts a binary clock signal to cause it to increment its binary counting. As output we have 8 digital outputs, each carrying a binary 1 or 0. There is no analog level anywhere there.

The D-A converter accepts these 8 binary lines, and produces a corresponding analog output, Vx.

See how you go with that.

13. Apr 2, 2012

### Staff: Mentor

The op-amp is a comparator, and its output is a digital signal, binary 1 or 0. The resistor to Vcc is symbolic of a dodge that the designer is using to match the comparator's output voltage levels (maybe -Vcc and +Vcc) to what the conventional AND gate expects/requires/demands as input (viz., 0v and +5v).

Everything following the op-amp O/P is digital.

14. Apr 5, 2012

### Femme_physics

I'm confused. You have a pulse coming in called Va, right? It goes into the AND gate...so it's in fact, the AND gate, who actually converts it to a digital signal. So, is an AND gate starts the process of analog to digital conversion. But an AND gate can only produce 1 or 0. Once the signal goes in the counter, the counter lets out a digital output.

The digital outputs is dependant on its count. It does not depend on Va (from what I understand). Va is only important to know till which number the counter counts to. Because when Vx = Va the system restarts the count.

Did I get it now? And, I still don't get the point of Vcc.

So Vcc is a supply voltage to the AND gate, and has been drawn just for general reference and not for practical usages?

15. Apr 5, 2012

### I like Serena

Va is not coming out of the op-amp.
The op-amp is a voltage comparator that compares Va to Vx.
It's output is either high voltage (digital '1') or low voltage, probably earth (digital '0').

Huh? :huh:

Why would the system restart the count if Vx = Va?

It is true that when Vx=Va, the digital output corresponds to the analog value Va.

Neither do I.

It doesn't seem to serve any purpose, but it won't hurt either.
It would be more logical if they had drawn Vcc as being connected as a power supply to the op-amp, and if they had drawn earth as being connected as a (negative) power supply as well.

16. Apr 5, 2012

### Femme_physics

Really? So the supply voltages for the op-amps are:

http://img856.imageshack.us/img856/821/vccvccvcc.jpg [Broken]

Because there's no voltage based on the comparator formula of (V+) - (V-)

Last edited by a moderator: May 5, 2017
17. Apr 5, 2012

### I like Serena

Yep!

If there is no voltage difference between V+ and V-, the op-amp's output is zero.
That wouldn't reset the counter, would it?

Last edited by a moderator: May 5, 2017
18. Apr 5, 2012

### Femme_physics

Reset...well, no, not reset, but it would stop the system, UNTIL someone or something resets it. It'd be stuck without a reset mechanism.

19. Apr 5, 2012

### I like Serena

Well worded!

20. Apr 5, 2012

### Femme_physics

Oohh... then I think I have it figured out... I'll try to put the whole explanation in place tomorrow... thank you very much ILS. You so smart

xoxo