# Homework Help: Trying to answer a successive approximation question

1. Apr 8, 2012

### Femme_physics

The following component is connected to a reference voltage of Vref = 6V. The component is given the value (2C)16 to convert. Calculate Vout.

http://img69.imageshack.us/img69/7396/figuringout.jpg [Broken]

Basically I treated this supposedly successive approximation converter like any other converter is this calculation..hope it's valid?

Last edited by a moderator: May 5, 2017
2. Apr 8, 2012

### I like Serena

Looks good to me.

3. Apr 11, 2012

### Femme_physics

Thank you!

4. Apr 11, 2012

### Staff: Mentor

One tiny error...

5. Apr 11, 2012

### Femme_physics

What? :uhh: What could that be?

6. Apr 12, 2012

### Staff: Mentor

7. Apr 12, 2012

### Femme_physics

Which? I made way too many heh...

8. Apr 12, 2012

### Staff: Mentor

I've suggested they create another sub-forum, Femmes_Forum.

9. Apr 12, 2012

### Femme_physics

Lol ;)

10. Apr 13, 2012

### Femme_physics

Did I actually made a tiny error? Can you please point it out, I can't see it.

11. Apr 13, 2012

### Femme_physics

I need to provide an explanation of how this A/D converter with 8 bits work based on successive approximation...but I'm still not sure I understand how successive approximation works. According to my notebook,
"
In this type of converter there is a logic system that's comprised of a shift register. The logic system provides a logic '1' to DAC from the MSB till the LSB, although if the comparator outputs '0' the logic system reboots the bit and this way you get binary digit that gets closer to the final number in each period. And Vout gets closer to Vin.
"

...

But if the logic system REBOOTS the bit how come we get a binary number closer to the final number?

If I reboot my computer it just reboots my computer, it doesn't get me closer to any value.

12. Apr 13, 2012

### I like Serena

For starters, it's a DAC and not an A/D converter.

"Rebooting" the bit only means that it sets the bit to zero.
It's similar to rebooting your computer, which sets ALL bits to zero. ;)

13. Apr 13, 2012

### Femme_physics

Well they've made a mistake in the question...don't look at me.

Yes, but Successive Approximation graph is like this:

http://img85.imageshack.us/img85/7590/sasan.jpg [Broken]

How...does...it do it?

The explanation doesn't let me know that

Last edited by a moderator: May 5, 2017
14. Apr 13, 2012

### I like Serena

I am looking at you. :)
Your problem statement shows a DAC, but a successive approximation converter is an ADC...

It starts with the most significant bit, which corresponds to the number 128.
In your case the input has a higher voltage, so this bit is set, and we move on the the second bit.

The second bit corresponds to the value 64, for a total of 128+64=172.
In your case the input has a lower voltage, so this bit is reset (or "rebooted" if you will).
So we remain at 128 and try the 3rd bit next (corresponding to 32, for a total of 128+32=160).

And so on...

Last edited by a moderator: May 5, 2017
15. Apr 13, 2012

### Femme_physics

So it's basically doing guesswork based on constant numbers?

I don't see how could it be necessarily faster than the continuous method?

Ah..you're right, the question is not about the pic I posted

16. Apr 13, 2012

### I like Serena

Not guess work, but successive measurements which determine each bit.

In 8 steps the analog input signal is converted into the 8-bit number 175.

The alternative method that counts from 0 up, would need 175 steps to count up to 175.

Now I'd say that 8 steps is a marked improvement over 175 steps.
(It's a gem in electronic engineering. ;)

17. Apr 13, 2012

### Femme_physics

Why do we even use the continuous one if it's so crappy? A matter of cost?

18. Apr 13, 2012

### I like Serena

I think it's a matter of exercise and education.

If they'd give you the schematic of a direct conversion ADC, that'd be pretty overwhelming.
But the principle given by a count-from-zero circuit makes the material understandable.
And ultimately it does the same thing, just slower.

19. Apr 14, 2012

### Femme_physics

Ah...understood. :) Perfectly.... I like it. Thanks. Do you know what made Nascent Oxygen say my calculation was wrong?

20. Apr 14, 2012

### I like Serena

I believe NascentO2 meant there should be (28-1) in the denominator instead of 28.
As far as I'm concerned both would be okay.

Since your teacher gave you 28 in the denominator, I'd say there was no error.

21. Apr 14, 2012

### Femme_physics

Just to clarify,

8 bits =256

10 bits = 1024

etc etc...?

Gotcha :)

PS how fast am I?

22. Apr 14, 2012

### I like Serena

Yes.

Wheeeeeeeeeeeeeeeeeeee! :D

23. Apr 14, 2012

### Femme_physics

hehe..you rock!

When I say:

8 bits =256

10 bits = 1024

I know the left side is called bits...what do you call the right side? Number of values?

24. Apr 14, 2012

### I like Serena

Yes.

8 bits means that the number of possible values is 256.

Furthermore the most significant bit represents the value 128.
And the least significant bit represents the value 1.

25. Apr 14, 2012

### Femme_physics

Thanks for the clarification...ohh a big blog update is coming up soon :)