# a/d converter,

Discussion in 'Electronic Basics' started by [email protected], Jun 28, 2006.

1. ### Guest

Hi, i am trying to create an a/d converter, however i don;t know where
to start, I know digital logic and i know electronics, i' have already
done this in the past for school lab but i just can't seem to get it to
get all the part required. i need to create a a/d converter with a
resolution of 3 bits after the decimal.
thank you

2. ### John PopelishGuest

3 bits resolution means that the converter divides the input signal
into one of 2^3 or 8 quantizations. For instance it might measure a 0
to 5 volt signal in .63 volt steps.

If, instead, you need a converter that quantizes a voltage to .001
volt (3 decimal places) is another thing, entirely (unless the full
range to be quantized is only .008 volts).

Tell us more about what you are trying to accomplish.

3. ### EeyoreGuest

" with a resolution of 3 bits after the decimal "

You're mixing decimal and binary.

I suspect that your statement " I know digital logic and i know electronics " is
lacking merit.

Graham

4. ### prateekGuest

You need a counter type ADC which essentially is
1. a pulse generator. The pulse duration determines the conversion
time.
2. Gate and control cct.
3. binary counter. No of bits is determined by the maximum voltage to
be digitised and the resolution required.ie. one count will be
equivalent to the resolution voltage.
4. level amplifier.
5. binary ladder of the same no of bits as the counter.
6. one comparator.
Suppose you want to digitise 5 volts with a resolution of 0.001V, so no
of levels is 5/.001= 5000. so you need 13 bit counter. (2 ^12 = 4096
and 2^13 =8192).
hope this helps
njoi
prateek

5. ### BobGGuest

You can make an a/d converter out of a microcontroller, a comparator,
and an RC circuit. You time how long it takes the rc voltage to charge
up to the voltage being measured (the comparator sets at this point).
The equation for rc charging is known, so the voltage can be computed.
Of course, no one does this since the microcontroller has a nice 10 bit

6. ### Bob MastaGuest

Please explain more: Are you looking for a circuit that
uses an A/D converter chip, or do you really want to
do the conversion "from scratch" as others have assumed?

Best regards,

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator

7. ### Guest

Well i am going to have to start from scratch.
I would have POT that would regulate 5 volts and i need to get it as
accurate as possible. The resolution of the the ADC should be 2^9=512,
which would give me a quantizes a voltage to .01Volts

Diagram:

POT ----> ADC converter Chip ----> BCD priority encoder -------->
Parallel port.

Does this make sense ?
Does he ADC need a frequency to operate. i am not sure because the ADC
would requires sample therefore by increasing the frequency you have
increasing the sample rate or can you get away without adding a
frequency to the IC ?

8. ### Bob MastaGuest

The problem with A/D-to-port schemes is usually that they
require a lot of tricky clocking schemes. For example, I
have an old (obsolete) Adnav port A/D that uses a MAX398
chip. But it sends its data out serially, so the port lines
have to be toggled laboriously to fetch each bit.

If you had a parallel output A/D you'd still need to do some
handshaking since the port normally wants to see only
8 bits at a time (though you can use the Status port to
gain extra bits).

If this is for a production application where you want to
keep the cost down, and you have a microcontroller
at your command, there are some pretty simple
dual-slope schemes. A single-slope scheme may be
all you need, however: Make a simple current source that can
charge a capacitor linearly. Feed the capacitor voltage to
one input of a comparator; the other input is the voltage
to be measured. The port must supply a line to short out
the capacitor (through a NPN transistor, say). When you
turn this line off, start a counter in the microprocessor.
When the comparator trips, it should cause an interrupt
whereupon you read the counter. Depending on the
charge rate of the capacitor and the internal counter
rate, you can get really high resolution from this simple
circuit. (A dual-slope is a bit more involved, but has
much better rejection of power-line harmonics if you
do it right.)

Hope this helps.

Bob Masta  