Analog to digital converters

There are several designs. Why not start with the obvious place...

for an over view of the varios techniques..

Dave

Thank you.

I like to figure things out. And as these ideas occur to me, I like to see if I'm correct. Wikipedia locks up my Dolphin web browser. :-(

Thank you, Rick C. Hodgin

I was able to look at it on my desktop computer. Thank you for the assistance. :-)

Thank you, Rick C. Hodgin

You can see there are many methods - none of which fits very well with the description you wrote, as far as I can see.

Successive approximation ADC's are the most common for general purpose ADCs. Flash (direct conversion) ADC's are used for very high speed, and Sigma-Delta is the usual method for high resolution (such as audio ADC's).

Single or Dual-slope integrating ADCs (very slow) are more like the

555 timer approach (if I understand the OP correctly). These are used where speed of conversion is not important, but high resolution and noise immunity are. You might find them in a digital multimeter. I used one in a blood chemistry analyzer. Single-slope is less common. Dual slope works by integrating up on a reference input (constant up slope), then down using the input signal (variable down slope. The time to integrate down to the low threshold is measured to give the ADC output. The up integration time is fixed (there is no high threshold). This allows the output value to be independant of the integrating capacitor value as long as the cap is large enough to prevent saturation during ramp up.

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Gabor

Oops! I got that backwards. Up slope is fixed time but rate is based on the input signal. Down slope is fixed rate set by a reference and time is measured to get ADC value. The way I originally described it would end up with a 1/x factor in the conversion value. In either case, the circuitry is quite simple since the control logic is mostly a big counter. The timing is generally provided externally, as is the integrating capacitor, which can be quite large and generally wants to be polypropylene or similar dielectric to reduce effects of surface charge storage (rebound).

Last time I used one of these was back in the early 1980's.

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Gabor

Here's a data sheet link for a more recent dual-slope ADC chip. It's a bit fancier than the one we had in the 1980's but the main idea is the same:

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Gabor

One of the advantages of dual slope conversion is that zero input gives a zero reading. Or do they have a separate offset circuit for this? I know many tout such in their designs.

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Rick C

[snip]

The MicroChip part in the link has an additional "auto-zero phase" that removes any offset. See section 3.1.1 in the data sheet.

Another advantage for DC measurement is that the conversion is taken on the integral of the input voltage over some fixed time period. This makes it easy to filter out AC noise including power line noise. The the blood chemistry analyzer we used a multiple of the line period (50 or 60 Hz) as the integration time. A sampling time of

0.1 second would remove line-frequency noise from 50 or 60 Hz power. This is useful in a handheld multimeter, where the leads are long and can pick up noise easily.

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Gabor

Pure flash ADCs are rare these days; they need too many power-hungry comparators. Most fast ADCs are pipeline architectures with radical levels of digital calibrations.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics