Sigma Delta ADC or Dedicated ADC Chip

william chan · 2008-03-12 01:14

This may seem like a stupid question....

Which is better, more accurate, faster, more flexible, less noise and uses less code?

1. Sigma Delta ADC using 2 pins on the Propeller or
2. Using a dedicated ADC chip connected to the Propeller.

I am very surprised with the flexibility of the Sigma Delta ADC code to change the sample bits easily.
Will the feedback pulses from the Sigma Delta ADC add noise to the source signal? ( Mic )

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Mike Green · 2008-03-12 01:34

The Sigma-Delta ADC using 2 pins on a Propeller works pretty well, but is not optimized.
A dedicated ADC will always be better, can be faster, probably has less noise. More flexible
depends on what you're comparing. Some external ADCs may not be very flexible at all.

Neither the Propeller-based ADC nor most external ADCs will take much code to implement.
It's hard to compare them on this.

The feedback pulses will add noise to the signal source. How much noise and whether that
will make much difference will depend as much on layout and care in filtering and shielding
as it does on the method used for ADC.

Paul Baker · 2008-03-12 01:53

For AC coupled signals such as a microphone (ac coupled means there is a capacitor between the pin and signal), Sigma Delta pretty well. But for signals which don't change or change slowly the sigma delta is quite noisy and really only suitible for ballpark measurments such as reading a pot. It's a little hard to measure bit accuracy in ac since the signal is always changing, but as you've noted you have control over the speed vs bits with sigma delta. If you have a demo board you can take a look at the actual data generated in one of the microphone_to_... examples on the object exchange.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Post Edited (Paul Baker (Parallax)) : 3/12/2008 1:58:29 AM GMT

william chan · 2008-03-12 02:54

I would think that using I2C or SPI to read samples from a dedicated ADC chip would be slower than doing the Sigma Delta ADC itself.

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Peter Jakacki · 2008-03-12 03:04

I have use an 8 channel 12-bit ADC in one application where I wrote a driver in assembler to scan the inputs and write the results to a buffer. I have some extra delays in there but if I take them out it can run at about 100,000 samples/second spread over the number of channels selected. There are some other tricks to make it go faster but the A/D chip max's out at 100ksps and is certainly needs to be slower at 3.3V. So for 8 channels this requires only 3 I/O pins and also has the advantage that it is DC accurate.

*Peter*

Rayman · 2008-03-12 14:58

I just want to point out that many dedicated ADC chips use the sigma-delta technique...· So, it's really a question of a homemade DIY circuit with discrete components versus a monolithic commercial product...

mynet43 · 2008-03-12 15:04

I need a simple adc input to sample a volume control pot.

I'd rather not use a dedicated chip like the MCP3208.

Would someone please point me to the simplest circuit and supporting code?

Thanks for your help.

Jim

Robot Freak · 2008-03-12 15:39

A while ago, I wanted the same thing with a Javelin Stamp.
Peter Verkaik then posted the following circuit:

                    +---------------o Javelin ADC in
                    |
Vin o-----[noparse][[/noparse]10k]-----+-----[noparse][[/noparse]10k]-----o Javelin ADC out
                    |
                 [noparse][[/noparse]0.1uF]
                    |
                   -+- GND

So somebody with knowledge of the Propeller could write code to generate a PWM signal on the ADC out, and then read the state of ADC in.

Kind regards,
Robot Freak

Mike Green · 2008-03-12 15:46

mynet43,
Have you looked at the AN001 application note? This describes the sigma-delta ADC we've been talking about including code to make it work. You can find this off Parallax's Propeller page. Go to that, then Downloads, then you'll see the link.

Robot Freak,
william chan also posted a slow simple ADC that works much the same way: http://forums.parallax.com/showthread.php?p=714954.

Post Edited (Mike Green) : 3/12/2008 3:53:59 PM GMT

Rayman · 2008-03-12 16:14

I think you want 2 caps where the resistors meet. One to Vdd and one to Gnd. This reduces effects of noise on power supply rail...

Beau Schwabe · 2008-03-12 16:42

"I think you want 2 caps where the resistors meet. One to Vdd and one to Gnd. This reduces effects of noise on power supply rail..."

It also has a way of balancing out any leakage effects within the capacitors when you use two·that are identical.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Peter Jakacki · 2008-03-12 21:59

mynet43 said...
I need a simple adc input to sample a volume control pot.

I'd rather not use a dedicated chip like the MCP3208.

Would someone please point me to the simplest circuit and supporting code?

Thanks for your help.

Jim

Although you could use an ADC to read a pot there is another way and that is simply to add a capacitor and treat the circuit as an RC timing circuit. There are numerous examples of doing this on the prop but simply it's a matter of either charging/discharging the cap and measuring the time it takes to switch levels. So your pot would be a two terminal device variable resistor in this instance instead of a three terminal voltage divider. BTW, only one I/O pin is required.

*Peter*

P.S. mynet43, you should ask this on a new thread as this thread was started by William and you could end up "hijacking" the thread which is a big no-no.

Post Edited (Peter Jakacki) : 3/12/2008 10:54:07 PM GMT

RinksCustoms · 2008-03-13 05:38

Peter Jakacki said...

mynet43 said...
I need a simple adc input to sample a volume control pot.

I'd rather not use a dedicated chip like the MCP3208.

Would someone please point me to the simplest circuit and supporting code?

Thanks for your help.

Jim

Although you could use an ADC to read a pot there is another way and that is simply to add a capacitor and treat the circuit as an RC timing circuit. There are numerous examples of doing this on the prop but simply it's a matter of either charging/discharging the cap and measuring the time it takes to switch levels. So your pot would be a two terminal device variable resistor in this instance instead of a three terminal voltage divider. BTW, only one I/O pin is required.

*Peter*

P.S. mynet43, you should ask this on a new thread as this thread was started by William and you could end up "hijacking" the thread which is a big no-no.

look for the·labs sticky in the main prop forum, then go down to the counters lab. One experiment goes into detail about using the counters for sigma delta in an RC/Time sort of way, minimal coding, good results.

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william chan · 2008-03-13 06:46

Quote :
P.S. mynet43, you should ask this on a new thread as this thread was started by William and you could end up "hijacking" the thread which is a big no-no.

Don't worry, any topics about ADC is fine on this thread.

Does anyone know what happens if the capacitor value is too small?
Answer : The sampled value will hover around 0.5 of max value.

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mynet43 · 2008-03-13 15:15

I certainly didn't mean to hijack the thread. This thread seemed pretty close to what I needed.

Mike's answer pointed me to the answer I was asking for. Other answers pointed me to even simpler solutions.

Thanks all for the great support!

Jim

Sigma Delta ADC or Dedicated ADC Chip

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