What's the cheapest way to make a dithered-input ADC?

Question

I know that some of these PICs have ADCs built in, and I may want to use that.

I am wondering if there is a way I can effectively dither the input to a low-bit (like 8 bits) ADC with the fewest parts (like no unnecessary op-amp) and all cheap and widely-available vanilla-flavored parts.

The voltage measured might be DC or very low frequency, likely always less than 10 Hz. The effective sample rate might be as low as 50 Hz, but this is after averaging the 500 kHz samples from the ADC.

The ADC was just randomly picked because it looked cheap. The Zener was chosen to be less than 5v breakdown. The resistors are guesses.

My questions reside among the Zener and associated resistors. What's a good cheap Zener to use for a noise source? What kinda r.m.s. AC noise output can I get from some cheap Zener? I want this voltage to be somewhere around the LSB voltage of the ADC which is VREF\$\cdot 2^{-8}\$ or VDD\$\cdot 2^{-8}\$ I do not want to need more than +5v and GND power supply.

The idea is to get, cheaply, much better than 8 bit precision, by dithering and averaging. Anyone do this before? What can I gain from your experience?

^{simulate this circuit – Schematic created using CircuitLab}

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Denouement:

Just FYI: I finally found a reference of someone measuring noise output of zener diodes. It looks like the noise from a zener in breakdown is about 1.5 µV which is far less (about 80 dB) than the 19 mV LSB and won't do much to dither unless one were to amplify it, which makes it less cheap (and I wanted to do this on the cheap).

So Tony EE "RocketScientist", that is the reason the circuit will "certainly not" work. And you didn't say a word why that's the case.

Answer 1

This is not a direct answer to your question, but addresses your actual problem.

This is probably not the best way to achieve what you want:

Many, probably even most, PICs come with internal A/Ds. Very likely you can find a PIC with built-in A/D that is cheaper than your "cheap PIC" plus external A/D. The result will also be smaller, and the A/D easier to drive in the firmware.

Before you think about dithering a low resolution A/D, consider:

1. Do you really need to? Noise is everywhere. Often there is enough random noise, or even non-random noise below half your sampling frequency, so that explicit dithering is unnecessary. Just sample a lot faster than you need, and low pass filter the result. I do this routinely. And yes, this is usually with A/Ds built into PICs.

2. Use a higher resolution A/D. You say you are using a 8 bit A/D. Very low end PICs come with 8 bit A/Ds, but many also have 10 bit A/Ds available. Instead of hoping to pick up another 2 bits with dithering, just get a A/D with two more bits in the first place. Most newer PICs nowadays even have 12 bit A/Ds, although usually not in the absolute lowest cost models.

Answer 2

You don't need to use a noise source, you can generate a square wave and filter it to be a triangle wave. Make the amplitude just a little more than 1 LSB and make the frequency significantly above your highest to be digitized but not so high that you lose time resolution with your base sample rate: -

The triangle wave causes the ADC's D0 bit to be 1 for about 20% of the time when Vin is zero. If Vin increased from 0 volts to say 0.25 LSB, D0 would be 1 for a longer period of time. This is a form of dithering and is probably more easily translated to a practical solution because you can use a spare IO generating a square wave and RC filter to a approximate a low amplitude triangle wave.

There are many variations of what I've decribed above and my suggestion is just an example. For instance, here's an article that gives a little more detail: -

It shows an analogue signal of amplitude 116.357 dithered with a triangle wave producing a count of 117 for 35.7% of the time and 116 for 64.3% of the time. So if you have good time resolution in your sampling you can trade this against poor amplitude resolution.

It's the same idea for sigma delta ADCs - you counter poor amplitude resolution by having great time resolving ability. It also goes right back to early ADCs that used a digital ramp and a comparator: -

In effect, the digital ramp generator is the dither signal and the comparator is resolving whether the net signal of dither plus Vin is above a certain threhold or not.