simplest possible voltage doubler?

I can't figure out how the O.P. is worrying about fractional percent resolution and accuracy with the signals coming from pots. I haven't seen many pots with fractional percent linearity, repeatability or noise. Seems like garden variety 1% parts should keep up with the pots.

Like I said, Farnell carry these parts.

When I went to the Farnell website -

The great advantage of the thin film dividers is the ratio tracking - the gain is about ten times more stable than you will get from discrete

----------- Bill Sloman, Nijmegen

Like I said, Farnell carry these parts.

When I went to the Farnell website -

The great advantage of the thin film dividers is the ratio tracking - the gain is about ten times more stable than you will get from discrete

----------- Bill Sloman, Nijmegen

Could you change the ADC reference to 2.048V ? Then you wouldn't need an amplifier at all You'd need lots of local C to get the noise down, but since the signal is from a pot I doubt the time constant would matter

Look into Microchip MCP6S26

It has a programmable gain of 1, 2, 4, 5, 8, 10, 16, 32 and has a front end

They also come in single channel MCP6S21.

Paul E. Schoen, President P S Technology, Inc.

Vishay says they'll guarantee 5 ppm tc tracking on their 0603's if all the parts come from the same reel. But their delivery is insane, too. So we'll just buy a reel of somebody else's 10 ppm parts.

We were playing with the math of making various inverting and non-inverting gains from a given number of identical resistors; lots of combinations emerge.

John

The spectroscopy boys have another cute trick: use a dac, and add a random offset to the analog signal at each sample, and then subtract it out digitally. This results in sub-bit resolution and essentially perfect differential linearity, even from a fast sar adc that's not inherently very good.

John

The spectroscopy boys might have got this from the audio boys.

John Watkinson's "The Art of Digital Audio" (ISBN 0-240-51270-7), which was published in 1988, credits this to

Blesser B. "Advanced A-D conversion and filtering: data conversion" in Digital Audo, edited by BA. Blesser, B. Locanthi and T.G. Stockman Jnr., pages 37-5, published in New York by the Audio Engineering Society, in 1983. Stockman has published some good stuff in the area.

------------- Bill Sloman, Nijmegen

Can you sketch a schematic of this? I think that if you use only two resistors, with no active part, you are going to lose even more dynamic range. Besides, don't you need some electronic switch to alternate between the two configurations (+0 mV and +2 mV)?

In article , John Larkin wrote: [...]

If you add a high frequency boost to the anti-alias filter, you can compensate for the digital side's low pass. This high frequency boost is often enough to boost the noise above one LSB.

A high frequency boost also tends to give you the best sort of noise for the job. You really want all of the noise's energy to be above the cut off frequency of the low pass filter you are using. Any noise below the cut off just adds noise to the measurement without helping to smear out the lower bits.

If the ADC is part of a servo system, there normally is a natural low passing action as part of the design. In these cases, you may not even have to add a filter to the design. The zero you add in analog land can help to make the phase margin of the servo loop as well as removing the cogging effects of the LSB.

In article , John Larkin wrote: [...]

Yes but there is a down side to this. The spectral resolution is degraded as the linearity is improved. The analog and digital values do not match to the same degree as there is a nonlinearity in the converter.

Never tried that; wouldn't the 10-bit quantizer have to be accurate to

I have dithered a 7-bit ADC to well over 16 bits accuracy in an electrical metering system, digitizing AC waveforms.

You're not flakey when everything that you do works.

John

"Seems like" is what fooled the first people who tried it. Read "Dithered Quantizers" by Robert M.Gray and Thomas G.Stockham Jnr. in the IEEE Transactions on Information Theory vol. 39 pages 805-812 May

--------- Bill Sloman, Nijmegen

Seems like the uniform distribution would in effect be a real time Monte Carlo experiment allowing you to interpolate the sample estimate using Mahalanobian weighting. Maybe you are averaging a different statistic. Maybe flakey-pooh can add a single comparator and RC for a delta-sigma modulator with 10b quantizer which ought to be good for 14b effective resolution.

You can improve DNL if you dither a lot, several LSBs at least, and average samples. Without the averaging, you obviously can't fix gross errors like missing ADC codes. The dither-average thing is often convenient when you have an ADC that can sample a lot faster than you really need to acquire the signal.

The other place where dithering is good is in electric meters, where the dynamic range requirement for current measurement is extreme.

John

That sounds like sliding scale A/D conversion which uses DAC summation to step the input sample through each and every amplitude bin for total elimination of the differential non-linearity error, resolution remains the same. The goal is to avoid going to a higher resolution converter in order to achieve a given THD specification, which would be the only other option for non-linear distortion reduction.

In article , Fred Bloggs wrote: [...]

There is another method that reduces THD:

Let say you have a fast ADC that has to scan a bunch of inputs and convert each channel. By adding dithering, you can remove the non-linearity caused by the LSB but not the differential non-linearity of the ADC.

You can add an extra channel to the scan and use it to bring in a very good quality ramp with the same dithering. Based on this extra input, you create a table of actual values.

When you ADC gives you, lets say, 2F7H, you look in this table for the

The best ramp is going to be one produced by a DAC because you have tight tolerances on precision of voltage versus time, so it sounds like you're going to naturally evolve this into a sliding scale conversion anyway.

In article , John Larkin wrote: [...]

There is a limit on the dithering placed by the allowed noise. When you make the dithering many LSBs tall, it and the differential non-linearity of the ADC ends up raising the noise floor.

Yes and many meter like situations. Any place where the bandwidth of the final output is low and lots of bits are wanted are good cases for dithering.

Those folks that do the super low frequency seismic monitoring most likely do dithering. Chances are they don't actually inject the dither. They just allow the higher frequency noise from the environment do it for them.