Accurate 0.1X and 0.01X Calibrations

[Conrad Hoffman]

Not quite audio, but we all have to calibrate things, or at least verify calibration, and having an accurate 0.1X and 0.01X divider goes a long way towards that effort. A KVD is nice, but out of range for many people. Here's the first draft of a new page that tells you how to build a Hamon resistive divider good to a few PPM if you use good resistors.

A Hamon Resistive Divider

Comments welcome-

Conrad

[johnferrier]

I just ordered a resistor kit with 4,000+ parts to build a pair of precision stepped attenuators using series and parallel connections. There is probably something to learn from your articles. Thanks for posting a link!

[johnferrier]

Learned that Excel 2010 x64 is the only version not bound by 2G of working memory by creating spreadsheets of 100,000 rows by 100,000 columns.

[Conrad Hoffman]

That's more cells than I want to contend with! At least the Hamon resistor in its simpler forms can be calculated with about a dozen cells. Not sure the configuration has any level control applications as I've always been able to do plenty accurate ratios with just a cheap DVM. OTOH, for calibrating the ranges of that cheap DVM, the Hamon is the way to go.

Conrad

[Conrad Hoffman]

Fixed a couple transposed digits!

CH

[johnferrier]

Okay, got a chance to read your web page and will reread with updates. I haven't determined how I can apply this yet, but it's interesting. Plan to read your published articles . . . perhaps this weekend.

-

Actually, have a pair of LTZ1000s and Vishay bulk foil resistors/trimmers (AD797s, MAT0x's) that could be used to build a nice mini-lab. However, only have inexpensive handheld DVMs. This could be interesting.


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[Conrad Hoffman]

The LTZ1000 is excellent but IMO it's hard to determine if the associated circuitry is up to the task unless you have something else very good to compare it too, possibly an ovenized reference bank of at least 3 standards. As far as the Hamon resistor, I used very good wire wound resistors, but my values are low enough that there's a few PPM drift if I put more than a couple volts on it. If/when I build another, I'll probably go for about 5X the impedance. The Vishay bulk metal foil resistors are better than anything out there, so they'd be the logical choice.

Conrad

[johnferrier]

Darn, I'd like to use the LTZs in something. I printed the page for the main circuit (Fig. 2) and look forward to reading more. I'm trying to think of a way to use this to tune the selection for resistors for the stepped attenuators. What that is, hasn't fully struck me . . . lol. What kinds of things do you use a mini-lab for? There must be a few non-obvious uses.

[johnferrier]

Okay, interesting stuff. Wonder though (besides whether you still have circuit boards), does the padding and/or using divide by four, rather than divide by five, degrade the performance much. Regardless, nice work.

[Conrad Hoffman]

Remember that quite a few years have passed since I wrote that. At the time I compared the reference to an ovenized standard cell bank for about a year. The only way to know what the long term drift is is to wait for it! The chip I used isn't made in a metal can anymore and the performance of plastic isn't nearly as good. I'm not sure what I'd use today, but usually if you take advantage of nulling and adjustment pins, the performance is degraded. Simplicity is usually your friend, so I wouldn't use any trims or padding. I want to know the exact value of a reference but it's not that important it be anything specific. Same thing with resistance and capacitance standards. They need to be rock stable but I don't care if they're some round number.

Today I'd probably spring for the LTZ1000 and just take super care with the passive parts. If feeling a bit poorer I'd try to scrounge up an LM399, as that has served very well in many decent instruments.

My boards were serviceable but not great works of art as they were hand made. Best bet is to roll your own.

CH