HCT4051 leakage

Clock it?

I don't think I have a charge injection problem. After I configure the switches to put the two resistors in series, I wait 4 milliseconds before I digitize Rx (which takes 125 millisec) and then digitize Rref, another 125. Since the worst node has an impedance near 100 ohms, and the capacitances are small, the RC tau is way under 1 usec. My calculator won't do e^-4000 but I'm guessing it's pretty small.

Hmmm... connecting the resistors does apply a load, about 6 mA, to my

2.5 volt reference. The same ref drives the ADC, so it's all ratiometric, *unless* the ref has a transient bounce from the 6 mA load step. Now I must need one of those super-low-noise multi-fet amps to analyze my voltage reference bounce!

John

(Hmmm, my last post got lost somewhere in usenet limbo for a week or two)

This is an AD7793 24-bit delta-sigma. It has a mode where you can trigger it to digitize. It takes twice as long to deliver data as in the continuous mode, but has no memory of the previous input.

We're not seeing crosstalk between channels, just nonlinearity. We'll get a few more boards nest week and, if they all have the same trend, I'll just stick a polynomial into the firmware to uncurve the data and never understand it.

John

If you do an experiment where you greatly slow down the cycling of the MUX, is it still bad? I thought that Sigma-delta ADCs aren't generally supposed to work well when muxed, their digital filters contain history about old input signals.

Chris

Fair enough it sounds like the fact that it is sigma-delta is nothing to do with the problems you are seeing.

By the way, are you able to measure the voltage of the RTD and precision resistor with no current flowing also? Perhaps there is a thermal EMF or something like that. If it is leakage in the ESD diodes, then if the voltages were similar leading to similar leakage with and without the current flowing (hard to arrange?) then it might be possible to compensate for the leakage that way.

Chris

Followup: we never found the cause of the nonlinearity in the resistance measurement. It's not leakage, because it's not very temperature dependent (our resistance measurement tc is 3 PPM/K!)

So we just fixed it:

MOVE.L RTCAL.W, D7 ; CONTEMPLATE FINAL CAL FACTOR... MULU.Q D7, D3:D4 ; D3 IS RESISTANCE IN OUR FORMAT!

; OK, WE HAVE THE RESISTANCE IN D3 H:L AS OHMS:FRACTIONAL OHMS. ; BUT THERE'S AN AS-YET UNEXPLAINED ERROR CURVE: HIGHER ; RESISTANCES READ TOO LOW, BY ABOUT 120 PPM AT 1500 OHMS. ; SO LET'S DO AN R^2 KLUGE, WHICH SEEMS TO FIT PRETTY WELL.

; THIS IS A SMALL TWEAK, SO WE'LL JUST NAB THE INTEGER OHMS, ; SQUARE, SCALE, AND ADD THAT IN.

; 120 PPM OF 1500 OHMS IS 0.18 OHMS, WHICH LOOKS LIKE ; 11,796 IN OUR FORMAT. SO IF X IS THE FUDGEFACTOR,

; 1500^2 * X = 11796 ; ; X = 0.005243 AS AN UNSIGNED FRACTIONAL,

X = 22517998 ; AS A LONG

MOVE.L D3, D5 ; COPY RESISTANCE SWAP.W D5 ; MOVE 'OHMS' PART INTO D5.W MULU.W D5, D5 ; AND SQUARE THAT... LONGWORD NOW. MOVE.L # X, D7 ; NAME THE FUDGE FACTOR MULU.Q D7, D4:D5 ; DO A FRACTIONAL MULTIPLY INTO D4 ADD.L D4, D3 ; AND BLEND IN THE CORRECTION.

; NOW CONVERT RESISTANCE TO TEMPERATURE

That's the great thing about being an engineer: you don't have to understand it, you just have to make it work.

John

We've tried a large number of boards (three, actually) and they are all close. Seems to me that, even if parts change later on, doing this is better than not doing it. My error budget is 250 PPM, so buying back 120 is pretty good.

We test every board, so if things change we'll know it.

Spoilsport!

John

[snip]

Gaaack! Until the next production run doesn't work ?:-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Any "patch" that works, but I don't understand why, always scares the hell out of me. I don't make three at a time, I make thousands :-(

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

I could have put that X factor into the module cal table, which is poked during production test and saved in eeprom. But a certain lazy youth, who's writing the cal software, talked me out of it. Hell, I could have put a whole mess of polynomial terms into e-square, and tuned the hell out of it.

A gimmick like this can be handy for, say, halving a residual error. Trying for a factor of 10 improvement would be pushing our luck.

I think some fancy chips like delta-sigma adcs have similar hidden cal tables. I don't know if anybody goes for higher-order terms. Don't you do zener-zaps and equivalent things?

John