Re: HCT4051 leakage

On Wed, 30 Nov 2005 14:31:49 -0800, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

>Hi,
>
>Has anybody measured typical leakage currents for an HCT4051 analog
>mux? I'm wondering about both ESD diode leakage (ie, to rails) and
>leakage through open switches. I'm running 0 and +5V rails.
>
>Indications are that everybody's 0.1 uA max spec is wildly
>pessimistic, but I was wondering if anybody knows more, before I have
>to drag my *** into the lab and make actual measurements. It's a lot
>easier to sit here and type and eat bon-bons.
>
>We're scanning eight RTDs. A +2.5 volt reference goes through a
>precision 270 ohm resistor and gets mux'd to a selected RTD. The
>voltage drop across the RTD gets differentially mux'd, too. A 24-bit
>delta-sigma ADC digitizes the voltage drop across the 270, then the
>voltage across the RTD, and does the math. We're getting errors in the
>tens of PPM, tolerable, but we're curious where they're coming from.
>The sensitivity analysis math here is a nuisance.
>
>
>John


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



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