Re: HCT4051 leakage

On Wed, 30 Nov 2005 23:56:35 GMT, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

>Hello John,
>
>> 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.
>>
>
>Actually channel-to-channel it's 0.4uA for the '51, less for the '52 and
>'53. Sez TI.
>
>However, the difference in RDSon from channel to channel can be up to
>10ohms or so. I have never seen that much in practice but if this
>matters can't you sink in a reference current instead of a voltage?
>

Putting R(ref) and R(unknown) in series, and measuring their voltage
drops, is a neat way to compute R(unk). The resistance range we can
measure goes from zero to infinity, and it's purely ratiometric on a
single super-precision resistor. Plus, we're summing the two measured
voltages as a sanity check on the entire 4-wire loop.

The small errors we're seeing may be due to cmos switch leakage, but
the error TCs are looking linear enough that leakage is probably not a
big issue... it should be sorta exponential on temperature, and we're
not seeing that.

Switch resistance doesn't matter here as long as it's constant for the
duration of the two measurements. We're taking about 130 millisec for
each measurement, 260 total. It might be that the current (about 6 mA
when we're measuring a 100 ohm RTD) is heating the cmos switch enough
to give it a r-versus-t curve that matters. 6 mA, 75 ohms typ, gives
around 3 milliwatts in the switch. The HCT switch is about 75 ohms and
increases about 0.25 ohms/K. So, what's the thermal coefficient of one
fet in an HCT4051? 1000 k/w maybe?

If it increases 3K as a result of the switched current, we'll have
0.75 ohms increase, serious by our standards. But what's the thermal
tau?

John


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