Re: HCT4051 leakage
- From: Joerg <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx>
- Date: Thu, 01 Dec 2005 21:10:00 GMT
Hello John,
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.
I don't know what the leakage mechanism is, that would be Jim's domain. However, unless I misunderstood something then 0.4uA out of 6mA is already 66ppm. So a few ten ppm seems like you are getting a good deal from those '51 chips. If they spec them at 0.4uA max versus 0.1uA for the other muxes then it's unlikely you'd be getting 0.1uA from a '51.
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?
Maybe one way to find out is to scoot the measurement window back and forth a few tens of milliseconds.
Whenever I wanted to know those kind of things from app engineers I was often told that such data is not available and that I should use this newer xyz gizmo chip. Which, of course, was an order of magnitude more expensive.
Regards, Joerg
http://www.analogconsultants.com .
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