Why are you using 3V? It will make life harder.
As I said, I will use the comparators of a LPC1549 and this is a 3.3V device. So at least the analog switch has to be 3.3V compatible.
Analogue switches don't work if the voltages are outside the supply rails.
Sure, so I don't do that. Or do I?
HCT logic is compatible with 3V so you could use that to interface with the MCU.
Sure, I wrote that in my initial post I think. Problem is that are so many 74HCT4066 versions with different specs, I'd rather use the TS3A4751 anyway.
Why are you so bothered about the on resistance?
Only if the error is noticeable of course. Besides, I care more about a reliable/predictable resistance over the voltage and temperature range.
I could live with 60Ohm or so on resistance if they were stable. Then again, if I get 0.9Ohm for free, why would I use something higher instead?
The potential divider now has a total resistance of 12M at the maximum input voltage of 52.8V so the current is just 4.3583uA.
Are you referring to the 1st schematic? In the new one, it's like 3.2MOhm in den 52.8V case and 16.5µA for the right divider.
Even if the switch has a resistance of 400R, the voltage drop will be 1.743mV.
Yeah, but again: if I can get a TS3A4751 for about the same price and have an on resistance of 0.9Ohm, why would I use a 74HCT4066?
The leakage current is a much more important parameter, 1nA will cause a voltage drop of 3mV across a 3M resistor.
If the ADC is 12-bit, 1 count = 3/2^12 = 3/4096 = 732.421875uV
In order for the switch to drop under 1 count, its on resistance should be < 168R and the leakage current < 244pA.
Of course it may not need to be that good. It should be possible to calibrate the error out to some degree but it puts it into perspective.
Problem is that most analog switches have typical leakage currents of 0.5nA to 1nA. There is not really that much I can do about it.
Then again, I'm open to suggestions for better switches. I found one from AD, ADG712 as far as I recall, but it's much more expensive and has 0.5nA
leakage current instead of 1nA. However, tests with the spice model from AD were catastrophic.
I've just tested the old 2N7000 MOSFET. At room temperature the leakage current was difficult to measure, in the order of 500pA at a drain-source voltage of 51V.
A higher voltage MOSFET such as the BSS131 will have an even lower leakage current.
I'm too lazy/busy to look it up right now, but all the MOSFETs suggested up to now had somewhat gigantic maximum leakage currents compared to analog switches.
What about using a BJT? I tested the BC548 and could only just measure the leakage current (50pA) when the voltage was increased to over 50V:
the maximum rating is only 30V, a transistor rated to >50V would be better.
Honestly nothing I considered yet. Honestly I find it hard to believe that a plain vanilla BJT will have better parameters than a sophisticated analog switch.
Besides, there are so many BJT types on the market that I imagine it will be hard to find one with specifically low leakage current.
Problem is also that standard types are produced by so many vendors that it will be hard to make sure you really get the device fitting the data sheet
(just like for the 74HCxxx gates).
I can't be 100% certain of my readings because I don't have an ammeter that sensitive. I measured the current by connecting a Fluke 175 multimeter set to mV in series with the transistor. The meter has an input impedance of roughly 10M so 1mV of voltage drop corresponds to a current of 100pA. The resolution is 0.1mV which is a current of 10pA.
Well, measuring in the pA range is tricky. Besides, measuring won't give the worst case values (high temperature and such).