Ramp Type Oscillator with Bottom Release

Run it into some edge and sense dV/dt. (?)

George H.

What's wrong with doing it exactly the same way?

Gm is smaller so you need more gain on the resulting current signal. You're making a kind of comparison, regardless.

Tim

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"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

So don't use it to sense the cap voltage directly against a voltage reference that's near the rail. Make a copy of mirror/CCS Q7 and its resistor that just sends current to ground (or use it as the load for Q2), and sense charging current differentially wrt to the reference mirror, from the source resistor/source junctions. By adjusting the emitter resistor ratio you should be able to set the "trip point" anywhere you like...

Yay!

Q4, rather

Can you run a little charge pump with mux in there so a tiny capacitance gets charged up to a bandgap and then added to zero? That way you should be able get even closer to zero than with you bipolar circuit.

However, you used the word "dump", which is similar to the word "fired", which ... :-)

The "way" detected the jump in Ib. No such current exists in CMOS. ...Jim Thompson

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"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

So? Fake a BJT by adding a diode-strapped MOS in parallel with it? Add a diode-strapped MOS from gate to drain (Baker clamp, in a sense)?

Bias it up with diode-strapped-MOS's so you're not comparing against zero?

You tell me, you're the self-proclaimed expert. I thought only liberals wanted hand-outs. ;-)

Tim

I'm curious as to what Chrysler the BJT version was for

Fricking amateur. Plonk. ...Jim Thompson

Use diffamps and a flipflop? Transistors are free in an ic, right?

Sensing near ground with CMOS is iffy. The offset voltages aren't as stable/reasonable as with bipolar.

I'll post a bipolar example in the morrow. I was hoping that there'd be a _dual_ effect with CMOS that I was missing. ...Jim Thompson

Well I got me a Chrysler, it seats about 20 So hurry up, and bring your juke box money!

How low an offset do you need? A few millivolts ought to be feasible without any fancy tricks, in a decent old process (e.g. 0.25um) with moderately large devices. Below 180nm I'd agree that achieving good matching is difficult. Do you have matching data for your process so that you can calculate how big the devices need to be?

If you really need much better than a millivolt 1-sigma then some sort of auto-zero circuit should be able to achieve this at the cost of complexity. At least in an oscillator there are probably times in the oscillation cycle when you don't need the comparator and could switch it out to auto-zero it. There are quite a few tricks to getting switched-cap circuits working well though, and whilst I saw some of them I never learned them myself.

Maybe use the charge cap method I mentioned in my other post. Gets you away from the rail with the sense node.

Other than Vgs having a much lesser effect when reaching close to full channel conduction I wouldn't know any effect.

There are some damned good CMOS-front-end RRIO opamps.

This is an implanted medical application... I have only 500nA to run the whole she-bang :-( ...Jim Thompson

-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

I'm looking for work... see my website.

Thinking outside the box...producing elegant & economic solutions.

How insensitive does the bottom-sensing need to be to supply variation/different cap values?

That kind of rules out a charge pump solution. Can you have it measure the offset from time to time and store that in a cap if things must remain analog?

lookie, lookie, 8 MOSFETs. It "bottoms" at about 8 mV. But he never mentioned the 500nA thing.