Sample & hold error

The only issue I'm seeing running the cct single ended is the inability for Vout to go rail to rail, and of course the switching tr on the input means the input doesnt either. Those should be livable with in this app.

NT

Tr base R only allows very little i into the base & out of the emitter. It causes a little unwanted emitter i that's dealt with by the following R to ground. The current from the input terminal is a good 50x as high as Ibase. So with 0v on the input terminal and Vcc on S, opamp +in sits at under a volt, just creating an input offset.

NT

accuracy

apx

worse

feeling

to

and

four

ADC

An R-2R ladder avoids precision resistors.

?-)

Only that it can't work.

John

On Sun, 02 Oct 2011 10:02:44 -0700, John Larkin wrote:

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56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R50 SYMATTR Value 100k SYMBOL npn 880 -304 R270 SYMATTR InstName Q5 SYMBOL npn 880 -16 R270 SYMATTR InstName Q6 SYMBOL npn 880 272 R270 SYMATTR InstName Q7 SYMBOL npn 880 560 R270 SYMATTR InstName Q8 TEXT -232 792 Left 0 !.tran 3

-- JF

You went to a lot of effort to demonstrate that it doesn't work.

John

Please explain how it doesn't work, John. I'm interested.

Thanks, John S

. . .

You could do a nice analog version using three not-too-expensive parts. Relay, cap, opamp. A few more if you don't like the relay.

John

Certainly. A 5-state thermometer code driving a binary-weighted

And if ADCs were this simple, don't you think somebody would be using them by now?

John

People used to make pure flash ADCs, which were thermometer codes followed by binary encoders. I worked for a place that sold a 6-bit 50 MHz version, which took 63 separate comparators. I don't know how far anybody ever pushed an IC flash converter, 8 bits certainly (255 comparators) and maybe even 10. Nowadays most fast ADCs are pipeline types.

Well, it is a discussion group. Think how dull it would be if everybody always agreed.

John

uracy

=3D apx

e worse

eling

ime to

our

e to

es and

lers

four

g ADC

gate

eap

al

ith

as

g of

or

Oops! I'm working on a variation to see if something similar could work. No promises after that last attempt. :)

NT

When you get into really long hold times, you have to worry about the tempco of the capacitor. Since the charge is constant, temperature changes make the voltage go up and down. Over a day that can easily dominate the error budget.

Cheers

Phil Hobbs

Is that really going to hold to 1% per hour?

NT

I make errors now and then, and cheerfully admit to them. You must not have noticed.

When I do serious engineering, I check my work exhaustively, and check it again, and then I have other people check it, too. I spent the entire weekend checking a PC board schematic and layout. A total of five people are checking this board, and maybe we can Gerber on Wednesday. But this is just a newsgroup, and nothing really matters here.

Wanna see the board?

John

worse

to

and

It wouldn't be hard to do it analog. Otherwise, I'd go for an ADC and a DAC.

There is another trick: a DAC, a comparator, and either an up/down counter or some SAR logic. After all, an ADC plus a DAC totals up to two DACs, and you only really need one.

John