simplest possible voltage doubler?

Hi - I need to take a signal coming from a 3K potentiometer that ranges from 0.44-1.77V and double it (so I'd have an output of 0.88-3.54V). That output will be then fed into an ADC. I have available on the board a very stable 5V supply, and a less clean 6V supply. The ADC is 10b with a 4.096 reference voltage, and I'd like to keep this circuit accurate enough to a single bit. My first thought is a non inverting amplifier - but that's one chip and 4 resistors per circuit, and since I need three of these on the same board, 1 chip and 12 resistors total. That's quite a bit... Is there any better way? I've seen some dedicated chips designed for current monitoring that would amplify a signal by a much higher gain - 50 or 100 without any external components at all, is there anything like this that would work for me?

Thanks for your help!

-MJ Noone

mmm wrote in news:dbvel3$h0m$ snipped-for-privacy@news.unina.it:

Argh. That is what happens when I post when I'm about 90% asleep. I was thinking of a difference amplifier, which I had originally planned on using instead of a non-inverting amplifier.

-MJ

snipped-for-privacy@ieee.org wrote in news:1122189695.322675.36350 @f14g2000cwb.googlegroups.com:

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yes - 2 resistors, quite right. I think I need some sleep... If I were to go the op-amp route those vishay dale voltage dividers look pretty nice. Something in a DIP package would be much, much too big for this board.

Can you suggest an op-amp to use? Ideally a quad op-amp as I will need 3 channels. I'm just never sure on how to choose an op-amp... If it matters, I'm currently planning on sampling at 100hz, but it's quite possible that I'll increase that.

-MJ

The CLC417 is intended for video application. It is inappropriate for the OPs application - in order to maximise the bandwidth of the part, Comlinear ran a lot of curren through the input transistors, which leads to large input bias currents and input offset currents - not something you'd want in Michael Noone's application.

Video is not usually too picky about DC levels - not least because if the DC level is critical, they fix it with a "black level clamp" - which menas that the CLC417 has a pretty poor DC offset specification of +/-5mV at 25C. Not what you'd normally chose for an instrument application.

------------------- Bill Sloman, Nijmegen

why four resistor ?

a non inverting-amplifier is only 2 resistor and an OpAmp

you must use precise equal resistor or calibrate ( the gain stage ) in software

Isn't it 2 resistors per circuit, 6 total?

R +---/\\/\\---+ | _ | +--|- \\ | | |324>---+-->2x Vin Vin, 0.44-1.77V +------|+_/ | \\ /R \\ | 0v-----+------------0v

For a 324 make 2R low enough (1k-ish?) so that they provide an output pulldown current, keeping the o/p stage in Class A.

If it is not smd, reduce the floor plan by standing resistors on end, on a 0.1 pitch.

Try to power the 3k pots from a voltage derived from the ADC's 4.096V.

Sanity Check... Why do you need to double it? Can you halve the reference voltage? You say signal???? What's a signal??? AC? DC? Varying DC? How fast? The responses you've had so far assume a rock-solid non-varying DC voltage. Unless that's the case, you have little to gain by doubling the signal. Just multiply the ADC output by 2. The accuracy/stabiltiy of your gain stage plus the noise introduced by it may outweigh the additional bit of precision. If the signal is noisy, you may gain something by averaging the ADC output. You mentioned the pot, so it must be significant. If it's user settable, you may find that the mechanical stiction and hysteresis in the pot make it impossible to set it as accurately as you expect...especially as it ages.

Can't tell what you're trying to accomplish, but it sounds like you're trying to do it the hard way. mike

Your 3.54 max output from a 5 volt supply just squeaks you by with some non rail to rail types. Since the frequency appears to not be very high, offset voltage and output voltage range appear to be the main concerns.

Take a look a at the LT1014

A rail to rail type with a similar offset spec is the LT6012

To get a better offset than that would probably take an auto zero type. For instance the LTC2052 has a typical offset spec of +-0.5 uV.

Keep in mind, that when you take layout into account, the dual versions may actually save space compared to the quads because there may be less of a tangle of traces.

The problem with the SOT-23's is that they're almost impossible to get. We've given up on them for a few current designs where they would be ideal, and gone to discrete 0603 thinfilms.

Do you know of anybody who actually stocks parts? It would seem to us that a few basic ratios should be popular enough to be stocked, but we've run into absured leadtimes and quantity requirements on the SOT-23s.

Mouser and Digikey stock 0.1% 0603 thinfilms at 25 and 10 ppm/k.

John

Pooh Bear wrote in news: snipped-for-privacy@hotmail.com:

Yes - I meant 2. My bad on that.

Very much so!

-MJ Noone

MSC wrote in news: snipped-for-privacy@4ax.com:

Possibly - I'm currently looking into that. The ADC is multiplexed and 3 of the other signals going into it need the 4.096 reference voltage. I'm working on finding a very small shunt resistor for those circuits that would allow me to use a 2.048V reference. (I just made a new post about exactly this)

-MJ Noone

Even more fun is to sum with noise or an asynchronous triangle, smear the samples several LSBs, and software lowpass filter. If the native noise is enough (dicey at 10 bits, almost guaranteed at 14-16) just filter.

John

If you take the wiper of your pot directly to the op-amp +input ( without an R to ground ) you need only 2 resistors per amplifier stage.

How did you get 4 ?

Use a quad op-amp to keep the IC package count low.

I don't understand why you think this is 'large' number of components. Are you extremely short of space ?

Graham

Can't you just feed the ADC with half the reference voltage?

Mike

This is "dithering" and you need to be a bit picky about the probablity distribution of the dithering signal.

I listed a bunch of useful references in a note in Rev. Sci. Instrum. in 1999.

Sloman A.W. "Comment on 'Noise averaging and measurement resolution" Review of Scientific Instruments, 70 4734 (1999)

An asynchronous triangular wave would produce a uniform distribution of off-sets, and this isn't good - you ideally want a a triangular probability distribution, and random noise comes a lot closer to this.

---------- Bill Sloman, Nijmegen

In article , Michael Noone wrote: [...]

If you want to avoid 0.1% resistors, consider this:

-------------------------- ADC ! ---O --------- ------O ! ! --- ! --- C big, non-leaky POT--------+------O ! --------- GND--O 2/3 of CD4053

CLC417 would do, except that I don't know whether it would work from a single sided 5V supply.

Regards, Mikko

Just checked from the NSC website (who acquired Comlinear, the original maker of the CLC417, some years ago): they seem to have a number of new types, including the LMH6718 dual which is specified for a single sided 5V.

Regards, Mikko

There's that 3rd resistor, a pull-up to V+.