simplest possible voltage doubler?

[Michael Noone]

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]

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?

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

 

[[email protected]]

Theat's six resistors, not twelve - since your 3k potentiometer doesn't
present a constant to the amplifier, balancing the input impedances
doesn't win you much.

And they don't have to be discrete resistors - in fact if you want
precise and stable gain you are much better off using an array of thin
film precision resistors in a single package, or three precision
dividers.

Farnell carry the Vishay RMKMS816 with 8 10k 0.1% resistors in a 16-pin
DIP - order code 309-8667, for about $15, and the MPM range of
precision dividers in an SOT23 package - the 10k+10k 1:1 divider (order
code 309-8643) costs about $4 and you'd need three of them.

I'd go for the SOT23 dividers myself - they are marginally cheaper, and
the PCB layout would come out better.

Not all that cheap, but quite precise - the dividers offer a ratio
tolerance of +/-0.05%, witha temperature stability of +/-2ppm/C,
appreciably better than the eight resistor array.

 

[Michael Noone]

why four resistor ?

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

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

 

[Michael Noone]

[email protected] wrote in @f14g2000cwb.googlegroups.com:

Theat's six resistors, not twelve - since your 3k potentiometer doesn't
present a constant to the amplifier, balancing the input impedances
doesn't win you much.

And they don't have to be discrete resistors - in fact if you want
precise and stable gain you are much better off using an array of thin
film precision resistors in a single package, or three precision
dividers.

Farnell carry the Vishay RMKMS816 with 8 10k 0.1% resistors in a 16- pin
DIP - order code 309-8667, for about $15, and the MPM range of
precision dividers in an SOT23 package - the 10k+10k 1:1 divider (order
code 309-8643) costs about $4 and you'd need three of them.

I'd go for the SOT23 dividers myself - they are marginally cheaper, and
the PCB layout would come out better.

Not all that cheap, but quite precise - the dividers offer a ratio
tolerance of +/-0.05%, witha temperature stability of +/-2ppm/C,
appreciably better than the eight resistor array.

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

 

[Tony Williams]

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.

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.

 

[Kiviranta, Mikko]

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?

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

Regards,
Mikko

 

[Kiviranta, Mikko]

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

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

 

[[email protected]]

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.

 

[Pooh Bear]

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?

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

 

[MSC]

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

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

Mike

 

[John Popelish]

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.

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
http://www.ldeo.columbia.edu/~tolstoy/research/

A rail to rail type with a similar offset spec is the LT6012
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1021,P2315,D2626

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.
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1021,P1855,D1799

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.

 

[Ken Smith]

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

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

-------------------------- ADC
!
---O

 

[mike]

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

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

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[Michael Noone]

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 ?

Yes - I meant 2. My bad on that.

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 ?

Very much so!

-MJ Noone

 

[Michael Noone]

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

Mike

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

 

[John Larkin]

Theat's six resistors, not twelve - since your 3k potentiometer doesn't
present a constant to the amplifier, balancing the input impedances
doesn't win you much.

And they don't have to be discrete resistors - in fact if you want
precise and stable gain you are much better off using an array of thin
film precision resistors in a single package, or three precision
dividers.

Farnell carry the Vishay RMKMS816 with 8 10k 0.1% resistors in a 16-pin
DIP - order code 309-8667, for about $15, and the MPM range of
precision dividers in an SOT23 package - the 10k+10k 1:1 divider (order
code 309-8643) costs about $4 and you'd need three of them.

I'd go for the SOT23 dividers myself - they are marginally cheaper, and
the PCB layout would come out better.

Not all that cheap, but quite precise - the dividers offer a ratio
tolerance of +/-0.05%, witha temperature stability of +/-2ppm/C,
appreciably better than the eight resistor array.

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

 

[Fred Bloggs]

Your 3.54 max output from a 5 volt supply just squeaks you by with some
non rail to rail types.

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

 

[Fred Bloggs]

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.

4.096/1024=4mV resolution. Neglecting differential non-linearity, you
will measure voltage within the 0.44-1.77V to within 4mV accuracy.
Assuming perfectly errorless scaling into the 0.88-3.54V range, measured
with 4mV accuracy, you then get 2mV accuracy on the original signal. If
you want 2mV accuracy, then you can alternately sum 2mV and 0mV with the
original signal to obtain the same estimate. So you don't need a
doubler, you need a summer- and that is two resistors.

 

[John Larkin]

4.096/1024=4mV resolution. Neglecting differential non-linearity, you
will measure voltage within the 0.44-1.77V to within 4mV accuracy.
Assuming perfectly errorless scaling into the 0.88-3.54V range, measured
with 4mV accuracy, you then get 2mV accuracy on the original signal. If
you want 2mV accuracy, then you can alternately sum 2mV and 0mV with the
original signal to obtain the same estimate. So you don't need a
doubler, you need a summer- and that is two resistors.

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