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'[EE:] OP-AMP Question.'
2004\04\07@165646 by

Guys,

I have a linear signal that goes from +0.45V to +1.15V. I need to
convert it to
a signal that is +0.3V, when level is +0.45V and +10V when the signal
is +1.15V.
Could somebody suggest an op-amp circuit that would do this. Would I
need some type
of offset to get this "strange" range? I tried using a regular
"non-inverting op-amp circuit",
but I couldn't get the full range. I am using a R-R op-amp with +10V as
the positive supply.
Any suggestions would be greatly appreciated.

Thanks.
Steve

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> I have a linear signal that goes from +0.45V to +1.15V. I
> need to convert it to a signal that is +0.3V, when level
> is +0.45V and +10V when the signal is +1.15V.
> Could somebody suggest an op-amp circuit that would do
> this. Would I need some type of offset to get this
> "strange" range? I tried using a regular "non-inverting
> op-amp circuit", but I couldn't get the full range. I am
> using a R-R op-amp with +10V as the positive supply.
> Steve

I can't figure out why I see no replies. Anyway, to get started,
run the input signal to the non-inverting input, then create a
reference voltage (your "some type of offset") by making a voltage
divider with two resistors to get something in between. (.3+1.15)/2
Figures out to .72 V.  Tie that to the inverting input and now it
will switch as it crosses the reference point.

Did it need to still be linear?  Reduce the gain of the op amp
with a feedback resistor from the output to the inverting input.
This will upset the offset but it will get you started if you
are going nowhere.  There's more to do but maybe this will
kick off a discussion, if you still need it and I didn't miss
the whole thread somehow.

I just noticed you said you had a circuit.  Maybe post a link
to that and we can look at it.  Probably you just need to
up the gain by adjusting the resistors in the feedback path
from output to inverting input.

Barry

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pic microcontroller discussion list wrote:
>> I have a linear signal that goes from +0.45V to +1.15V. I
>> need to convert it to a signal that is +0.3V, when level
>> is +0.45V and +10V when the signal is +1.15V.

[snip]

> I can't figure out why I see no replies. Anyway, to get started,
> run the input signal to the non-inverting input, then create a
> reference voltage (your "some type of offset") by making a voltage
> divider with two resistors to get something in between. (.3+1.15)/2
> Figures out to .72 V.  Tie that to the inverting input and now it
> will switch as it crosses the reference point.
>

also see the thread "Voltage Conversion Circuit" a few days before
Valentine's Day. It covers this type of circuit.

You mentioned your op-amp was a R-R powered by 10V, and that you
weren't getting the full range. Even a R-R won't truly go to 10V,
but it should be pretty close. How close to 10V (and 0.3V for that
matter) do you need to get? Maybe you should tell us a little more
about what is supposed to happen to the output when the input is
between 0.45V and 1.15V.

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Everybody,

Spehro emailed me a circuit that worked perfectly. I appreciate all of
I am currently reading Art of Electronics, hopefully it will "shed some
light" on
my analog weaknesses!

Thanks again guys!
Steve

>>> peisermaRIDGID.COM 4/13/2004 1:33:09 PM >>>
pic microcontroller discussion list wrote:
>> I have a linear signal that goes from +0.45V to +1.15V. I
>> need to convert it to a signal that is +0.3V, when level
>> is +0.45V and +10V when the signal is +1.15V.

[snip]

> I can't figure out why I see no replies. Anyway, to get started,
> run the input signal to the non-inverting input, then create a
> reference voltage (your "some type of offset") by making a voltage
> divider with two resistors to get something in between. (.3+1.15)/2
> Figures out to .72 V.  Tie that to the inverting input and now it
> will switch as it crosses the reference point.
>

also see the thread "Voltage Conversion Circuit" a few days before
Valentine's Day. It covers this type of circuit.

You mentioned your op-amp was a R-R powered by 10V, and that you
weren't getting the full range. Even a R-R won't truly go to 10V,
but it should be pretty close. How close to 10V (and 0.3V for that
matter) do you need to get? Maybe you should tell us a little more
about what is supposed to happen to the output when the input is
between 0.45V and 1.15V.

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2004\04\14@121002 by
A simple way to analyze op amp circuits is to take advantage of the fact
that in an op amp negative feedback circuit, the differential input
voltage is ideally zero. I call this the "Theory of the Happy Op Amp."
Generally, you can determine the voltage at the non-inverting input and
assume the voltage at the inverting input is the same, then analyze the
rest of the circuit using Ohm's Law based on that assumption. A sample of
using this technique (analysis of an instrumentation amplifier) is at
http://kauko.hallikainen.org/cuesta/et113/InstrumentationAmpAnalysis.pdf

Harold

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>A simple way to analyze op amp circuits is to take advantage of the fact
>that in an op amp negative feedback circuit, the differential input
>voltage is ideally zero. I call this the "Theory of the Happy Op Amp."
>Generally, you can determine the voltage at the non-inverting input and
>assume the voltage at the inverting input is the same, then analyze the
>rest of the circuit using Ohm's Law based on that assumption. A sample of
>using this technique (analysis of an instrumentation amplifier) is at
>http://kauko.hallikainen.org/cuesta/et113/InstrumentationAmpAnalysis.pdf

This is usually summed up as follows:
In an op-amp circuit with negative feedback,
a) the difference in voltage between the positive and negative inputs
(or inverting and non-inverting, if you prefer) is ZERO.  This is called
VIRTUAL SHORT.
b) the current flowing into/out of either input is ZERO.  This is usually
referred to as VIRTUAL OPEN.

Caveats:  Real op-amps don't work quite that well.  Input bias currents
mean that the current into/out of the inputs is very small, but non-zero.
Many factors affect the voltage between the inputs.

In general, the op-amps output will slew in whatever direction is
needed to maintain the virtual open and virtual short, UP TO THE
LIMITS.  And bear in mind that "rail-to-rail" op-amps usually aren't;
they are just "almost rail-to-rail" (within a few tens of millivolts).

This e-mail got rather out of hand; I'm not exactly sure of the
original intent; at any rate, these are the things I wish someone
had explicitly told me when I was a student rather than leaving
them for me to discover in order to move beyond the "cookbook"
they gave me.  :-p

Mike H.

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