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PICList Thread
'Adding voltage offset'
1997\02\25@021335 by Clement W.L. Soo

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Hi Folks,

I have an analog signal source vary from -40mv to +1100mv, and I want to swift entire range from 0mv to 1400mv. It can be easily done by adding 40mv
by simple math. Yet, if I want to add the offset using some hardware, how can I do it? The signal that will eventually go through an op-amp (LM324)and the op-amp may have a gain of 3.57 to bump it up to a range of 0 to
5volt and feed it to Analog to Digital. The reason that I want to swift the signal to positive range cause my board was configured to only accept 0 to 5 volt (Vref-low and V ref-high).

Thanks,


Clement

1997\02\25@081422 by Jonathan King

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If you are going to gain up the signal by 3.57, you can introduce an offset in
that amplifier.

I'd assume you would use a non-inverting opamp configuration for the signal
gain of 3.57.  Normally the resistor going to the negative input is grounded,
but by putting it at -40mv/(Rf/R1) = -15.6mV, you will add the 40mv needed.

I assume you have negative voltages available, since the signal itself is
negative.  If you don't there are other ways to do it.



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Jonathan King                        |   spam_OUTkingTakeThisOuTspamuicc.com
Unitrode Corp.                       |   http://www.unitrode.com
7 Continental Blvd                   |   (603) 429-8715
Merrimack, NH   03054                |   (603) 424-3460

1997\02\25@131245 by Larry G. Nelson Sr.

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>I have an analog signal source vary from -40mv to +1100mv, and I want to
swift entire range from 0mv to 1400mv. It can be easily done by adding 40mv
>by simple math. Yet, if I want to add the offset using some hardware, how
can I do it? The signal that will eventually go through an op-amp (LM324)and
the op-amp may have a gain of 3.57 to bump it up to a range of 0 to 5volt
and feed it to Analog to Digital.

You can do this with an Op Amp adder circuit. You sum a 40mv reference with
the input signal using two input resistors to the same node of the same
value. Most data books show a simple summing amp circuit that will do what
you need.

Larry
Larry G. Nelson Sr.
.....L.NelsonKILLspamspam@spam@ieee.org
http://www.ultranet.com/~nr

1997\02\26@024142 by Clement W.L. Soo

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Hi,

Actually, I mistype couple of the requirements on my previous question. The signal is vary from -400mv to +1100mv, not -40mv to +1100mv. So, I need to shift it +400mv (0.4Volt). Now, the new adjust range should vary >from 0 to 1500mv (1.5Volt). After a gain of 3.33, it would give me an analog singal vary from 0 to 5volt and I can read it using A/D.  The question is how can I add 0.4Volt offset to my signal?  And I don't have any negative voltage available. The signal is from a temperature sensor.  


Thanks,

Clement

----------
From:   Jonathan King[SMTP:kingspamKILLspamuicc.com]
Sent:   Tuesday, February 25, 1997 7:12 AM
To:     pic microcontroller discussion list
Cc:     .....soo123wKILLspamspam.....MEENA.CC.UREGINA.CA
Subject:        Re: Adding voltage offset

If you are going to gain up the signal by 3.57, you can introduce an offset in
that amplifier.  

I'd assume you would use a non-inverting opamp configuration for the signal
gain of 3.57.  Normally the resistor going to the negative input is grounded,
but by putting it at -40mv/(Rf/R1) = -15.6mV, you will add the 40mv needed.

I assume you have negative voltages available, since the signal itself is
negative.  If you don't there are other ways to do it.



---------------------------------------------------------------------
Jonathan King                        |   EraseMEkingspam_OUTspamTakeThisOuTuicc.com
Unitrode Corp.                       |   http://www.unitrode.com
7 Continental Blvd                   |   (603) 429-8715
Merrimack, NH   03054                |   (603) 424-3460

1997\02\27@043606 by Mike Ghormley

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Clement wrote:
>
>Hi,
>
>Actually, I mistype couple of the requirements on my previous question. =
>The signal is vary from -400mv to +1100mv, not -40mv to +1100mv. So, I =
>need to shift it +400mv (0.4Volt). Now, the new adjust range should vary =
>from 0 to 1500mv (1.5Volt). After a gain of 3.33, it would give me an =
>analog singal vary from 0 to 5volt and I can read it using A/D.  The =
>question is how can I add 0.4Volt offset to my signal?  And I don't have =
>any negative voltage available. The signal is from a temperature sensor. =

In your first post you mentioned passing the signal thru an op-amp to boost
the 1.5Vpp signal to 5Vpp.  If you can afford for the signal to be inverted
you can try this:
                                 3.333K
                            ----/\/\/\---------|
                            |                  |
                            |                  |
                            |     |\           |
                  1K ohm    |     | \          |
       Vin >-----/\/\/\-----0-----|  \         |
       -0.4/1.1                   |   \________|_______\ Vout
                       +5V  |-----|   /                / 5.0/0.0
                        |   |     |  /
                       /    |     |/
              4582 ohm \    |
                       /    |
                       \    |
                       |    |
          0.89576V     0----|
                       |
                       /
                       \
                       /
             1000 ohm  \
                       |
                       |
                    ------
                    / / /


Of course most op-amps require negative voltage.

Hello, I must be going...

Michael

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