741 Op Amp
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Posts: 46,084
I have testing a 741 opamp to interface with
a max186 & 187 ADC I just received.
Problem I'm having is that the output of the
opamp will not follow the input like I
thought.
I am powering the opamp with 5 volts. Input
is from a 9 volt battery. I have the 9 volts
going though a resistor network to limit the
voltage input to the opamp. When I adjust the
input voltage down to 1 to 2 volts the output
of the opamp will stay at 2 volts. Is this
normal for the opamp?
What I was expecting was the output to follow
the input. Although it will follow the input
until the input voltage reaches about 2 volts
and that is where the output stays. Is this
normal for the opamp??
a max186 & 187 ADC I just received.
Problem I'm having is that the output of the
opamp will not follow the input like I
thought.
I am powering the opamp with 5 volts. Input
is from a 9 volt battery. I have the 9 volts
going though a resistor network to limit the
voltage input to the opamp. When I adjust the
input voltage down to 1 to 2 volts the output
of the opamp will stay at 2 volts. Is this
normal for the opamp?
What I was expecting was the output to follow
the input. Although it will follow the input
until the input voltage reaches about 2 volts
and that is where the output stays. Is this
normal for the opamp??
Comments
1) Are you operating the 741 op amp in the inverting or non-inverting mode?
2) Are you using it as a non-inverting voltage follower?
3) What happens to the 741 output when you remove the ADC that is driven by
the
741 op amp? Perhaps the 741 is being limited by the load it is triving,
i.e. the
741 output is being clamped by the input circuitry of the ADC.
4) Try running the 741 from a split supply, say plus and minus 5 volts.
5) Is the 741 a good device, i.e. functional and meets specs?
Burt
1) Are you operating the 741 op amp in the
inverting or non-inverting mode?
non-inverting mode
2) Are you using it as a non-inverting
voltage follower?
not sure about this????
I have read about the inverting and
non-inverting and the comparing but not about
the voltage follower. Maybe that's how I
should hook it.
I have attached a 741.bmp file, how I have
hooked it up.
3) What happens to the 741 output when you
remove the ADC that is driven by
the 741 op amp?
I have not hooked it up to the ADC yet, I
have just hooked up the opamp on the bench.
Perhaps the 741 is being limited by the load
it is triving,
i.e. the 741 output is being clamped by the
input circuitry of the ADC.
4) Try running the 741 from a split supply,
say plus and minus 5 volts.
5) Is the 741 a good device, i.e. functional
and meets specs?
Original Message
From: <burtsz@a...>
To: <basicstamps@yahoogroups.com>
Sent: Sunday, January 28, 2001 2:35 PM
Subject: Re: [noparse][[/noparse]basicstamps] 741 Op Amp
> Hi Gene,
>
> 1) Are you operating the 741 op amp in the
inverting or non-inverting mode?
> 2) Are you using it as a non-inverting
voltage follower?
> 3) What happens to the 741 output when you
remove the ADC that is driven by
> the
> 741 op amp? Perhaps the 741 is being
limited by the load it is triving,
> i.e. the
> 741 output is being clamped by the
input circuitry of the ADC.
> 4) Try running the 741 from a split
supply, say plus and minus 5 volts.
> 5) Is the 741 a good device, i.e.
functional and meets specs?
>
> Burt
>
>
>
>
The 741 opamp is not a good choice for low voltage rails, ie, +/-5 volts.
My old databook for the National LM741 only specs operation at +/-15 and
+/-20 volts. With 15 volt rails, it is only guaranteed to swing to within 3
volts of the supplies with only a 10k load ... worse with more loading. So
+/- 5 volt rails will probably only allow output to swing +/- 2volts.
The same thing applies to the input voltage range, it is only guaranteed to
accept input voltage within 3 volts of each rail. Extrapolating to +/-5
volts, it would only accept input between +/- 2 volts.
Can you switch to +/-12 or +/- 15 volts? The configuration you show is a
voltage follower, which gives a gain of unity, non-inverted. Is this what
you need?
Good luck,
Ray McArthur
use a duel power supply (+ & - 9 volts for example, using two 9 volt
batteries with one plus and one minus terminal for a ground). The
offset you described is typical for a 741 when using a single ended
power supply. You might try selecting an opamp that is intended for a
single power source, like the LM358. Good luck, Dave Thompson.
--- In basicstamps@y..., "Gene Shults" <gshults@o...> wrote:
> I have testing a 741 opamp to interface with
> a max186 & 187 ADC I just received.
> Problem I'm having is that the output of the
> opamp will not follow the input like I
> thought.
> I am powering the opamp with 5 volts. Input
> is from a 9 volt battery. I have the 9 volts
> going though a resistor network to limit the
> voltage input to the opamp. When I adjust the
> input voltage down to 1 to 2 volts the output
> of the opamp will stay at 2 volts. Is this
> normal for the opamp?
> What I was expecting was the output to follow
> the input. Although it will follow the input
> until the input voltage reaches about 2 volts
> and that is where the output stays. Is this
> normal for the opamp??
matter which input is higher voltage? I would like to
measure the difference in voltage at the inputs
without regard to which way they are biased so that my
output is always positive.
So basically you need to compute the absolute value of a signal? Instead of
using opamps would it be possible for you to use a bridge rectifier? Of course
your signal would have to be larger than ~1.4 volts but in my estimation that
would be a lot easier than the op amp way.
Good Luck!
Randy Knutson
Jeff Mitchell wrote:Can a 741 be made to output a non inverted pulse no
matter which input is higher voltage? I would like to
measure the difference in voltage at the inputs
without regard to which way they are biased so that my
output is always positive.
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You can use two op-amps to make a precision
rectifier, if you need to measure a low-voltage AC
signal.
If you use a rectifier, the threshold is about 1.2
volt, so you 'loose' a part of the information. If
the voltage is lower than 1.2 volt, you can't get
any output.
With an op-amp, this threshold is something like 3
microvolt... let me know if this is what you need.
Phil.
Original Message
From: "Jeff Mitchell" <EL-JEFE@P...>
Subject: [noparse][[/noparse]basicstamps] 741 Op Amp
| Can a 741 be made to output a non inverted pulse
no
| matter which input is higher voltage? I would
like to
| measure the difference in voltage at the inputs
| without regard to which way they are biased so
that my
| output is always positive.
Some theory on half and full-wave precision rectifiers can be found at the link
below:
http://www.educatorscorner.com/experiments/docs/exp88/exp88a.doc
Good Luck!
-=Randy Knutson
Philippe Derenne wrote:Hi Jeff,
You can use two op-amps to make a precision
rectifier, if you need to measure a low-voltage AC
signal.
If you use a rectifier, the threshold is about 1.2
volt, so you 'loose' a part of the information. If
the voltage is lower than 1.2 volt, you can't get
any output.
With an op-amp, this threshold is something like 3
microvolt... let me know if this is what you need.
Phil.
Original Message
From: "Jeff Mitchell"
Subject: [noparse][[/noparse]basicstamps] 741 Op Amp
| Can a 741 be made to output a non inverted pulse
no
| matter which input is higher voltage? I would
like to
| measure the difference in voltage at the inputs
| without regard to which way they are biased so
that my
| output is always positive.
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from the same email address that you subscribed. Text in the Subject and Body of
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