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opamp with 0-5vinput and +/-5v output

D

dimention11

Jan 1, 1970
0
Hello
This is my first post to sci.electronics. I have no formal electronics
training but have completed quite a few DIY projects ( Digital and Analogue)
from the net and elsewhere.

At the moment I am working on a Printerport (lpt) dac using the Quad 8bit
DAC Max506.
This chip has a rail-to-rail output voltage (0-5V) for the application I
need +/-5v. I know (think :) an opamp is the solution. However I am still
bamboozled by some of the tech jargon on the datasheets. If anyone knows of
a solution off the top of their heads I would really appreciate it. if not I
will report back with a couple of chips data sheets and ask for some
guidance.

If you need more info I can email my project so far :)

Kurt
 
J

John Popelish

Jan 1, 1970
0
dimention11 said:
Hello
This is my first post to sci.electronics. I have no formal electronics
training but have completed quite a few DIY projects ( Digital and Analogue)
from the net and elsewhere.

At the moment I am working on a Printerport (lpt) dac using the Quad 8bit
DAC Max506.
This chip has a rail-to-rail output voltage (0-5V) for the application I
need +/-5v. I know (think :) an opamp is the solution. However I am still
bamboozled by some of the tech jargon on the datasheets. If anyone knows of
a solution off the top of their heads I would really appreciate it. if not I
will report back with a couple of chips data sheets and ask for some
guidance.

If you need more info I can email my project so far :)

Kurt

Since the output voltage range is twice as big as the input range, you
need a voltage gain of 2. You will also need an amplifier ssupply of
at least +- 5 volts. Assuming you find a rail to rail opamp so that
you can get by with just +-5, you connect your input signal to the +
input, and use two equal resistors in a divider between the output and
a +5 reference voltage (the positive supply if it is accurate
enough). The center of the divider goes to the - input.

When the input is at 0 volts, the output must go to - 5 volts in order
for the divider to produce a matching 0 volts in the - input.

When the input is at +5 volts, the output goes to +5 volts and there
is no voltage across the divider, so the - input is also at +5.
 
D

dimention11

Jan 1, 1970
0
John Popelish said:
Since the output voltage range is twice as big as the input range, you
need a voltage gain of 2. You will also need an amplifier ssupply of at
least +- 5 volts. Assuming you find a rail to rail opamp so that you can
get by with just +-5, you connect your input signal to the + input, and
use two equal resistors in a divider between the output and a +5 reference
voltage (the positive supply if it is accurate enough). The center of the
divider goes to the - input.

When the input is at 0 volts, the output must go to - 5 volts in order for
the divider to produce a matching 0 volts in the - input.

When the input is at +5 volts, the output goes to +5 volts and there is no
voltage across the divider, so the - input is also at +5.


thanks John
I thought I would need a split power supply of at least +/-5v using a rail
to rail opamp.
(does not have to be exactly +/-5v just above +/-3.5 and not biased)


here is a rough ascii drawing of your description I hope.

------------------------------------------------+5v
| |
| R
| (+5v) +--------|
| R |
| | (out) |
------------------------ |
(+In) | | |
---------------| opamp | |
| | |
(-In) +-----| | |
| | | |
| ----------------------- |
| | (-5v) |
| |
+--------------------------------------+
 
J

John Popelish

Jan 1, 1970
0
dimention11 said:
thanks John
I thought I would need a split power supply of at least +/-5v using a rail
to rail opamp.
(does not have to be exactly +/-5v just above +/-3.5 and not biased)


here is a rough ascii drawing of your description I hope.

------------------------------------------------+5v
| |
| R
| (+5v) +--------|
| R |
| | (out) |
------------------------ |
(+In) | | |
---------------| opamp | |
| | |
(-In) +-----| | |
| | | |
| ----------------------- |
| | (-5v) |
| |
+--------------------------------------+

We normally use a fixed width font, like Courier to make these
sketches, since proportionally spaced fonts have such spacing variety.
But I think I understand what you drew, and if so, it is correct.

Do you understand the principle that negative feedback must produce
the desired output by matching the inputs?
 
D

dimention11

Jan 1, 1970
0
John Popelish said:
We normally use a fixed width font, like Courier to make these sketches,
since proportionally spaced fonts have such spacing variety. But I think I
understand what you drew, and if so, it is correct.

Do you understand the principle that negative feedback must produce the
desired output by matching the inputs?

If you are asking if I understand why using the -ve input for feedback from
the divider will change
my signal from a 0-5v to a +/-5v I am a little sketchy and will do a little
more reading.
I do understand the voltage divider and hazzard a guess that changing one of
the resistors
value will change my offset or bias. I think I will need this for
zeroing/calibrating if true.

I have a questions on the specs I will need(opamp)

Q1. what is the sign for voltage gain? (the symbol on datasheets)
 
P

Pooh Bear

Jan 1, 1970
0
dimention11 said:
Hello
This is my first post to sci.electronics. I have no formal electronics
training but have completed quite a few DIY projects ( Digital and Analogue)
from the net and elsewhere.

At the moment I am working on a Printerport (lpt) dac using the Quad 8bit
DAC Max506.
This chip has a rail-to-rail output voltage (0-5V) for the application I
need +/-5v. I know (think :) an opamp is the solution. However I am still
bamboozled by some of the tech jargon on the datasheets. If anyone knows of
a solution off the top of their heads I would really appreciate it. if not I
will report back with a couple of chips data sheets and ask for some
guidance.

You need to *offset* the DAC's 0-5V output to give you a -5V to +5V span ?

An op-amp will indeed help you. :) You won't need anything too exotic.

Sorry but I don't see all the posts in this thread ( news server playing up ).

You basically need a stage with a gain of 2 ( to increase the 5V span of the DAC
to the 10V total span required ) with a -2.5V offset ( which will offset the
DAC's output with an input code of 128 to 0V at your system output ).

Graham
 
J

John Popelish

Jan 1, 1970
0
dimention11 said:
If you are asking if I understand why using the -ve input for feedback from
the divider will change
my signal from a 0-5v to a +/-5v I am a little sketchy and will do a little
more reading.

The point is that any amplifier programmed with feedback must arrive
at the desired output by forcing the two inputs to the same voltage.
I do understand the voltage divider and hazzard a guess that changing one of
the resistors
value will change my offset or bias. I think I will need this for
zeroing/calibrating if true.

The ration of the resistors sets the gain. The formula for this non
inverting configuration is gain=1+Routput/Rreference.

You change the offset by varying the voltage the reference resistor
connects to.
I have a questions on the specs I will need(opamp)

Q1. what is the sign for voltage gain? (the symbol on datasheets)

The voltage gain magnitude and sign is set by the feedback network and
the connections. Any opamp that is unity gain stable will have gain
at DC of at least 100,000. you set the amplifier gain to something
less than this with the feedback network.

Here is a basic opamp tutorial:
http://www.national.com/an/AN/AN-20.pdf

If you pick one with a gain bandwidth product of 1 MHz, its gain will
fall to 1 by the time the frequency rises to 1 MHz, since the gain
times the frequency can;t be more than 1 million. You didn't mention
what frequency this amplifier has to reproduce.

Here is a low cost dual amplifier that has a 1.5 MHz gain bandwidth
product that works with 3 to 15 volts total supply:
http://cache.national.com/ds/LM/LMC6482.pdf
 
D

dimention11

Jan 1, 1970
0
The point is that any amplifier programmed with feedback must arrive at
the desired output by forcing the two inputs to the same voltage.


The ration of the resistors sets the gain. The formula for this non
inverting configuration is gain=1+Routput/Rreference.

You change the offset by varying the voltage the reference resistor
connects to.


The voltage gain magnitude and sign is set by the feedback network and the
connections. Any opamp that is unity gain stable will have gain at DC of
at least 100,000. you set the amplifier gain to something less than this
with the feedback network.

Here is a basic opamp tutorial:
http://www.national.com/an/AN/AN-20.pdf

If you pick one with a gain bandwidth product of 1 MHz, its gain will fall
to 1 by the time the frequency rises to 1 MHz, since the gain times the
frequency can;t be more than 1 million. You didn't mention what frequency
this amplifier has to reproduce.

Here is a low cost dual amplifier that has a 1.5 MHz gain bandwidth
product that works with 3 to 15 volts total supply:
http://cache.national.com/ds/LM/LMC6482.pdf


Thanks for the links especialy the tutorial I had found a similar one and
was
slowly chewing my way through, yours is better though! :)

This project is to ultimatly control some high speed galvo's which have
their
own amp that req a +/-5v input. galvos are capable of 60k points per second
(30khz i think) so anything in the mhz range should be sufficient. LOL.

Thank you so much on all your help you have realy set me in the right
direction.
One last question the two resistors have to be equal I understand this, as
for a
value anything in the range of 1K to 100K should not be to extreme? or does
this need to be calculated?

Thanks again for all your help.
 
J

John Popelish

Jan 1, 1970
0
dimention11 said:
One last question the two resistors have to be equal I understand this, as
for a
value anything in the range of 1K to 100K should not be to extreme? or does
this need to be calculated?

Anything between those two values should work. However, 1k resistors
will load the output quite a bit when it is trying to pull down to -5
(10 volts across 2 1k resistors in series takes 5 mA) so staying at
the high end will get you closer to full rail voltage out. The opamp
I suggested draws very tiny bias current, so even higher values could
work, but stray capacitance starts to interfere with the frequency
response. I think a pair of 100k 1% resistors are a good choice.
 
D

dimention11

Jan 1, 1970
0
Thanks once again.
I think I will go for the http://www-s.ti.com/sc/ds/tlc2272a.pdf
this has comparable specs and is a little cheaper and more available for me
at the moment.
Thanks for the tip on the resistors as well.
I think I am ready for the prototype now :)
 
J

John Fields

Jan 1, 1970
0
Hello
This is my first post to sci.electronics. I have no formal electronics
training but have completed quite a few DIY projects ( Digital and Analogue)
from the net and elsewhere.

At the moment I am working on a Printerport (lpt) dac using the Quad 8bit
DAC Max506.
This chip has a rail-to-rail output voltage (0-5V) for the application I
need +/-5v. I know (think :) an opamp is the solution. However I am still
bamboozled by some of the tech jargon on the datasheets. If anyone knows of
a solution off the top of their heads I would really appreciate it. if not I
will report back with a couple of chips data sheets and ask for some
guidance.


---
Geez, I posted this over on alt.electronics this morning and got
nothing back over there. I guess he took my advice about posting!^)

---
Try this: (view in Courier New)



0 to 5V in>------|+\
| >---+--->-5 to +5V out
+5V>---[10k]--+--| / |
| |
+--[10k]--+

Make sure your + and - supplies are high enough to give you the
headroom you need to get your full output voltage range.

BTW, in the future you may want to consider posting to
sci.electronics.basics or sci.electronics.design, since this group is
mostly a wasteland.
 
P

Pooh Bear

Jan 1, 1970
0
John said:
BTW, in the future you may want to consider posting to
sci.electronics.basics or sci.electronics.design, since this group is
mostly a wasteland.

Which group are you referring to being a wasteland ?

I'm already reading this in s.e.d

Graham
 
D

dimention11

Jan 1, 1970
0
I just saw your post in alt.electronics
Thanks for the tip :)
 
D

dimention11

Jan 1, 1970
0
Ok I have finished my design and I am wondering if anyone would like to
check it for me.
I used eagle to make the schematic. I will send it via email just leave a
post here or on my email.
Thanks.
 
F

Fred Bloggs

Jan 1, 1970
0
dimention11 said:
Ok I have finished my design and I am wondering if anyone would like to
check it for me.
I used eagle to make the schematic. I will send it via email just leave a
post here or on my email.
Thanks.

I wouldn't have thunk you could get 0.4% unadjusted initial accuracy off
the printer port...What exactly is a multi-MHz "galvo" anyway?
 
D

dimention11

Jan 1, 1970
0
Oh and also
The amp I will be driving from this has either
signal and gnd or signal and -ve signal as inputs.

If I wished use the -ve signal
I would hook the feedback resistors from the output and then to the +ve
input of the opamp. Is this correct?
 
F

Fred Bloggs

Jan 1, 1970
0
dimention11 said:
Oh and also
The amp I will be driving from this has either
signal and gnd or signal and -ve signal as inputs.

If I wished use the -ve signal
I would hook the feedback resistors from the output and then to the +ve
input of the opamp. Is this correct?

No. Never "hook" the feedback resistors to the (+) input of the op amp,
unless you want a comparator or something. Get a copy of :
http://www.amazon.com/exec/obidos/tg/detail/-/0750677015/002-6421345-1775238?v=glance
 
D

dimention11

Jan 1, 1970
0
Fred Bloggs said:
I wouldn't have thunk you could get 0.4% unadjusted initial accuracy off
the printer port...What exactly is a multi-MHz "galvo" anyway?

This project is for driving an amp that drives galvos capable of
60k Points per second (30Khz),
if you find some multi Mhz galvo's let them know at alt.lasers,
Trust me they will be excited ;).

as for 0.4%accuracy from a LPT port ....
I cannot remember this in our conversation perhaps you read this in another
thread
 
D

dimention11

Jan 1, 1970
0
Sorry my bad
Hook it to the -ve supply instead of the +ve supply.
Dunno where I got that last idea from must be time for breakfast :)
 
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