adding an enable to a bipolar op-amp, how???
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adding an enable to a bipolar op-amp, how???
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Hank
Guest
Posted: Thu Dec 08, 2005 1:35 am    Post subject: adding an enable to a bipolar op-amp, how??? Reply with quote
Hi,

I am converting the 0 to 4.096V output of a DAC into -2.048V to +2.048V
using an op-amp as a differential amplifier. On powerup, the DAC is reset
to 0V, which obviously makes the output of the op-amp -2.048V. I want the
output of the op-amp to be 0V on powerup until I enable it with 5V logic.

Is there a really easy way of implementing this that I am just missing? All
of my solutions seem too complicated for such a seemingly simple
requirement. :-(

Thanks,

Hank

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Jim Thompson
Guest
Posted: Thu Dec 08, 2005 1:35 am    Post subject: Re: adding an enable to a bipolar op-amp, how??? Reply with quote
On Wed, 07 Dec 2005 20:02:36 GMT, "Hank" <lightdoesnotage@yahoo.com>
wrote:

Quote:
Hi,

I am converting the 0 to 4.096V output of a DAC into -2.048V to +2.048V
using an op-amp as a differential amplifier. On powerup, the DAC is reset
to 0V, which obviously makes the output of the op-amp -2.048V. I want the
output of the op-amp to be 0V on powerup until I enable it with 5V logic.

Is there a really easy way of implementing this that I am just missing? All
of my solutions seem too complicated for such a seemingly simple
requirement. :-(

Thanks,

Hank


Inverting with plus input tied to ground? Or full differential?

How are you adding in the offset?

Post a drawing... "real" preferred on

alt.binaries.schematics.electronic

but ASCII will do if you really must ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
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Hank
Guest
Posted: Thu Dec 08, 2005 1:35 am    Post subject: Re: adding an enable to a bipolar op-amp, how??? Reply with quote
Quote:

Inverting with plus input tied to ground? Or full differential?

How are you adding in the offset?

Post a drawing... "real" preferred on

alt.binaries.schematics.electronic

but ASCII will do if you really must ;-)

...Jim Thompson
--

I thought all you guys draw in ASCII :-)

Ok, I did a quick drawing and posted to alt.binaries.schematics.electronic
under the title "Bipolar output from single supply DAC"

Thanks,

Hank

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Jim Thompson
Guest
Posted: Thu Dec 08, 2005 1:35 am    Post subject: Re: adding an enable to a bipolar op-amp, how??? Reply with quote
On Wed, 07 Dec 2005 23:08:32 GMT, "Hank" <lightdoesnotage@yahoo.com>
wrote:

Quote:

"Jim Thompson" <To-Email-Use-The-Envelope-Icon@My-Web-Site.com> wrote in
message news:2fpep19105pr2epf4sqpk9cvksgv11reg5@4ax.com...
On Wed, 07 Dec 2005 20:02:36 GMT, "Hank" <lightdoesnotage@yahoo.com
wrote:

Hi,

I am converting the 0 to 4.096V output of a DAC into -2.048V to +2.048V
using an op-amp as a differential amplifier. On powerup, the DAC is reset
to 0V, which obviously makes the output of the op-amp -2.048V. I want the
output of the op-amp to be 0V on powerup until I enable it with 5V logic.

Is there a really easy way of implementing this that I am just missing?
All
of my solutions seem too complicated for such a seemingly simple
requirement. :-(

Thanks,

Hank


Can you enable Vref? If it's zero at power-up, so is the OpAmp
output.


mmm, it is a three terminal part (+5V, Gnd, and 4.096V output) but I suppose
I could put
a smallish logic level P-channel fet between its VCC terminal and +5V...

that seems easy enough, huh?

Here I was trying to muck with the final output...

guess that is why I pay you the big bucks...oh wait... :-)

Thanks,

Hank


It probably draws so little current that you could power it via a
74HC04 output. That way VDD will be forced to zero when off, rather
than floating.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
Back to top
John Larkin
Guest
Posted: Thu Dec 08, 2005 1:35 am    Post subject: Re: adding an enable to a bipolar op-amp, how??? Reply with quote
On Wed, 07 Dec 2005 23:55:17 GMT, "Hank" <lightdoesnotage@yahoo.com>
wrote:

Quote:

"Jim Thompson" <To-Email-Use-The-Envelope-Icon@My-Web-Site.com> wrote in
message news:2fpep19105pr2epf4sqpk9cvksgv11reg5@4ax.com...
On Wed, 07 Dec 2005 20:02:36 GMT, "Hank" <lightdoesnotage@yahoo.com
wrote:

Hi,

I am converting the 0 to 4.096V output of a DAC into -2.048V to +2.048V
using an op-amp as a differential amplifier. On powerup, the DAC is reset
to 0V, which obviously makes the output of the op-amp -2.048V. I want the
output of the op-amp to be 0V on powerup until I enable it with 5V logic.

Is there a really easy way of implementing this that I am just missing?
All
of my solutions seem too complicated for such a seemingly simple
requirement. :-(

Thanks,

Hank


Can you enable Vref? If it's zero at power-up, so is the OpAmp
output.


Actually, upon further thought, I am going to switch to a shunt type Vref.
The part I was going to use, AD158x has an inrush current of 15mA or more.
It has a low overhead requirement of only 200mV, but if the pfet I choose
has
a typical Rdson of ~100 ohms, that is 1.5V dropped across the fet...oops!

I think a typical resistor used with a shunt is about 1k, meaning only 1mA
through
the pfet, or a drop of 100mV. The Vref input to the DAC has a 160k
impedance,
the input impedance of my op-amp configuration is going to be equal to R3,
so
probably about 20k...I guess I should buffer the output from the shunt?

I guess there never is a really simple solution...



You could use a tiny SPDT analog switch to open the 4-volt path, or
even switch it to ground. Like an Sn74LVC1G66 or something.

Or use an opamp that has a powerdown input.

John
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Hank
Guest
Posted: Thu Dec 08, 2005 1:35 am    Post subject: Re: adding an enable to a bipolar op-amp, how??? Reply with quote
"Jim Thompson" <To-Email-Use-The-Envelope-Icon@My-Web-Site.com> wrote in
message news:2fpep19105pr2epf4sqpk9cvksgv11reg5@4ax.com...
Quote:
On Wed, 07 Dec 2005 20:02:36 GMT, "Hank" <lightdoesnotage@yahoo.com
wrote:

Hi,

I am converting the 0 to 4.096V output of a DAC into -2.048V to +2.048V
using an op-amp as a differential amplifier. On powerup, the DAC is reset
to 0V, which obviously makes the output of the op-amp -2.048V. I want the
output of the op-amp to be 0V on powerup until I enable it with 5V logic.

Is there a really easy way of implementing this that I am just missing?
All
of my solutions seem too complicated for such a seemingly simple
requirement. :-(

Thanks,

Hank


Can you enable Vref? If it's zero at power-up, so is the OpAmp
output.


Actually, upon further thought, I am going to switch to a shunt type Vref.
The part I was going to use, AD158x has an inrush current of 15mA or more.
It has a low overhead requirement of only 200mV, but if the pfet I choose
has
a typical Rdson of ~100 ohms, that is 1.5V dropped across the fet...oops!

I think a typical resistor used with a shunt is about 1k, meaning only 1mA
through
the pfet, or a drop of 100mV. The Vref input to the DAC has a 160k
impedance,
the input impedance of my op-amp configuration is going to be equal to R3,
so
probably about 20k...I guess I should buffer the output from the shunt?

I guess there never is a really simple solution...
Back to top
Hank
Guest
Posted: Thu Dec 08, 2005 1:35 am    Post subject: Re: adding an enable to a bipolar op-amp, how??? Reply with quote
"Jim Thompson" <To-Email-Use-The-Envelope-Icon@My-Web-Site.com> wrote in
message news:2fpep19105pr2epf4sqpk9cvksgv11reg5@4ax.com...
Quote:
On Wed, 07 Dec 2005 20:02:36 GMT, "Hank" <lightdoesnotage@yahoo.com
wrote:

Hi,

I am converting the 0 to 4.096V output of a DAC into -2.048V to +2.048V
using an op-amp as a differential amplifier. On powerup, the DAC is reset
to 0V, which obviously makes the output of the op-amp -2.048V. I want the
output of the op-amp to be 0V on powerup until I enable it with 5V logic.

Is there a really easy way of implementing this that I am just missing?
All
of my solutions seem too complicated for such a seemingly simple
requirement. :-(

Thanks,

Hank


Can you enable Vref? If it's zero at power-up, so is the OpAmp
output.


mmm, it is a three terminal part (+5V, Gnd, and 4.096V output) but I suppose
I could put
a smallish logic level P-channel fet between its VCC terminal and +5V...

that seems easy enough, huh?

Here I was trying to muck with the final output...

guess that is why I pay you the big bucks...oh wait... :-)

Thanks,

Hank
Back to top
Jim Thompson
Guest
Posted: Thu Dec 08, 2005 1:35 am    Post subject: Re: adding an enable to a bipolar op-amp, how??? Reply with quote
On Wed, 07 Dec 2005 20:02:36 GMT, "Hank" <lightdoesnotage@yahoo.com>
wrote:

Quote:
Hi,

I am converting the 0 to 4.096V output of a DAC into -2.048V to +2.048V
using an op-amp as a differential amplifier. On powerup, the DAC is reset
to 0V, which obviously makes the output of the op-amp -2.048V. I want the
output of the op-amp to be 0V on powerup until I enable it with 5V logic.

Is there a really easy way of implementing this that I am just missing? All
of my solutions seem too complicated for such a seemingly simple
requirement. :-(

Thanks,

Hank


Can you enable Vref? If it's zero at power-up, so is the OpAmp
output.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
Back to top
Tony Williams
Guest
Posted: Thu Dec 08, 2005 7:33 am    Post subject: Re: adding an enable to a bipolar op-amp, how??? Reply with quote
In article <wdHlf.21032$Oq3.17353@trnddc05>,
Hank <lightdoesnotage@yahoo.com> wrote:

Quote:
I am converting the 0 to 4.096V output of a DAC into -2.048V to
+2.048V using an op-amp as a differential amplifier. On
powerup, the DAC is reset to 0V, which obviously makes the
output of the op-amp -2.048V. I want the output of the op-amp
to be 0V on powerup until I enable it with 5V logic.

Maybe think about switching the bias on the
differential amplifier.

Option1.
~~~~~~~~
Switch 2R R
4.096Vref -+----+/+------/\/\---+---/\/\--+--> Vout
| _ |
+--|- \ |
|Op >--+
+--|+_/
|
DAC(at 0V) +------------/\/\----+---/\/\--+0V
2R R

At power-up Switch= OFF, so there is no bias on
the diff-amp. Needs a good switch because the
switch-resistance is in circuit during normal operation.

Option2.
~~~~~~~~

2R R
4.096Vref -+-------------/\/\---+---/\/\--+--> Vout
| | _ |
| +--|- \ |
| Switch 4R/3 |Op >--+
+---+/+------/\/\----+--|+_/
|
DAC(at 0V) +------------/\/\----+---/\/\--+0V
2R R

At switch-on DAC=0V, Switch= ON, and the 4R/3 is in
circuit. This moves the diff-amp into equal 2R/R
on both legs, with both inputs the same. Vout= 0.
Switch= OFF for normal operation.

--
Tony Williams.
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Pooh Bear
Guest
Posted: Thu Dec 08, 2005 8:14 am    Post subject: Re: adding an enable to a bipolar op-amp, how??? Reply with quote
Hank wrote:

Quote:
Hi,

I am converting the 0 to 4.096V output of a DAC into -2.048V to +2.048V
using an op-amp as a differential amplifier. On powerup, the DAC is reset
to 0V, which obviously makes the output of the op-amp -2.048V. I want the
output of the op-amp to be 0V on powerup until I enable it with 5V logic.

Is there a really easy way of implementing this that I am just missing? All
of my solutions seem too complicated for such a seemingly simple
requirement. :-(

Use an analog switch on the op-amp output ?

Graham
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Guest
Posted: Thu Dec 08, 2005 8:43 am    Post subject: Re: adding an enable to a bipolar op-amp, how??? Reply with quote
Hank wrote:
Quote:
"Jim Thompson" <To-Email-Use-The-Envelope-Icon@My-Web-Site.com> wrote in
message news:2fpep19105pr2epf4sqpk9cvksgv11reg5@4ax.com...
On Wed, 07 Dec 2005 20:02:36 GMT, "Hank" <lightdoesnotage@yahoo.com
wrote:

Hi,

I am converting the 0 to 4.096V output of a DAC into -2.048V to +2.048V
using an op-amp as a differential amplifier. On powerup, the DAC is reset
to 0V, which obviously makes the output of the op-amp -2.048V. I want the
output of the op-amp to be 0V on powerup until I enable it with 5V logic.

Is there a really easy way of implementing this that I am just missing?
All
of my solutions seem too complicated for such a seemingly simple
requirement. :-(

Thanks,

Hank


Can you enable Vref? If it's zero at power-up, so is the OpAmp
output.


Actually, upon further thought, I am going to switch to a shunt type Vref.
The part I was going to use, AD158x has an inrush current of 15mA or more.
It has a low overhead requirement of only 200mV, but if the pfet I choose
has
a typical Rdson of ~100 ohms, that is 1.5V dropped across the fet...oops!

I think a typical resistor used with a shunt is about 1k, meaning only 1mA
through
the pfet, or a drop of 100mV. The Vref input to the DAC has a 160k
impedance,
the input impedance of my op-amp configuration is going to be equal to R3,
so
probably about 20k...I guess I should buffer the output from the shunt?

I guess there never is a really simple solution...

There are some amplifiers around with clamping terminals. The
Comlinear CLC502

http://cache.national.com/ds/CL/CLC502.pdf

comes to mind, but National Semiconductor has discontinued it.

The compensation terminal on the LM301 and the (rather nasty) uA748 can
be used as a clamping input - in addition to the 30pF to ground
compensation capacitor (built into modern op amps) you can hook up a
pair of small diodes (!N918 or similar) that draw current if the
compensation point goes above some positive voltage, or sink current if
the output goes below some negative voltage.

I last tried this around 1975, so the details are a little vague in my
mind.

--------------
Bill Sloman, Nijmegen
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Tony Williams
Guest
Posted: Thu Dec 08, 2005 1:24 pm    Post subject: Re: adding an enable to a bipolar op-amp, how??? Reply with quote
In article <4398268A.6010003@nospam.com>,
Fred Bloggs <nospam@nospam.com> wrote:


Quote:
.
. +5
. |
. |
. 4.096V >--[R3]-+-[R4]------|----.
. | | |
. | | OA |
. '---------|-\ |
. | >---+-> Out
. .---------|+/
. | |
. Vin >--[R1]-+-[R2]------+--- 0v.
[snip]


A caveat Fred. A 'RRIO' opamp whose o/p stage is a
bipolar transistor with a parallel c-c FET for the final
pulldown will get it's Vout lifted by the current through
R3+R4. The '324-type is quite an ambush for this effect
when running on single rail. Must use an opamp with a
genuine CMOS o/p stage.

--
Tony Williams.
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Fred Bloggs
Guest
Posted: Thu Dec 08, 2005 5:35 pm    Post subject: Re: adding an enable to a bipolar op-amp, how??? Reply with quote
Quote:

I am converting the 0 to 4.096V output of a DAC into -2.048V to +2.048V
using an op-amp as a differential amplifier. On powerup, the DAC is reset
to 0V, which obviously makes the output of the op-amp -2.048V. I want the
output of the op-amp to be 0V on powerup until I enable it with 5V logic.

Is there a really easy way of implementing this that I am just missing? All
of my solutions seem too complicated for such a seemingly simple
requirement. :-(


It would be simpler to keep the Enable control out of the precision
signal paths which would mean something like controlling the V- to the
OA, a RRIO type will then simply rail out at 0V. A Tiny Logic switch
will do.
View in a fixed-width font such as Courier.

..
.. +5
.. |
.. |
.. 4.096V >--[R3]-+-[R4]------|----.
.. | | |
.. | | OA |
.. '---------|-\ |
.. | >---+-> Out
.. .---------|+/
.. | |
.. Vin >--[R1]-+-[R2]-. |
.. | |
.. | |
| |
| |
| |
| |
.. +5V .----+ |
.. | | | |
.. | GND | |
.. __ |- | |
.. EN >--o| | |
.. |- | |
.. | | |
.. [1M] | |
.. | | \ |
.. +---------| o---'
.. | | /
.. [1M] | GATE
.. | |
.. '-----------+
.. |
.. -5V
..
..
..
..
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Spehro Pefhany
Guest
Posted: Thu Dec 08, 2005 5:35 pm    Post subject: Re: adding an enable to a bipolar op-amp, how??? Reply with quote
On Thu, 08 Dec 2005 09:40:22 -0500, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

Quote:
On Wed, 07 Dec 2005 20:02:36 GMT, the renowned "Hank"
lightdoesnotage@yahoo.com> wrote:

Hi,

I am converting the 0 to 4.096V output of a DAC into -2.048V to +2.048V
using an op-amp as a differential amplifier. On powerup, the DAC is reset
to 0V, which obviously makes the output of the op-amp -2.048V. I want the
output of the op-amp to be 0V on powerup until I enable it with 5V logic.

Is there a really easy way of implementing this that I am just missing? All
of my solutions seem too complicated for such a seemingly simple
requirement. :-(

Thanks,

Hank

How about this one?

This is probably a bit better in general, and perhaps better for this
application- it outputs zero rather than Vdac*(2/3) when 'disabled'
and still has cancellation of Ib* switch resistance and switch
resistance variations over input voltage.


R3 R4
___ ___
o-|___|-------.-----------------|___|-----.
+4.096V | |
| |
.------------. '
| | +------|----------------+
| | | | Iba |
| | 'o | <- |\ |
o-----------| | __--o-|------|-\ ' Vo
Enable | '----o | | >------+----o
| | .---|+/
GND-|------o | | |/
___ | __--o-|--+
o-|___|---+-|-- ---o | <- Ibb
R1 | '------------'
DAC 0-4V | 2/3 4053
.-.
R2 | |
| |
'-'
|
GND

R2 = 2*R1
R3 = 2*R4
R1||R2 ~= R3||R4



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
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Spehro Pefhany
Guest
Posted: Thu Dec 08, 2005 5:35 pm    Post subject: Re: adding an enable to a bipolar op-amp, how??? Reply with quote
On Wed, 07 Dec 2005 20:02:36 GMT, the renowned "Hank"
<lightdoesnotage@yahoo.com> wrote:

Quote:
Hi,

I am converting the 0 to 4.096V output of a DAC into -2.048V to +2.048V
using an op-amp as a differential amplifier. On powerup, the DAC is reset
to 0V, which obviously makes the output of the op-amp -2.048V. I want the
output of the op-amp to be 0V on powerup until I enable it with 5V logic.

Is there a really easy way of implementing this that I am just missing? All
of my solutions seem too complicated for such a seemingly simple
requirement. :-(

Thanks,

Hank

How about this one?


R3 R4
___ ___
o-|___|-------+-----------------|___|-----.
+4.096V | |
| |
.------------. '
Enable | | +------|----------------+
o-----------| | | | |
| | 'o | |\ | +/-2.048V out
| | __--o-|------|-\ '
| '----+ | | >------+----o
R1 | | .---|+/
___ +---|------o | | |/
o-|___|-+ | __--o-|--+
+---|------o |
DAC 0-4V | '------------'
.-. 2/3 4053
| |
R2 | |
'-'
|

GND



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
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