op amp specs question

[panfilero]

Hello,

I need an op amp that can handle a 800mA current surge at it's output
for a few mS.... it will be operating as a unity gain buffer, with
negative feedback... it there a spec on the datasheets that tell you
how much of an inrush current an op amp can handle at the output? I
see that most the opamps can supply around 60mA but I can't tell about
what kind of surge they can handle....

Reason for the surge is a capacitive load

Thanks!

 

On Apr 23, 9:41 am, panfilero <panfil...@gmail.com> wrote:

Hello,

I need an op amp that can handle a 800mA current surge at it's output
for a few mS.... it will be operating as a unity gain buffer, with
negative feedback... it there a spec on the datasheets that tell you
how much of an inrush current an op amp can handle at the output?  I
see that most the opamps can supply around 60mA but I can't tell about
what kind of surge they can handle....

Reason for the surge is a capacitive load

Thanks!

Sounds like you need a high current opamp. I like the OPA544 for
+/-15V and the OPA569 for 5V. If the output is single ended you could
always buffer the low current opamp output with a transistor.

George Herold

 

[panfilero]

On Apr 23, 12:36 pm, ggher...@gmail.com wrote:

On Apr 23, 9:41 am, panfilero <panfil...@gmail.com> wrote:

Hello,

I need an op amp that can handle a 800mA current surge at it's output
for a few mS.... it will be operating as a unity gain buffer, with
negative feedback... it there a spec on the datasheets that tell you
how much of an inrush current an op amp can handle at the output?  I
see that most the opamps can supply around 60mA but I can't tell about
what kind of surge they can handle....

Reason for the surge is a capacitive load

Thanks!

Sounds like you need a high current opamp.  I like the OPA544 for
+/-15V and the OPA569 for 5V.  If the output is single ended you could
always buffer the low current opamp output with a transistor.

George Herold

Thanks George, so I'm worried about current surges in and out of my
opamp output... does this mean I could handle this with a totem pole
transistor set up at the output of my opamp?

 

[Eeyore]

panfilero wrote:

Hello,

I need an op amp that can handle a 800mA current surge at it's output
for a few mS.... it will be operating as a unity gain buffer, with
negative feedback... it there a spec on the datasheets that tell you
how much of an inrush current an op amp can handle at the output? I
see that most the opamps can supply around 60mA but I can't tell about
what kind of surge they can handle....

They usually have current limiting built in. If you don't mind your cap
taking a few tens of ms perhaps to charge the caps to some quiescent
voltage at turn on then it's not a problem. What would be a problem is
high desired dV/dt at the op-amp output.

Also most op-amp don't like heavy capacitive loads, it changes the phase
response and at unity gain that's exactly what you don't want for
stability.

You could also add a discrete output stage within the feedback loop..

Graham

 

[Eeyore]

panfilero wrote:

On Apr 23, 12:36 pm, ggher...@gmail.com wrote:
On Apr 23, 9:41 am, panfilero <panfil...@gmail.com> wrote:

Hello,

I need an op amp that can handle a 800mA current surge at it's output
for a few mS.... it will be operating as a unity gain buffer, with
negative feedback... it there a spec on the datasheets that tell you
how much of an inrush current an op amp can handle at the output? I
see that most the opamps can supply around 60mA but I can't tell about
what kind of surge they can handle....

Reason for the surge is a capacitive load

Thanks!

Sounds like you need a high current opamp. I like the OPA544 for
+/-15V and the OPA569 for 5V. If the output is single ended you could
always buffer the low current opamp output with a transistor.

George Herold

Thanks George, so I'm worried about current surges in and out of my
opamp output... does this mean I could handle this with a totem pole
transistor set up at the output of my opamp?

A schematic is needed. So many variables.

Graham

 

[Eeyore]

panfilero wrote:

Eeyore <rabbitsfriendsandrelati...@hotmail.com> wrote:
panfilero wrote:
On Apr 23, 12:36 pm, ggher...@gmail.com wrote:
On Apr 23, 9:41 am, panfilero <panfil...@gmail.com> wrote:

Hello,

I need an op amp that can handle a 800mA current surge at it's output
for a few mS.... it will be operating as a unity gain buffer, with
negative feedback... it there a spec on the datasheets that tell you
how much of an inrush current an op amp can handle at the output? I
see that most the opamps can supply around 60mA but I can't tell about
what kind of surge they can handle....

Reason for the surge is a capacitive load

Thanks!

Sounds like you need a high current opamp. I like the OPA544 for
+/-15V and the OPA569 for 5V. If the output is single ended you could
always buffer the low current opamp output with a transistor.

George Herold

Thanks George, so I'm worried about current surges in and out of my
opamp output... does this mean I could handle this with a totem pole
transistor set up at the output of my opamp?

A schematic is needed. So many variables.

Graham

ok, here's a link to the schematic

http://s555.photobucket.com/albums/jj477/panfilero/?action=view&current=OpAmpsDrivingCap.jpg

basically the point of my opamps is to sink and source current and
protect my DAC.... so they are just voltage followers... I was
thinking maybe I'm overdoing it with these two opamps and could just
use two transistors in a totem pole arrangement to source and sink the
current... but I've never done that and I'm not sure if it's a good
solution

Forget totem-pole, it's 'old-hat'

U1B is almost superflous unless you need to sink current into it ( I presume you do ).

What value of capacitor do you have in mind and why is it there ( what is its purpose, since
being driven by voltage sources it will do nothing other than be a pain in the ass and limit
dV/dt ) ? What's going to be connected to it ? Where did you get this circuit ?

Graham

 

[panfilero]

On Apr 23, 4:18 pm, Eeyore <rabbitsfriendsandrelati...@hotmail.com>
wrote:

panfilero wrote:
On Apr 23, 12:36 pm, ggher...@gmail.com wrote:
On Apr 23, 9:41 am, panfilero <panfil...@gmail.com> wrote:

Hello,

I need an op amp that can handle a 800mA current surge at it's output
for a few mS.... it will be operating as a unity gain buffer, with
negative feedback... it there a spec on the datasheets that tell you
how much of an inrush current an op amp can handle at the output?  I
see that most the opamps can supply around 60mA but I can't tell about
what kind of surge they can handle....

Reason for the surge is a capacitive load

Thanks!

Sounds like you need a high current opamp.  I like the OPA544 for
+/-15V and the OPA569 for 5V.  If the output is single ended you could
always buffer the low current opamp output with a transistor.

George Herold

Thanks George, so I'm worried about current surges in and out of my
opamp output... does this mean I could handle this with a totem pole
transistor set up at the output of my opamp?

A schematic is needed. So many variables.

Graham

ok, here's a link to the schematic

http://s555.photobucket.com/albums/jj477/panfilero/?action=view&current=OpAmpsDrivingCap.jpg

basically the point of my opamps is to sink and source current and
protect my DAC.... so they are just voltage followers... I was
thinking maybe I'm overdoing it with these two opamps and could just
use two transistors in a totem pole arrangement to source and sink the
current... but I've never done that and I'm not sure if it's a good
solution

Thanks!

 

[panfilero]

On Apr 23, 7:07 pm, Eeyore <rabbitsfriendsandrelati...@hotmail.com>
wrote:

panfilero wrote:
Eeyore <rabbitsfriendsandrelati...@hotmail.com> wrote:
panfilero wrote:
On Apr 23, 12:36 pm, ggher...@gmail.com wrote:
On Apr 23, 9:41 am, panfilero <panfil...@gmail.com> wrote:

Hello,

I need an op amp that can handle a 800mA current surge at it's output
for a few mS.... it will be operating as a unity gain buffer, with
negative feedback... it there a spec on the datasheets that tell you
how much of an inrush current an op amp can handle at the output?  I
see that most the opamps can supply around 60mA but I can't tell about
what kind of surge they can handle....

Reason for the surge is a capacitive load

Thanks!

Sounds like you need a high current opamp.  I like the OPA544 for
+/-15V and the OPA569 for 5V.  If the output is single ended you could
always buffer the low current opamp output with a transistor.

George Herold

Thanks George, so I'm worried about current surges in and out of my
opamp output... does this mean I could handle this with a totem pole
transistor set up at the output of my opamp?

A schematic is needed. So many variables.

Graham

ok, here's a link to the schematic

http://s555.photobucket.com/albums/jj477/panfilero/?action=view&curre....

basically the point of my opamps is to sink and source current and
protect my DAC.... so they are just voltage followers... I was
thinking maybe I'm overdoing it with these two opamps and could just
use two transistors in a totem pole arrangement to source and sink the
current... but I've never done that and I'm not sure if it's a good
solution

Forget totem-pole, it's 'old-hat'

U1B is almost superflous unless you need to sink current into it ( I presume you do ).

What value of capacitor do you have in mind and why is it there ( what is its purpose, since
being driven by voltage sources it will do nothing other than be a pain in the ass and limit
dV/dt ) ? What's going to be connected to it ? Where did you get this circuit ?

Graham

So is this better than pursuing a two transistor solution like totem
poles?

You are right I will be sinking into U1B, because my voltage across
the load is bipolar

Its for a lab experiment at school, we don't have the test article
yet... it's a piezo element which was described by the vendor as a
capacitive load... but they couldn't give me any specific values for
it (I don't know how to model it) so I'm trying to make my circuit for
the worst case scenario... which would be a purely capacitive load...
but I'm worried about my design cause I don't want to fry my opamps
with the surge current from the cap...

I made up the circuit

 

[Bob Masta]

On Thu, 23 Apr 2009 18:22:35 -0700 (PDT),
panfilero <panfilero@gmail.com> wrote:

Its for a lab experiment at school, we don't have the test article
yet... it's a piezo element which was described by the vendor as a
capacitive load... but they couldn't give me any specific values for
it (I don't know how to model it) so I'm trying to make my circuit for
the worst case scenario... which would be a purely capacitive load...
but I'm worried about my design cause I don't want to fry my opamps
with the surge current from the cap...

I made up the circuit

Not sure what sort of piezo element you will be
using, but the ones I have used require high
voltages (several hundred to 1000 volts). You may
want to look at the high voltage / high power
op-amps by Apex.

Best regards,




Bob Masta

DAQARTA v4.51
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
FREE Signal Generator
Science with your sound card!

 

[panfilero]

On Apr 24, 6:39 am, N0S...@daqarta.com (Bob Masta) wrote:

On Thu, 23 Apr 2009 18:22:35 -0700 (PDT),

panfilero <panfil...@gmail.com> wrote:
Its for a lab experiment at school, we don't have the test article
yet... it's a piezo element which was described by the vendor as a
capacitive load... but they couldn't give me any specific values for
it (I don't know how to model it) so I'm trying to make my circuit for
the worst case scenario... which would be a purely capacitive load...
but I'm worried about my design cause I don't want to fry my opamps
with the surge current from the cap...

I made up the circuit

Not sure what sort of piezo element you will be
using, but the ones I have used require high
voltages (several hundred to 1000 volts).  You may
want to look at the high voltage / high power
op-amps by Apex.

Best regards,

Bob Masta

              DAQARTA  v4.51
   Data AcQuisition And Real-Time Analysis
             www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
           FREE Signal Generator
        Science with your sound card!

per manufacturer I need to send at the most -1V to 1V across it, and I
need to be ready for a current surge...

I was thinking... if I put a resistor at the output of each opamp,
before the load (and in series with the load) then... when the
capacitor is first energized it would be like a short and basically
there would be a resistaqnce between my opamp outputs... therefore if
I have say 6V at one output and 5V at the other then the most current
I will see (per ohms law) would be (6-5)/(R1+R2) where R1 and R2 are
the resistors at the outputs of the opamps that I just mentioned...
does this make sense? Would this solve my current surge issue?

Thanks!

 

[Eeyore]

panfilero wrote:

Eeyore <rabbitsfriendsandrelati...@hotmail.com> wrote:
panfilero wrote:
Eeyore <rabbitsfriendsandrelati...@hotmail.com> wrote:
panfilero wrote:
On Apr 23, 12:36 pm, ggher...@gmail.com wrote:
On Apr 23, 9:41 am, panfilero <panfil...@gmail.com> wrote:

Hello,

I need an op amp that can handle a 800mA current surge at it's output
for a few mS.... it will be operating as a unity gain buffer, with
negative feedback... it there a spec on the datasheets that tell you
how much of an inrush current an op amp can handle at the output? I
see that most the opamps can supply around 60mA but I can't tell about
what kind of surge they can handle....

Reason for the surge is a capacitive load

Thanks!

Sounds like you need a high current opamp. I like the OPA544 for
+/-15V and the OPA569 for 5V. If the output is single ended you could
always buffer the low current opamp output with a transistor.

George Herold

Thanks George, so I'm worried about current surges in and out of my
opamp output... does this mean I could handle this with a totem pole
transistor set up at the output of my opamp?

A schematic is needed. So many variables.

Graham

ok, here's a link to the schematic

http://s555.photobucket.com/albums/jj477/panfilero/?action=view&curre...

basically the point of my opamps is to sink and source current and
protect my DAC.... so they are just voltage followers... I was
thinking maybe I'm overdoing it with these two opamps and could just
use two transistors in a totem pole arrangement to source and sink the
current... but I've never done that and I'm not sure if it's a good
solution

Forget totem-pole, it's 'old-hat'

U1B is almost superflous unless you need to sink current into it ( I presume you do ).

What value of capacitor do you have in mind and why is it there ( what is its purpose, since
being driven by voltage sources it will do nothing other than be a pain in the ass and limit
dV/dt ) ? What's going to be connected to it ? Where did you get this circuit ?

Graham

So is this better than pursuing a two transistor solution like totem
poles?

I'd use complementary emitter followers.

You are right I will be sinking into U1B, because my voltage across
the load is bipolar

Its for a lab experiment at school, we don't have the test article
yet... it's a piezo element which was described by the vendor as a
capacitive load... but they couldn't give me any specific values for
it (I don't know how to model it) so I'm trying to make my circuit for
the worst case scenario... which would be a purely capacitive load...
but I'm worried about my design cause I don't want to fry my opamps
with the surge current from the cap...

I made up the circuit

Right, now it makes sense. What size piezo and we can give you an estimate of the likely C.

Graham

 

[Eeyore]

panfilero wrote:

On Apr 24, 6:39 am, N0S...@daqarta.com (Bob Masta) wrote:
On Thu, 23 Apr 2009 18:22:35 -0700 (PDT),

panfilero <panfil...@gmail.com> wrote:
Its for a lab experiment at school, we don't have the test article
yet... it's a piezo element which was described by the vendor as a
capacitive load... but they couldn't give me any specific values for
it (I don't know how to model it) so I'm trying to make my circuit for
the worst case scenario... which would be a purely capacitive load...
but I'm worried about my design cause I don't want to fry my opamps
with the surge current from the cap...

I made up the circuit

Not sure what sort of piezo element you will be
using, but the ones I have used require high
voltages (several hundred to 1000 volts). You may
want to look at the high voltage / high power
op-amps by Apex.

Best regards,

Bob Masta

per manufacturer I need to send at the most -1V to 1V across it, and I
need to be ready for a current surge...

I was thinking... if I put a resistor at the output of each opamp,
before the load (and in series with the load) then... when the
capacitor is first energized it would be like a short and basically
there would be a resistaqnce between my opamp outputs... therefore if
I have say 6V at one output and 5V at the other then the most current
I will see (per ohms law) would be (6-5)/(R1+R2) where R1 and R2 are
the resistors at the outputs of the opamps that I just mentioned...
does this make sense? Would this solve my current surge issue?

Putting an R in series with the C would be very effective but may slightly
reduce the acoustic output.

Graham

 

[MooseFET]

On Apr 23, 3:54 pm, panfilero <panfil...@gmail.com> wrote:

On Apr 23, 4:18 pm, Eeyore <rabbitsfriendsandrelati...@hotmail.com
wrote:



panfilero wrote:
On Apr 23, 12:36 pm, ggher...@gmail.com wrote:
On Apr 23, 9:41 am, panfilero <panfil...@gmail.com> wrote:

Hello,

I need an op amp that can handle a 800mA current surge at it's output
for a few mS.... it will be operating as a unity gain buffer, with
negative feedback... it there a spec on the datasheets that tell you
how much of an inrush current an op amp can handle at the output?  I
see that most the opamps can supply around 60mA but I can't tell about
what kind of surge they can handle....

Reason for the surge is a capacitive load

Thanks!

Sounds like you need a high current opamp.  I like the OPA544 for
+/-15V and the OPA569 for 5V.  If the output is single ended you could
always buffer the low current opamp output with a transistor.

George Herold

Thanks George, so I'm worried about current surges in and out of my
opamp output... does this mean I could handle this with a totem pole
transistor set up at the output of my opamp?

A schematic is needed. So many variables.

Graham

ok, here's a link to the schematic

http://s555.photobucket.com/albums/jj477/panfilero/?action=view&curre....

basically the point of my opamps is to sink and source current and
protect my DAC.... so they are just voltage followers... I was
thinking maybe I'm overdoing it with these two opamps and could just
use two transistors in a totem pole arrangement to source and sink the
current... but I've never done that and I'm not sure if it's a good
solution

If you want the results to be linear, you better off with op-amps.
You haven't said much about the signal. The op-amp may have to handle
some serious power if you are swinging a capacitive load up and down
very much.