Function Generator Current limiting

Thread Starter

dotbot

Joined Mar 29, 2019
8
Hi all
I'm in the process of designing a bench top function generator (for the learning experience more than anything else) and I'm having some trouble understanding output current characteristics of op amps.

My design draws heavily from the example DIY function generator by Cezar Chirilla here on All About Circuits (however i plan on using the AD5932 chip and add a few more bells and whistles)
https://www.allaboutcircuits.com/projects/how-to-DIY-waveform-generator-analog-devices-ad9833-ATmega328p/

For some context
The circuit below takes the output from the AD9833 (i will use a AD5932) and first adjusts the amplitude of the signal (op amp in non-inverting config) and the second adjusts the offset (op amp as a summing amp). My confusion is around how much current can be drawn from the output of the second op-amp.
upload_2019-3-30_14-33-29.png
I haven't chosen an op-amp yet but lets say i use the LM318N, how can I determine the maximum output current if I have a very small load or short the output to ground?
upload_2019-3-30_15-11-6.png
I have seen that some op-amp data sheets have values for max output current (like the LM124N), but this one only seems to have supply current, is that output current?

Essentially I'm trying to work out if I need to put some circuitry into limiting the output current (only way I could think to do this is an AB amplifier because I want to be able to output negative and positive voltages) or if I can just heat sink the opamp enough that it will survive the output being shorted to ground while a sine wave with a +12V and -12V peaks is being outputted (or close to that).

In an ideal world, I would only attach the output to high impedance loads but will all do silly things sometimes and I want to protect my device from said silliness.

This is my first post on AAC so if i've broken any rules with this post please let me know and i will try and rectify them.

Thanks
 

danadak

Joined Mar 10, 2018
3,577
It does state in data sheet output can support a continuous short. Page 2.

http://www.ti.com/lit/ds/symlink/lm318-n.pdf

Supply current is NOT output current. There are instances in some parts
where device current is so small next to load specified in specs that supply
current could be interpreted as output current, so read datasheet carefully.

Normally one wants output to be 50 ohms. The LM318 is limited to 500 mW of dissipation, so you have to pay attention to non shorted conditions as well.


Regards, Dana.
 

OBW0549

Joined Mar 2, 2015
2,984
Very often the graphs shown in a data sheet will contain lots of good information not given in the tables. In the case of the LM318, Figure 8 shows the part features a very sharp output current limiting, at around +/- 20 mA:
Screen Shot.png
 

Thread Starter

dotbot

Joined Mar 29, 2019
8
It does state in data sheet output can support a continuous short. Page 2.

http://www.ti.com/lit/ds/symlink/lm318-n.pdf

Supply current is NOT output current. There are instances in some parts
where device current is so small next to load specified in specs that supply
current could be interpreted as output current, so read datasheet carefully.

Normally one wants output to be 50 ohms. The LM318 is limited to 500 mW of dissipation, so you have to pay attention to non shorted conditions as well.


Regards, Dana.
Thanks for the reply danadak, I will endeavor to read datasheets much more carefully from now on (and look more closely at the graphs thanks OBW0549)

I am not quite sure about your last paragraph thou, what do you mean by "one wants the output to be 50 ohms" ?

also, could you give an example of in what circumstances you could dissipate more than 500mW through the op-amp?

Maybe I'm miss understanding but if the typical voltage swing is +-13
upload_2019-3-31_11-43-9.png
and the current limiting is such that the max current possible is +- 21mA or so (and that's for voltage swings of +-11 or less)
upload_2019-3-31_11-45-56.png
Wouldn't the max possible power dissipation be somewhere in the order of 90mW or so?
(voltage swing of +-12V the output current would be limited to about 7ma giving a power dissipation of 84mW)

Regards dotbot
 

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danadak

Joined Mar 10, 2018
3,577
My error,. not paying attention to output I limit. You will be fine on Pdiss.

50 ohms is standard function generator output z. So your design with LM318
will not be able to drive current necessary to swing a 50 ohm load appreciably.
50 ohms important if driving into 50 ohm systems thru coax to minimize reflections.
10's of Mhz kinds of applications.

http://www.rfcafe.com/references/electrical/history-of-50-ohms.htm

https://hackaday.com/2016/07/11/whats-special-about-fifty-ohms/

https://www.belden.com/blog/broadcast/50-ohms-the-forgotten-impedance


Regards, Dana.
 
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