Author Topic: Square wave generation & current  (Read 2819 times)

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Offline zanzeooTopic starter

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Square wave generation & current
« on: September 30, 2018, 07:03:17 am »
Hello

I'm trying to desing a square wave generator ( or function generator amplifier)  with these features :

Pure resistive load (33 ohms)
Ouput from 1Vpp to 5 VPP
Frequency 2Mhz


Here is what i've done :(see attachement)


TC4425 with a clock from function generator (Dclock) 0-5Vdc
the Ouput Ur1 works fine with a 5VPP but not below ( because of VDD of TC4425)
VDD is done with an external power supply

Is there a better way to do the same with the possibility to choose VPP output below (1 VPP for exemple )
I guest with external low VGSth mosfet  ??
« Last Edit: September 30, 2018, 07:06:51 am by zanzeoo »
 

Offline zanzeooTopic starter

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Re: Square wave generation & current
« Reply #1 on: September 30, 2018, 07:05:06 am »
ERRATA : R1 is 33 ohm , not 10K
 

Offline iMo

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Re: Square wave generation & current
« Reply #2 on: September 30, 2018, 10:00:56 am »
There are high-speed opamps, capable of driving 50ohm loads. I would try with those. You may set the gain with a resistor in the feedback.
Readers discretion is advised..
 
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Offline zanzeooTopic starter

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Re: Square wave generation & current
« Reply #3 on: September 30, 2018, 10:10:16 am »
well , do you have a reference of these opamps ? , i need a 33ohms load , not 50 ... ;)
 

Online David Hess

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Re: Square wave generation & current
« Reply #4 on: September 30, 2018, 07:06:03 pm »
Is the output suppose to be balanced or single ended?  A variable balanced output would be somewhat unusual.  Your example actually shows a balanced 10 volt peak-to-peak output.

Is this intended to drive a transmission line?  Usually a series resistance is used to set the output impedance although a parallel resistance and current source can be used also.
 

Offline zanzeooTopic starter

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Re: Square wave generation & current
« Reply #5 on: September 30, 2018, 07:10:06 pm »
Is the output suppose to be balanced or single ended?  A variable balanced output would be somewhat unusual.  Your example actually shows a balanced 10 volt peak-to-peak output.

Is this intended to drive a transmission line?  Usually a series resistance is used to set the output impedance although a parallel resistance and current source can be used also.


not to drive a transmission line , but a pure résistive 33 ohms load ,  so yes i need a balanced output which can vary from 1VPP to 5 VPP
 

Online David Hess

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Re: Square wave generation & current
« Reply #6 on: September 30, 2018, 07:18:19 pm »
not to drive a transmission line , but a pure résistive 33 ohms load ,  so yes i need a balanced output which can vary from 1VPP to 5 VPP

Ok, but your example shows a 10 volt peak-to-peak signal.

Is the only requirement a square wave?
 

Offline zanzeooTopic starter

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Re: Square wave generation & current
« Reply #7 on: September 30, 2018, 07:21:35 pm »
not to drive a transmission line , but a pure résistive 33 ohms load ,  so yes i need a balanced output which can vary from 1VPP to 5 VPP :palm:

Ok, but your example shows a 10 volt peak-to-peak signal.

Is the only requirement a square wave?



sorry for the misunderstood , yes from 2 Vpp to 10 VPP

 

Online David Hess

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Re: Square wave generation & current
« Reply #8 on: October 01, 2018, 01:40:37 am »
So that leaves 10 volts peak-to-peak across 33 ohms or 303 milliamps.  The 2 MHz requirement is not particularly difficult.

A simple analog solution using power operational amplifiers could be made with something like LT1206s.  Current boosted operational amplifiers could also work but can be tricky.

The pulse generator solution is much faster and could amount to a MOSFET h-bridge but the gate drive to the MOSFETs needs to be configured to be independent of the supply voltage which is the problem with the TC4425.
 

Offline zanzeooTopic starter

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Re: Square wave generation & current
« Reply #9 on: October 01, 2018, 08:34:40 am »
So a simple follower with à lt1206 would do the job ?
 

Offline zanzeooTopic starter

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Re: Square wave generation & current
« Reply #10 on: October 02, 2018, 10:09:18 am »
No more idea ?
 

Offline T3sl4co1l

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Re: Square wave generation & current
« Reply #11 on: October 02, 2018, 05:10:47 pm »
What is the load actually?

If it's literally a 33 ohm resistor, why not use DC?

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Offline zanzeooTopic starter

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Re: Square wave generation & current
« Reply #12 on: October 09, 2018, 12:39:06 pm »
Yes its 33  ohms but i need 2Mhz freq...
 ???


 

Offline mikerj

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Re: Square wave generation & current
« Reply #13 on: October 09, 2018, 04:18:16 pm »
Yes its 33  ohms but i need 2Mhz freq...
 ???

Why do you need a differential output?  Can a single ended drive be used (one side of the resistor be connected to 0v the other other side driven by 10Vpp)?
 

Offline zanzeooTopic starter

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Re: Square wave generation & current
« Reply #14 on: October 09, 2018, 04:23:54 pm »
Yes its 33  ohms but i need 2Mhz freq...
 ???

Why do you need a differential output?  Can a single ended drive be used (one side of the resistor be connected to 0v the other other side driven by 10Vpp)?

Yes because it's suppose to simulate a sensor so , differential is needed .
 

Offline T3sl4co1l

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Re: Square wave generation & current
« Reply #15 on: October 09, 2018, 09:17:26 pm »
Again, what is the driver connected to?
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Offline bson

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Re: Square wave generation & current
« Reply #16 on: October 09, 2018, 10:44:25 pm »
You can use differential output amplifier, for example http://www.ti.com/product/THS4532/description (or the slightly cheaper THS4531A).  Use it as a 1x single ended to differential buffer and attenuate the input as needed to get the right output level.  If you need to shape the square wave put an inverter in front of the attenuation, and if polarity matters simply reverse the outputs.
« Last Edit: October 09, 2018, 10:53:01 pm by bson »
 
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Offline zanzeooTopic starter

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Re: Square wave generation & current
« Reply #17 on: October 09, 2018, 10:50:28 pm »
Nice but not sure it can drive a 33 ohms load  alone ??? i guess it needs a transformer at output ?
 

Offline bson

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Re: Square wave generation & current
« Reply #18 on: October 09, 2018, 11:01:16 pm »
Nice but not sure it can drive a 33 ohms load  alone ??? i guess it needs a transformer at output ?
The datasheet states 50mA max, so it should be fine for 1Vpp, although I suppose only with the single unit THS4531A.

 

Offline bson

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Re: Square wave generation & current
« Reply #19 on: October 09, 2018, 11:06:04 pm »
Ah, nvm, it's +/-25mA for the two outputs, so 50mA total, while you need a 31mA output.  What's up with the 33Ω load?  That's pretty weird.  Maybe just some low voltage bridge driver?
 

Offline bson

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Re: Square wave generation & current
« Reply #20 on: October 09, 2018, 11:12:30 pm »
DRV8838?

Just looking through TI's product selector since I'm familiar with it, but I'm sure there's a gazillion of these out there.
 
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