Square wave generation & current - Page 1
Forum section: electronics / beginners

author: zanzeoo
author_rank: Contributor
timestamp: September 30, 2018, 07:03:17 am
content: HelloI'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 VPPFrequency 2MhzHere is what i've done :(see attachement)TC4425 with a clock from function generator (Dclock) 0-5Vdcthe Ouput Ur1 works fine with a 5VPP but not below ( because of VDD of TC4425)VDD is done with an external power supplyIs 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 ??

author: zanzeoo
author_rank: Contributor
timestamp: September 30, 2018, 07:05:06 am
content: ERRATA : R1 is 33 ohm , not 10K

author: iMo
author_rank: Super Contributor
timestamp: September 30, 2018, 10:00:56 am
content: 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.

author: zanzeoo
author_rank: Contributor
timestamp: September 30, 2018, 10:10:16 am
content: well , do you have a reference of these opamps ? , i need a 33ohms load , not 50 ...

author: David Hess
author_rank: Super Contributor
timestamp: September 30, 2018, 07:06:03 pm
content: 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.

author: zanzeoo
author_rank: Contributor
timestamp: September 30, 2018, 07:10:06 pm
content: Quote from: David Hess on September 30, 2018, 07:06:03 pmnot 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

author: David Hess
author_rank: Super Contributor
timestamp: September 30, 2018, 07:18:19 pm
content: Quote from: zanzeoo on September 30, 2018, 07:10:06 pmOk, but your example shows a 10 volt peak-to-peak signal.Is the only requirement a square wave?

author: zanzeoo
author_rank: Contributor
timestamp: September 30, 2018, 07:21:35 pm
content: Quote from: David Hess on September 30, 2018, 07:18:19 pmsorry for the misunderstood , yes from 2 Vpp to 10 VPP

author: David Hess
author_rank: Super Contributor
timestamp: October 01, 2018, 01:40:37 am
content: 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.

author: zanzeoo
author_rank: Contributor
timestamp: October 01, 2018, 08:34:40 am
content: So a simple follower with à lt1206 would do the job ?

author: zanzeoo
author_rank: Contributor
timestamp: October 02, 2018, 10:09:18 am
content: No more idea ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 02, 2018, 05:10:47 pm
content: What is the load actually?If it's literally a 33 ohm resistor, why not use DC?Tim

author: zanzeoo
author_rank: Contributor
timestamp: October 09, 2018, 12:39:06 pm
content: Yes its 33 ohms but i need 2Mhz freq...

author: mikerj
author_rank: Super Contributor
timestamp: October 09, 2018, 04:18:16 pm
content: Quote from: zanzeoo on October 09, 2018, 12:39:06 pmWhy 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)?

author: zanzeoo
author_rank: Contributor
timestamp: October 09, 2018, 04:23:54 pm
content: Quote from: mikerj on October 09, 2018, 04:18:16 pmYes because it's suppose to simulate a sensor so , differential is needed .

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 09, 2018, 09:17:26 pm
content: Again, what is the driver connected to?

author: bson
author_rank: Unknown
timestamp: October 09, 2018, 10:44:25 pm
content: You can use differential output amplifier, for examplehttp://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.

author: zanzeoo
author_rank: Contributor
timestamp: October 09, 2018, 10:50:28 pm
content: Nice but not sure it can drive a 33 ohms load alonei guess it needs a transformer at output ?

author: bson
author_rank: Unknown
timestamp: October 09, 2018, 11:01:16 pm
content: Quote from: zanzeoo on October 09, 2018, 10:50:28 pmThe datasheet states 50mA max, so it should be fine for 1Vpp, although I suppose only with the single unit THS4531A.

author: bson
author_rank: Unknown
timestamp: October 09, 2018, 11:06:04 pm
content: 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?

author: bson
author_rank: Unknown
timestamp: October 09, 2018, 11:12:30 pm
content: 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.