Electronics > Projects, Designs, and Technical Stuff

Voltage comparator

(1/8) > >>

OM222O:
I'm becoming interested in making a frequency counter and the software part is all done.
I just needed a way to convert any incoming signal to a clean square wave so my first idea was to use an op amp / voltage comparator in schmitt trigger mode to do that. my main questions is: what do I need to look out in the op amp spec for high frequency applications? let's say I want about 20MHz bandwidth. should I care about slew rate as it's purely just switching between two states? is gain bandwidth product important in this configuration? would a dedicated voltage comparator be more suitable in this application?
And finally if op amps / comparators can't provide that much bandwidth, what's the simplest way I can make a voltage comparator using BJTs?

ArthurDent:
The way it was explained to me was that you can view the rise time of a square wave as a clipped sine wave of infinite frequency. The faster rise time will give a more precise trigger point so slew rate is important. Have you considered a high speed amp followed by a fast logic gate.

OM222O:
I won't need a logic gate as I'm currently running on an LPC1768 uC which runs at about 100 or 140MHz (can't remember but it's plenty fast enough) with interrupts on rising edge and a 5 second rolling average for that extra bit of accuracy! that being said, yes I had a look at op amps and fast ones seem to be application specific, for example driving a video signal. This one from TI seems overkill and costs about 5$ which is acceptable.
http://www.ti.com/lit/ds/symlink/opa855.pdf
but again I'm not sure what spec to look for? Recovery time? slew rate? bandwidth?

here are a few cheaper alternatives that I can use for testing but I'm not sure which one to order due to not knowing which spec I should optimize for.
http://www.ti.com/lit/ds/symlink/opa355.pdf
https://www.mouser.co.uk/datasheet/2/609/ADA4851-1_4851-2_4851-4-878231.pdf

David Hess:
Do not use an operational amplifier; they are only suitable for low speed comparator applications.

20 MHz will require higher speed than common comparators like the 150 nanosecond LM311 or 300 nanosecond LM339/LM393 will support.  The 10 nanosecond LT1016 would be a good choice or the 12 nanosecond LT1116 if single supply operation is required.

Faster designs would likely use discrete differential amplifier stages followed by just a suitably fast logic gate.

OM222O:
did you even look at the data sheets of the op amps I posted? the first one has 8GHz bandwidth and 2.7KV/uS slew rate, not to mention 3nS recovery time   :wtf::palm: what part of that is "low speed"?  :-//

also the parts you mentioned seem fine but this TI one seems to have just 4.5nS rise and fall time?
http://www.ti.com/lit/ds/symlink/tlv7031.pdf?HQS=TI-null-null-mousermode-df-pf-null-wwe&DCM=yes&ref_url=https%3A%2F%2Fwww.mouser.co.uk%2F
I don't strictly care about propagation delay as the period is the most important thing here, I don't even require the output to have a balanced duty cycle (i.e: close to 50%). I'm not sure if I'm missing something here (for example if I should care about propagation delay as it affects frequency response somehow? )

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod