Electronics > Projects, Designs, and Technical Stuff
Voltage comparator
<< < (8/8)
duak:
You've got rather a lot of hysteresis there OM.  Try varying the value of R1 between 100R and 1K0.

The rise and fall times of the outputs are about what the datasheet shows.

I used the hp 5315B counter a number of times and found its triggering to be quite useable.  The LED indicator was very handy for setting the threshold.  I looked at the hp 5316B manual and see that it has some test point connections to observe trigger level.  If I were to rebuild my counter, I would add an LED indicator as well as a test point or connector to allow a scope to show the comparator output and actually see the trigger, sort of like the trigger view on some scopes.  I would also consider adding a multirange low pass filter tied to the frequency range selector to attenuate noise above the expected range.
OM222O:
you are absolutely right  :-DD
I added a third resistor to add a DC offset to the hysteresis and it works very well now with input voltages as low as 0.2v.
I also added a common collector amplifier which increases input impedance by beta x Remitter which should be at least 100x10k which is 1Meg, plenty good enough for now.



Again I'm debating if I should add a common emitter amplifier for the gain, or just use a high speed op amp ... fiddling with CE amplifier and biasing point etc has proven to be a nightmare  :-DD :-// |O
David Hess:

--- Quote from: OM222O on May 01, 2019, 04:43:38 pm ---is there something wrong with my circuit? or are these comparator just not fast enough?
--- End quote ---

The impedance at the non-inverting input is too high.

Also, add a little bit of AC hysteresis at high frequencies with a small capacitor in parallel with R2.  Just 10s of picofarads is sufficient.

I forgot to mention it but there are some comparators with adjustable hysteresis via an external input.  I perused a few but none of them struck me as particularly suitable.

If I was going to add variable hysteresis, I would use the comparator's output to drive a diode bridge biased by adjustable currents which then drives the appropriate input.  Old fast function generators work this way.  If the hysteresis only goes in one direction, then the bridge can be replaced with just two diodes or transistors and one current source.  Despite having used this feature on the HP counters I mentioned, I am dubious that it is worth putting that much effort into but it a place in automated test gear.

Another intriguing possibility is to drive one of the offset null pins for hysteresis but most comparators lack them.


--- Quote ---I ran some simulations using LTSpice and the LT1116 seems to be totally useless at 20Meg (lower frequencies are ok)

So I tried the LT1016 which seems pretty bad too
--- End quote ---

The LT1016 is good up to about 50 MHz.
Navigation
Message Index
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod