usual fast preamp questions

[Carl-NC]

If i want to reject targets such us medium size nails with TC say ~80uS, my amplifier minimum TC should be 80/5=16 uS.

No, if you set the TC to 16us then it won't detect targets less than 16us. (Ok, it will, but in a diminished way.)

And finaly In classical one stage amplifier for TC 16 uS i should place ~15 pF capacitor and 1000k feadback resistor.

Yes.

[Vadim18]

Thanks for clarification, Carl!
This thread just pointed my attention to amplifier tau, since i commonly use 45-80uS for first delay settings, and my current amplifier TC is 2uS, wich is not optimised.
So now i have plans to retune amplifier TC from 2 to 16uS, with hope what 5TC will not affect signal at 80uS delay.

[Carl-NC]

Yes, if you are sampling at 45-80us you certainly don't need a 2us preamp!

[Mr.Jaick]

What is the advantages and disadvantages of AC coupling the pre-Amp?

what i think right now:
advantages:
1) reducing DC offset picked up from ground

dis:
1) not being able to take EF sample? because of capacitor discharge

and when used with the inverting input of the OP-AMP, which resistor sets the cutoff frequency?

A, B or C?

[lucifer]

What is the advantages and disadvantages of AC coupling the pre-Amp?

what i think right now:
advantages:
1) reducing DC offset picked up from ground

dis:
1) not being able to take EF sample? because of capacitor discharge

and when used with the inverting input of the OP-AMP, which resistor sets the cutoff frequency?
[ATTACH]54880[/ATTACH]
A, B or C?

AC coupling the preamp is required to remove the DC offset of the input signal (usually when coil voltage decays to 12V and not to GND). You are still able to take an EF sample. In the attached circuit R10 and C11 form a high pass filter while R9 and C10 form a low pass filter. So both resistors A and C set cutoff frequencies of those filters.

[lucifer]

Yes, if you are sampling at 45-80us you certainly don't need a 2us preamp!

Hi Carl,
How about when preamp is done in two stages? It appears to me that the preamp overall tau is the sum of the tau of first and second stage. Am I right?

[Mr.Jaick]

AC coupling the preamp is required to remove the DC offset of the input signal (usually when coil voltage decays to 12V and not to GND). You are still able to take an EF sample. In the attached circuit R10 and C11 form a high pass filter while R9 and C10 form a low pass filter. So both resistors A and C set cutoff frequencies of those filters.

Thank you lucifer
basically it's needed when the coil decays to another voltage level than the analog ground (op amp ground)
I was wondering if there is any other use for it because I have seen in other places that they use it but doesn't seem to be any reason for it.
or maybe they didn't know what they're doing.
example:

signal will decay to GND (+12) here