SD2000 easy 10% greater depth mod.

[Carl-NC]

Hey Carl,

On the op amp inputs its not the same thing
and you have to calculate both.
And then add them.
Right?

Opamp input current noise is not the same as the feedforward R noise. See my old noise analysis thread, click here.

[Mr. Bean]

I was double spacing back then !!!

Opamp input current noise is not the same as the feedforward R noise. See my old noise analysis thread, click here.

Holy crap I was a double spacing back then too.

This is going to be harder to break then I thought.

Anyway looked up the AD797 bipolar op amp and it is

2 pA/rtHz, so not sure exactly what Woody was trying to

say, thought I could figure it out, add some bandwidth

and get the numbers in the nA, but barely.

Comparing the "current" noise in the 1 kohm to the bipolar

input of op amp.

Resistors have Power Noise. Same thing.

Op Amps current noise and voltage noise effects calculated

seperately and sumed with sum of the ol squares routine.

[Mr. Bean]

Hope you know its me

Hey Carl,

You should know I am using windows and firefox and

have my DNS address, and know it is me.

I like Mr. Bean.

He's Funny.

[Carl-NC]

Holy crap I was a double spacing back then too.

I just now looked at the old thread, had forgotten you were a major participant in that conversation. Since you've been around so long, why don't you go ahead and register? Use Mr Bean as an avatar.

[Mr. Bean]

Oct 2005

I just now looked at the old thread, had forgotten you were a major participant in that conversation. Since you've been around so long, why don't you go ahead and register? Use Mr Bean as an avatar.

Thanks for the invite Carl, maybe sometime, for now

probably should get my tail back to work and quit playing

on the internet.

Here's a post of mine from that thread Oct. 2005.

{Fet front end, sound familiar?}


Hi Eric,

Hope you're getting this.

Part of Classwork.

There will be a test.

Performance.

So build it yourself, lots of time, hmmmm, Receipe.

Take a 1.0 nV/rtHz or less JFet, add a drain resistor, add opamp, close loop on whole thing.

No not that easy, but not that bad either.

Good start?

[sharky]

Noise

Hi Reg,

You changed your email as it keeps bouncing.....

The N channel fet on the front end is switched off during flyback and decay then switched on to allow the receive signal to pass. This is then connected to a P channel fet that feeds the input of the first ampifier...SD2000 -2100 is a LM394 and later versions to a AD797. The P channel also feeds a timing pulse to the timing electronics.

Minelab make most of their gains via getting rid of the series 1K ohm dropping resistor that is used on other P.I front end designs. The noise from a 1K resistor would be 4nv/hertz/2 but a bipolar input stage needs to take into account current noise that can be up to 100nv/hz/2. Even the AD797 is not optimum because when the device is open circuited due to turn off via the front end fet the AD797 goes into unstable oscillation and noise generation that is not rapidly brought under control by sourcing a low impedance path.

There is plenty to do on a P.I front end but the Candy approach is in the right direction.

Just by shunting the input of a AD797 with 5K ohms the noise decreases somewhat. There is plenty of scope for more experimentation on P.I front ends.

Now u guys really talk of noise in front end for better depth which our guy MAX rejects

[Max]

Now u guys really talk of noise in front end for better depth which our guy MAX rejects

I don't reject anything... expecially lemon sized diamonds PIKE.

PIKE HERE, where are pictures of your lemons !

Or you need a guru or something...to increase your S/N ratio ?

Kind regards,
Max

[Aziz]

Here is a link about keeping the old supply lines clean

might be important after low noising the front end, maybe

not, maybe there is enough PSRR, could be dangerous on

the discrete stuff, who knows?

http://www.wenzel.com/documents/finesse.html

Hi,

this could be much better perhaps:

Ultra-Low-Noise LDO Achieves 6nV √ Hz Noise Performance
http://pdfserv.maxim-ic.com/en/an/AN3657.pdf

[JC1]

WoW!

Hi Aziz,

WoW !!! that is one low noise low dropout regulator.

Seems it would be extreme overkill

and for most things I am sure it is.

And yet someone designed it for something

and Maxim promotes it for some reason.

Anyway, good find on your part.

And keep up the good posts !!!

[Carl-NC]

It was designed for ultrasound applications. I'm using it in 2 Maxim ultrasound reference designs.

- Carl

[Aziz]

It was designed for ultrasound applications. I'm using it in 2 Maxim ultrasound reference designs.

- Carl

Hi Carl,

during the search for ultra low noise LNO, I explored that many high definition audio freaks also use such techniques. But most used by the RF engineers to get VCO and PLL most stable to meet the requirements.

Getting this technique to the PI pre-amp could bring more SNR and stability.

I found also that different battery types causes different noise behaviour. So it is also a matter of using the right batteries.

Aziz