Preamp

[Idefix]

Preamp and noise

Mainly from this:

The higher the resistance, the higher the noise (increase follows a root square law)

[ATTACH]34790[/ATTACH]

Hello,

About the noise, does anybody tries to make some changes in the main oscillator? like using a Xtal with divider (hef4060, etc ) , instead of RC and 555 ?
It seems to me , that an RC oscillator, because its is a free oscillator, runs with a lots of jitter. Sample after sample, the timing of sampling may vary, and the sample will sometimes be a bit later or a bit earlier then the previous one. (considering that the operator moving if very slow compared to the repeat period )
So, this will make and amplitude variation of the same target signal. Making an indesirable noise. May be I'm wrong.
The test can be done on one kind of pulse, just for testing. I will do it as soon as I have a real PI running. For the moment i only do some tries on Spice.

Also this is the preamp thread, I have do some spice runs with a THS4031 OPA, (just a bit expensive about $4) instead of classic NE5534, and with schottky diodes instead of 1n4148, it makes the sampling possible more sooner , and 6 to 8 µs seems to be possible , after the beginning of the flyback impulse.
I will post them , as soon as I can post pictures.

(It is my first post, so I will try to explain later my points of interest in this very good forum. )

[Sean_Goddard]

Back, sorry for the absense

[green]

Tried the circuit. This one worked. Looks like the 3Hz integrator filter is to low. Need to work on the amplifier noise. New schematic(gain jumper wrong on 4538 noise test_2)

Tried a differential amplifier and different integrator. How low a noise level should we expect?

[green]

My thoughts on signal to noise, hope some are close to correct. First, need to compare peak to peak not RMS. Sweep coil over target and miss the next sweep. Have a pulse .1 to.2 seconds long. RMS is going to approach zero. Peak to peak white noise typically 6 times RMS. When does the target signal disappear. Searched minimum signal to noise, 3 to 1 was a suggested value. Example: scales, lsd 1 gram, noise 0 to 5 grams. Add 5 grams, displays 5 to 10 grams. If the weight was added and removed when the no weight reading was 0 the reading would be in the noise band. Signal needs to be greater than noise to have signal reading greater than noise reading every time. 3mv noise doesn't tell me what I need to know. Eric was probably referencing a test point on a particular detector which would if I could calculate the gain. Noise and signal should be referenced to the input(coil volts). I'm thinking if I could detect a 1 uvolt change in coil volts that would be good. Did another schematic for a 1 and 10 uvolt test signal. Reply #67 didn't work, missed the obvious reason why. Appreciate comment if I missed something this time.

Tried the circuit. This one worked. Looks like the 3Hz integrator filter is to low. Need to work on the amplifier noise. New schematic(gain jumper wrong on 4538 noise test_2)

[localbug]

Ahh....I see
Why didn't I think of that.
Thank you Davor

[Davor]

Every PI with EF pulse is in effect an auto-zero configuration.

[localbug]

Does anyone have been experimenting with Auto Zero Amplifier? Is it going to make any improvement to change manually offset nulling on pre amp with AZA?

Regards
John

[Sean_Goddard]

Qiaozhi, are you SURE that isn't Minelabs latest machine but designed for housewives (apologies to any female members we have)?

[Teleno]

I am curious, what function in the MCU will you use for outputing the target signal after it has gone through the ADC
are you going to use simple PWM audio ?

Yes since it's just a prototype. Later on the MCU can eventually be used to play waveforms stored in the EEPROM, for instance, but audio is not the priority right now.

[6666]

I am curious, what function in the MCU will you use for outputing the target signal after it has gone through the ADC
are you going to use simple PWM audio ?

[Sean_Goddard]

Only one way to find that out I'd say ;-)

[Sean_Goddard]

How low can YOU go? Check out the example circuit in this App note 0.95nV/Hz!!

http://cds.linear.com/docs/en/design-note/dn254f.pdf

Any use?

[Old cart]

The MOSFET current decreases with temperature (Rds_on increases), so an NTC would reduce the source resistance by the same ratio (hopefully), achieving temperature independency.

Auto cal. is another option, but not exclusive, since both methods can be implemented. Calibration would adjust to different coils and Tx currents.

I suspect, but do not know for sure, that it is unlikely that an NTC would match the slope of the current change with temperature. Maybe it could be partially corrected and the rest done in software.
In a more advanced application software and a DAC could be used to correct for all errors, not just those created by temperature change.

[Sean_Goddard]

OR you could sample every alternate pulse and use the delta as a reference (would that work I wonder)? I know that if you "ping" the coil with a low level pulse you can use that. It's been done but I can't remember where and it was used to take into account the sun heating up the windings, thermal heating due to current flow and a whole host of other things. Done in real time too!

[Achillion]

If used as a replacement for the two stage opamp MiniPulsePlus front end, do you reckon this front end could improve the sensitivity and noise rejection?