Eric. Re disconnecting fet?

[Anonymous]

Hi Eric, since seeing your post re disconnecting
the power mosfet to test noise levels, I have tried to come up with a system of doing this and
can't see a way it could be done. Care to give
me a few clues on a circuit or device that would handle this? I keep getting caught up by the reverse bias caused by the back emf and the fact that any other device would also be left present
in place of the fet??? My main reason for asking is that I need to switch different damping resistors on every other pulse and as this needs to be done right at the front end, it has me beat.
I figure a clue to the way that you disconnect the fet would help with this.
TIA,
Rob.

 

[Anonymous]

I just unsoldered the series resistor connected to the Mosfet drain. Not all PI's use a series resistor so just unsolder the drain leg.
I know what you are getting at though; to switch the Mosfet out during the read period. Don't know either, without introducing another device. For normal PI's, these circuit noise sources are way below the noise level picked up by the coil, even when way out in the country.
Eric.

 

[Anonymous]

What a let down. Thanks for the reply tho.
Cheers.

 

[Anonymous]

Eric,
Has anyone ever used a differential input with the RX coil as a separate one. The common mode noise would be cancelled out to a large degree, and you could also have more turns on the RX.
Excuse my possible (given some of my previous posts) ignorance here, but surely this would help with the noise problem.

 

[Anonymous]

Using a mono coil, you wouldn't be able to design a receiver that common-modes the noise without also common-moding the signal. You could design a differential coil, but even then it would common-mode only the correlated noise (i.e., from external sources such at powerline) and not the uncorrelated noise (such as from internal thermal sources).
The "Twinloop" design on Geotech is an example of the latter.
- Carl

 

[Anonymous]

Ah, Yes Carl, I see that, but what i meant was you would be reducing the noise induced by the FET, and other front end components. Granted not a lot of reduction, but surely every little helps?