Balance pot??

[Anonymous]

Can someone help with the purpose of the balance pot that proceeds the differential integrator? I know I have seen it described in previous postings before but I can't seem to find the description. Also, how do you correctly adjust it?
Thanks

 

[Anonymous]

Hi,
The link below may be the post you were looking for. I don't ever recall seeing any posts describing how to adjust the balance pot. I don't recall seeing any posts on adjusting any pots such as compensation either. Maybe Eric will answer your questions.
Good luck,
Charles

 

[Anonymous]

Hi Charlie,
The purpose of the balance pot is to equalise the ON resistance of the two JFets. These could, of course, be matched for ON resistance but even then, some small adjustment is often desirable. Balancing the two time constants in the differential integrator is important so as to minimise the signal caused by movement of the searchcoil in the earth's magnetic field. The easiest way of setting the pot is to wave a small ferrite magnet over the search coil so as to give + and - 0.5V amplitude signal at the output of the front end amplifier. Connect the other channel of the scope to the integrator output and adjust the pot for minimum deflection of the baseline. You will see the signal ramp up and down at, and between, the sampling points, but the long interval between should not move. Set scope sensitivity to 50mV/div to do this. By the way, you need the coil in a metal free area to do this, otherwise the integator output will be way off zero and you may not be able to adjust the vertical shift on the scope to compensate. Using a small coil for this test can help in this respect.
Eric.

 

[Anonymous]

Thanks Eric and Charles for the quick response.