Circuits for magnetic pickups

[Terry Sare]

     To cancel the voltage induced in both line you would probably need to 
     ground the sensor side. Use a twisted pair to the sensor to have a 
     balance line, otherwise common mode noise cancellation will not be as 
     effective. You should also have some degree of hystresia in this as it 
     is a high input impedance and will pickup non common mode voltage and 
     oscillate.
     
        TS

______________________________ Reply Separator _________________________________
Subject: Re: Circuits for magnetic pickups
Author:  owner-diy_efi-outgoing at coulomb.eng.ohio-state.edu at dell_unix
Date:    8/1/96 1:23 PM


>If anyone get a good pickup going (no extra pulses) that uses the simple 
>op-amp, let me know. I would love to remove the lm1815 from the design. I am 
>far into the routing of the board, but I have a couple of extra amps in the 
>LMC6494 that I am using for O2 buffers. So I you guys get some good thig 
>working, shoot me some mail.
     
Sandy,
     
I always got good noise immunity by doing this:
     
     
                        |-------- gain ----------
                        |                        |
   [pickup]-----------------  +                  |
       |                          opamp  >--------- 
       ---------------------  -
                  |
                  |
                 GND
     
This circuit uses an op-amp like a comparator (high pos-feedback). 
But what is important is using the other lead from the pickup as
a ground reference and feeding it into the negative side of the 
op-amp.  Any noise in the system (eg. ignition) will appear on 
both leads of the pickup and be cancelled out inside the op-amp.
     
Here is something to consider:  Does it matter where you ground the 
lead?  At the sensor, or at the op-amp?
     
-tim