I would consider using a decent-quality audio transformer for=20 isolation into your circuit. That means that you can use a normal=20 single-ended (not differential) amplifier for signal conditioning. The transformers that I use are a semi-custom part (standard part=20 number but manufactured when the order is placed) that I purchase in=20 thousands. They are relatively small (similar in size to Mouser's=20 TM016) but with super high inductance and darned decent low-frequency=20 response. Provided that your amplifier has relatively high input=20 impedance, the truck's sensor and electronics should not even notice=20 the addition of your circuit. I'd be happy to send you a couple of transformers for you to try and=20 I can supply as many as you need, up to the point where your=20 quantities justify you ordering them yourself. Something like Mouser's TM016 is most likely NOT suitable because the=20 inductance is fairly low - the low frequency response of those=20 transformers is rated at 200Hz when fed with a 600R source. They are=20 OK for telco-type circuits but not for Hi-Fi audio and certainly not=20 for a variable-reluctance sensor where the speed approaches zero. dwayne At 12:29 PM 2/8/2012, Bob Blick wrote: >I need to design a device that interfaces to the speed sensor of heavy >trucks. These are variable reluctance sensors and will remain connected >to the vehicle's ECU, my circuit will piggyback. These are two-wire, >differential signals full of common-mode noise and vary in amplitude >greatly with frequency and also position as the tailshaft bearing wears >in the vehicle. --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .