Message	Date	From	Subject
3748	2009-11-07 12:29:21	KK7B	Single supply op-amps
Referring to the post: "Here's a little tip you don't see, for the best noise and common mode (balance) performance you must use a dual supply for the op amps. Single supply op amps are always noisier and always sacrifice common mode rejection. Noise specs in the data sheets ALWAYS are based on a dual supply, in fact I've yet to see data sheet that show the noise in a single supply setup. The reason a single supply op amp is noisier is simple, by biasing the non-inverting input you are hooking a noise source to an input" If you study the op-amp circuits in EMRFD and other high-performance circuits using op-amps, you will see that the above issues are addressed--often with a single 12 volt supply. The statement "you must use a dual supply for the op amps" with the argument that that's how the data sheet measurements are made is highly misleading. Components are seldom used in data sheet test circuits. Op-amps are wonderful components, and those of us who design them, and design with them, have many techniques for dealing with noise and common mode rejection. It is important to design the circuit based on the signals at the inputs and the desired performance, not by blindly applying rules based on lore. Statements with ALWAYS in them are always wrong--including this one. Best Regards, Rick KK7B
3759	2009-11-09 02:49:23	jr_dakota	Re: Single supply op-amps

Referring to the post:

"Here's a little tip you don't
see, for the best noise and common mode (balance) performance you must use a
dual supply for the op amps. Single supply op amps are always noisier and always
sacrifice common mode rejection. Noise specs in the data sheets ALWAYS are based
on a dual supply, in fact I've yet to see data sheet that show the noise in a
single supply setup. The reason a single supply op amp is noisier is simple, by
biasing the non-inverting input you are hooking a noise source to an input"

If you study the op-amp circuits in EMRFD and other high-performance circuits using op-amps, you will see that the above issues are addressed--often with a single 12 volt supply.

The statement "you must use a dual supply for the op amps" with the argument that that's how the data sheet measurements are made is highly misleading. Components are seldom used in data sheet test circuits. Op-amps are wonderful components, and those of us who design them, and design with them, have many techniques for dealing with noise and common mode rejection. It is important to design the circuit based on the signals at the inputs and the desired performance, not by blindly applying rules based on lore.

Statements with ALWAYS in them are always wrong--including this one.

Best Regards,

Rick KK7B