Sean, I may have been a bit misleading. Thank you for catching it. I meant to say near zero volts output, as you pointed out. Most op amps do not have the very high open loop gain, which unity gain buffers do have. The buffer example I gave was for the LM310. The only spec I could find on open loop gain was was super OLG. Op amps can be configured as unity gain easily by connecting the output directly to the inverting input. Sometimes resistors are placed in the feedback loop and in the non-inverting input to balance the currents. That configuration is generally stable for relatively large signal, and the higher the open loop gain of the device the better. At small signals I believe the jth valley currents can contribute to drift. When one is operating in the low millivolts (near zero) there can develop a small drift band which is okay if your measurement is not small enough to be compromised. Of course, we are talking about a DC configuration. The OP37 is a precision op amp which is fairly reliable as a unity gain buffer. It has a 1.8 million open loop gain. Drift is 0.2 uV/C where the LM310 is 10 uV/C so it is really stable. Even the long term drift spec is good. I have designed amplifiers that operate in the microvolts and even the board leakage can cause drift and affect the performance so the offset currents need to be balanced. I designed a 50 femtoamp amplifier once and I could not even mount the parts in a PCB. So I have become keenly aware of the problems of small signal and drift and configuration. ----- Original Message ----- From: "Sean Breheny" To: "Microcontroller discussion list - Public." Sent: Tuesday, May 11, 2010 7:48 PM Subject: Re: [EE] Opamp design doubt Hi Rich, Not stable near zero what? I don't understand your comment. Near zero volts output? Sean On Tue, May 11, 2010 at 3:56 PM, Rich wrote: > That is true, Russell. In fact most op amps configured as unity gain are > not stable near zero. That is why there are special very high open loop > gain devices called buffers or followers, like the LM 310, for example. > > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist