Dave, I have tested some weeks ago an astable oscillator circuit based on a 555CMOS ( http://www.emesystems.com/OL2mhos.htm ) but didn't tested this aspect (frequency linearity) very well. Then, I don' t have chosen this circuit in the conviction that a 555 has a lot of noise in the comparisons of others chips, but perhaps I'm in mistake.. many thanks Marco ----- Original Message ----- From: "Dave Dilatush" To: Sent: Wednesday, April 16, 2003 1:24 PM Subject: Re: [EE]: OP-AMP squarewave oscillator > Marco Genovesi wrote... > > >Is it correct that the frequency values aren't linear? > >I imagined a frequency formula "similar" to this: F = 1/(1.4*R*C) > > The reason why your oscillator period is not exactly proportional > to feedback resistance is because the opamp is not an "ideal" > (i.e., "textbook") opamp. > > I suspect the biggest source of the discrepancy you are seeing is > due to the fact that the opamp output does not swing all the way > between VDD and GND: especially in the high state, the opamp > output saturates at around a volt or so below the VDD rail. > Also, this saturation voltage is dependent on load resistance, > which is mostly the feedback resistor itself. So the > relationship between feedback resistance and oscillator period is > not going to be very linear. > > You might get better results with a CMOS 555-type oscillator, > like National's LMC555 or Texas Instruments' TLC555. > > Hope this helps a bit... > > Dave D. > > -- > http://www.piclist.com hint: To leave the PICList > mailto:piclist-unsubscribe-request@mitvma.mit.edu -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu