Author Message
 slow_rider
 Posted: Mar 13, 2012 - 08:26 PM
 Joined: Feb 16, 2007 Posts: 698 Location: Israel
 I have a port which needs to be biased with 4.5V. I'm generating 0-5V DC using PWM + LP filter and need to add that to the bias voltage. I know how to do this using a non-inverting OPAMP based summing amplifier. Any simpler way to do it?

 hugoboss
 Posted: Mar 13, 2012 - 09:22 PM
 Joined: Feb 19, 2010 Posts: 507 Location: Montreal, QC, CA
 You could supply 4.5V to the AVR ground and 9.5V to its VCC, in theory it should work but I have not tried it and will not be held responsible if it burns out.

 ka7ehk
 Posted: Mar 13, 2012 - 09:29 PM
 Joined: Nov 22, 2002 Posts: 12049 Location: Tangent, OR, USA
 You can use any resistor network as a (scaled) summer. The output will be some factor smaller than any input. Just N resistors for N voltages connected to a common point. The output R of the LP filter can be one of those Rs. Then, you will have Vout = K * (V1+v2+...) where K is less than 1. Its simple voltage divider arithmetic. If you are just trying to bias a filtered PWM signal, then use two voltage sources (often one is ground and one is some V). Make a network consisting of your filter resistor tied to the tap point of the voltage divider. Cap from the junction point of the network to ground. It is left as an exercise to figure out the resistor values. Jim _________________Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA "The only thing standing between us and victory is defeat" P.G.Wodhouse in Wooster & Jeeves series

 Chuck-Rowst
 Posted: Mar 14, 2012 - 04:03 PM
 Joined: Jul 19, 2011 Posts: 460
 slow-rider, What exactly are you trying to accomplish here? Is your final output 0-5 Volts? Or, 4.5 to 9.5 volts? Or another range? Your PWM signal is 0-5 Volts from a micro GPIO pin?

 Chuck-Rowst
 Posted: Mar 14, 2012 - 04:03 PM
 Joined: Jul 19, 2011 Posts: 460
 slow-rider, What exactly are you trying to accomplish here? Is your final output 0-5 Volts? Or, 4.5 to 9.5 volts? Or another range? Your PWM signal is 0-5 Volts from a micro GPIO pin?

 jan_dc
 Posted: Mar 14, 2012 - 04:25 PM
 Joined: Feb 09, 2011 Posts: 322 Location: Turnhout, Belgium
 hugoboss wrote: You could supply 4.5V to the AVR ground and 9.5V to its VCC, in theory it should work but I have not tried it and will not be held responsible if it burns out. Sure this works. You'll have 5V between GND and VCC on the AVR so no problem. You just must be sure that your output is never connected to anything lower than 4.5V This also means that the GND is at 4.5V so anything else connected to the MCU must have approriate GND level (4.5V). And thus the whole circuit might be more complex than using an opamp.

 slow_rider
 Posted: Mar 18, 2012 - 12:29 PM
 Joined: Feb 16, 2007 Posts: 698 Location: Israel
 jan_dc wrote: hugoboss wrote: You could supply 4.5V to the AVR ground and 9.5V to its VCC, in theory it should work but I have not tried it and will not be held responsible if it burns out. Sure this works. You'll have 5V between GND and VCC on the AVR so no problem. You just must be sure that your output is never connected to anything lower than 4.5V This also means that the GND is at 4.5V so anything else connected to the MCU must have approriate GND level (4.5V). And thus the whole circuit might be more complex than using an opamp. I don't really want to run the AVR's low rail on anything higher than GND level because that would mean debugging will be a nightmare if at all possible. Not to talk about having to pre-program the chip. A simple non-inverting summing amplifier for two voltages is 4 resistors and a single OA so I think I'll use that. Thanks for the help guys!

 bobgardner
 Posted: Mar 18, 2012 - 09:36 PM
 Joined: Sep 04, 2002 Posts: 21263 Location: Orlando Florida
 RuggedCircuits.com has a couple dozen circuit diagrams for signal conditioning, and opamp gain and bias is one of them. Worth a look. _________________Imagecraft compiler user

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