Op-amp input impedances and a pot/attenuator
 User Name Stay logged in? Password
 Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read Search

 Chip Amps Amplifiers based on integrated circuits

 Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
breez
diyAudio Member

Join Date: Jul 2004
Location: Finland
Op-amp input impedances and a pot/attenuator

Hello

I have been wondering. If I have a potentiometer/attenuator in the usual configuration as a voltage divider in front of the op-amp input and there is also an input cap (series) and a shunt resistor from the input to ground:

What is the impedance seen by the source? I presume it will be the pot value in parallel with the shunt resistor. Correct?

What happens to the DC offset of the amp when turning the pot the resistance from wiper/input pin to ground changes? At the other end of the pot the resistance reaches very low values. Wouldn't this unbalance the input impedances of the amp and therefore cause large DC offset at output?

Thanks,
Lauri

edit: If the second point is correct, is there any easy way out of it?
Attached Images
 circ.gif (2.9 KB, 237 views)

 31st January 2005, 05:48 PM #2 macboy   diyAudio Member   Join Date: Oct 2003 Location: Ottawa, Canada Here's the trick, the AC Impedance is the parallel combination of the input resistor and both halves of the pot, just as you suspected. But the DC impedance as seen from the input of the chipamp/opamp is only the input resistor. Why? Because of the input cap, which blocks DC. So what is the effect to the DC offset of changing the position of the pot? None.
 31st January 2005, 11:03 PM #3 TwoSpoons   diyAudio Member     Join Date: Oct 2004 With the pot set for zero output the source will see just the pot impedance. At max setting it will see pot in parallel with shunt R. In between it will see [input->wiper] in series with ( [wiper->ground] in parallel with shunt R). Make sense? Its easier to see if you draw it out as three resistors.
 31st January 2005, 11:33 PM #4 azira   diyAudio Member   Join Date: Dec 2003 Location: Near Seattle DC impedance it'll just see the full value of the POT, however, the AC impedance it'll see is: lets call R1 resistance above the wiper, and R2 below the wiper, so R1 + R2 = Rpot, Xcap is the AC impedance of the cap which is frequency dependant. Xcap = 1/jwC; R1 + ( R2 || (Rshunt + Xcap) ) If C is sufficiently large so that it's impedance is perhaps 1/10th or so of Rshunt, the it can be safely calculated as R1 + (R2 || Rshunt) Which by the way, varies as you adjust R1, at 0-volume, you'll have Rpot as your input resistance, at full volume, you'll have Rpot || (Rshunt+Xcap) -- Danny
 1st February 2005, 12:11 AM #5 homer09   diyAudio Member     Join Date: Jan 2004 Location: Montreal What if one does not have any input cap or input resistor, just a pot and a shunt resistor? any problems there?
macboy
diyAudio Member

Join Date: Oct 2003
Quote:
 What if one does not have any input cap or input resistor, just a pot and a shunt resistor? any problems there?
Yes, there is a potential for problems. The DC impedance, looking back from the opamp input (which is a source of leakage current) to the audio source, will be all over the place. That will affect the DC offset, because the leakage current will induce a different voltage depending on the position (resistance) of the pot. Even though this voltage is small, it is amplified by the gain of the opamp.

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Site     Site Announcements     Forum Problems Amplifiers     Solid State     Pass Labs     Tubes / Valves     Chip Amps     Class D     Power Supplies     Headphone Systems Source & Line     Analogue Source     Analog Line Level     Digital Source     Digital Line Level     PC Based Loudspeakers     Multi-Way     Full Range     Subwoofers     Planars & Exotics Live Sound     PA Systems     Instruments and Amps Design & Build     Parts     Equipment & Tools     Construction Tips     Software Tools General Interest     Car Audio     diyAudio.com Articles     Music     Everything Else Member Areas     Introductions     The Lounge     Clubs & Events     In Memoriam The Moving Image Commercial Sector     Swap Meet     Group Buys     The diyAudio Store     Vendor Forums         Vendor's Bazaar         Sonic Craft         Apex Jr         Audio Sector         Acoustic Fun         Chipamp         DIY HiFi Supply         Elekit         Elektor         Mains Cables R Us         Parts Connexion         Planet 10 hifi         Quanghao Audio Design         Siliconray Online Electronics Store         Tubelab     Manufacturers         AKSA         Audio Poutine         Musicaltech         Aussie Amplifiers         CSS         exaDevices         Feastrex         GedLee         Head 'n' HiFi - Walter         Heatsink USA         miniDSP         SITO Audio         Twin Audio         Twisted Pear         Wild Burro Audio

 Similar Threads Thread Thread Starter Forum Replies Last Post slor Tubes / Valves 7 17th March 2008 08:56 PM duffy Chip Amps 4 2nd December 2004 07:08 AM Dominique Parts 5 5th August 2004 12:19 PM ElectroNick Pass Labs 1 16th September 2003 09:28 PM Saurav Everything Else 3 16th May 2003 05:10 PM

 New To Site? Need Help?

All times are GMT. The time now is 11:20 AM.