*technical elec question* DC offset/input impedance relation
 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
homer09
diyAudio Member

Join Date: Jan 2004
Location: Montreal
*technical elec question* DC offset/input impedance relation

Ok im trying to learn the technical aspect of DC offset and its relation to input impedance. Ive read in the forums, but found no clear explanation of the following question:

How is DC offset and input impedance related? What variations in input impedance increase/decrease DC offset?

also, How does one keep DC offset low with a pot as a volume control?

I attached the circuit diagram of the gain clone im thinking of building.
Attached Images
 schem_2b.gif (10.3 KB, 695 views)

 9th February 2005, 03:21 PM #2 richie00boy   Did it Himself diyAudio Member     Join Date: Nov 2003 Location: Gloucestershire, England, UK It just comes down to simple ohm's law. The input draws a current (input bias current) 'I' which, when drawn through an impedance of 'R' ohms, creates a voltage drop 'V' of I*R. As the signal is referenced to ground, the actual level the input sees will be offset by this amount. Ideally, you try to null out the effects of the input bias current by providing both inverting and non-inverting inputs with the same impedance to ground. In the schematic you posted, the impedance (and thus the offset voltage) seen by the non-inverting input will vary with the position of the pot wiper. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, modules and more.
 9th February 2005, 03:34 PM #3 homer09   diyAudio Member     Join Date: Jan 2004 Location: Montreal Richie, i understand how the imput impedance value in ohms is what determines the voltage drop to ground and it is what the opamp sees as input. But, isnt the input signal AC, how is this converted to a DC offset? In my circuit, will there be large DC offset at the output (at any pot wiper position)? and maybe what forumla i need to caluclate it at any wiper position
 9th February 2005, 04:08 PM #4 richie00boy   Did it Himself diyAudio Member     Join Date: Nov 2003 Location: Gloucestershire, England, UK DC offset is just that: DC. Therefore your signal -- which is AC -- 'rides' on top of the DC offset. The offset is there under DC or no signal conditions as a constant voltage at the output of your op-amp and, when a signal is running through the op-amp it is simply in addition to the DC offset. Forget the input signal causing DC offset, as I mentioned in my earlier post it is the input bias current that cause it. The offset is worsened by using large value resistors to bias the inputs. The circuit you posted will have a DC offset but, like any circuit of that type it will be harmlessly small. You already have the formula in my earlier post to work out the DC offset. Find out the input bias current for the chip from the datasheet and it should be obvious what to do Just remember to work in ohms, amps and volts. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, modules and more.
 10th February 2005, 08:18 PM #5 Evan Shultz   diyAudio Member   Join Date: Jun 2004 Location: California, USA the largest factor in a circuit of this type, once the compensation resistor is added, will be the value of Rf. the bias and offset currents that flow into the inverting pin must also flow through the feedback resistor creating DC at the op amp output. So, the larger Rf is the more DC offest will be created. Richie's first post explains that Ohm's Law is all you need to calculate it. He also points out that is should be harmlessly small with nearly any op amp.
homer09
diyAudio Member

Join Date: Jan 2004
Location: Montreal
Quote:
 Originally posted by Evan Shultz Richie's first post explains that Ohm's Law is all you need to calculate it.
Well i beg to differ. a formula is useless if you don't know what values to plug in or how to apply it to the situation.
So, for the non EE here, can you guide me through calculating the DC offset for the schematic i posted.

i have:

input bias current: 0.2 uA (typical) *from datasheet as richie asked
pot value: 0 to 250K ohms
feedback: 22K ohms
shunt (NI to ground): 12K ohms
inverting to ground: 680 ohms

and of course, ohms law: V = IR

i understand better through formulas and numbers than through wordy explanations, so dont be shy with the numbers. Thanks for the crash course!

ps. i know DC offset wont be a problem, this is more for my personal understanding, i would like to build something i understand, not limit myself to following a schematic and procedure blindly.

 11th February 2005, 12:27 AM #7 Stocker   diyAudio Member   Join Date: Dec 2003 Location: Austin so you gave the formula. Mix it up with gain of the amplifer. right guys? or am I off(set) in my thinking? __________________ Jesus loves you.
 11th February 2005, 04:31 AM #8 theChris diyAudio Member   Join Date: Dec 2001 Location: SIUE, Illinois, USA DC offset: assume you have a offset voltage. in you case, this offset is multiplied by the gain of 22/0.68, not too good. this means a 10mV offset becomes over 220mV of offset. second: all amps draw some "Input Bias Current" and "Input Offset Current" the latter refers to the matching of bias current. if the impedance of non-inverting to ground is different then the impedance of inverting to ground, this input bias current will induce unequal voltages on each input, leading to DC offset. input offset explains how likely matching of both resistances is to correct DC offset. many amp circuits use capacitors to prevent the source from providing bias current to the amplifier, and in the feedback loop to reduce DC gain to 1. in your example, chaning the pot changes the impedance of one input, thus chaning the DC offset due to input bias currents. as for AC-DC offset well, it should be very small. such an effect is possible from the non-linear characteristics that are minimized in a good amplifier (or amplifier chip) design. this kinda thing can affect oscillators though. __________________ if only it could be used for good, not evil...
 11th February 2005, 06:54 AM #9 Yoghourt   diyAudio Member   Join Date: Jun 2004 Location: Alpes Opamp caracteristics are always related to input. Dc voltage error on (Ve+ - Ve-) is Vio - Ib*(Re+ - Re-) +Iio*(Re+ + Re-)/2 + (Vdd+Vss)/(2*CMRR) Namely: Vio: input offset voltage Ib: input bias current Re+, Re-: DC impedance seen by e+ and e- inputs Iio: input offset current Vdd: supply positive rail Vss: supply negative rail CMRR: common mode rejection ratio. This last error expresses that ground is not always at the equilibrium point of opamp, which is at mid-point of supplies. In other words: dc offset error related to asymetry of supply. On a big low-frequency transient, one rail of chipamp supply can swing. Effect is that DC offset seems to "jump". To relate the dc voltage error to output, mutiply by DC gain. For more precisions, see on-line doc at texas instruments called "opamp for everyone". __________________ Dzharis: raaaaaah lovelyyyy!
Keld
diyAudio Member

Join Date: Jan 2002
Location: Borås, Sweden, Tellus
Quote:
 Originally posted by Yoghourt For more precisions, see on-line doc at texas instruments called "opamp for everyone".
Any chance for a link to that doc. Mysearch came up with a lot of interesting stuff, but not that one!

 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         Holton Precision Audio         CSS         Dx Classic Amplifiers         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 MisterTwister Multi-Way 3 16th July 2009 02:37 AM nobody special Solid State 8 29th August 2005 09:23 PM sam9 Solid State 9 21st April 2005 02:28 PM sobazz Solid State 1 14th July 2003 05:45 AM

 New To Site? Need Help?

All times are GMT. The time now is 02:38 AM.