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Kai Klaas
02/24/06 20:00
  02/24/06 20:17

Read: 1577 times

#110740 - Impedance matching
Responding to: Prahlad J. Purohit's previous message
Prahlad said:
Why are these resistor not required with CMOS OpAmps can you please give some more information on this.

Assume you have a LM324. This OPamp contains bipolar transistors at inputs. To work properly an input bias current of about 45nA is flowing out of each input, remember that input transistors are PNP. This current is the base current of PNP, which must flow to make the OPamp to work properly.
As the LM324 has (almost) identical transistors at each input (non-inverting and inverting), like most of all OPamps available, out of both inputs a current of about 45nA is flowing. The two currents are nearly identical, but can differ by about +-5nA. The difference of both currents is called input offset current.

If you have now high ohmic resistors at inputs, the input bias current will cause relevant voltage drops, but their effect at output will cancel, if you keep them identical. This can be done by chosing the resistors in such a way, that each input bias current sees the same effective ohmic resistance from according input to ground or virtual ground. Then only the difference of input bias currents, the input offset current leads to an error. This error has the same effect like the original input offset voltage and so, it must be kept low.

An example:
LM324 has at inverting input a resistance of 100kOhm to ground, non-inverting input is grounded directly (zero impedance). Then, input bias current causes a voltage drop of 45nA*100kOhm = 4.5mV. This voltage drop has the same effect as the original input offset voltage. Remarkably is, that original input offset voltage of LM324 is only 2mV typically. So, the too high resistance at inverting input heavily increases offset voltage of OPamp.
But if you have also at non-inverting input a 100kOhm resistance to ground, then only the input offset current of 5nA must be taken into consideration: The additional offset voltage is less than 5nA*100kOhm = 0.5mV!

It's clear, that this cancelling methode is only efficient, if input bias current of an OPamp is rather high. If it's low, like with JFET-OPamps or CMOS-OPamps, then the impedance matching is not required.

An example:
Input bias current of LF356, which is an JFET-OPamp, is only 30pA typically. Voltage drop across above 100kOhm resistance is only 3ÁV, which is 1000 times smaller than the original input offset voltage of LF356. So, there's no advantage at all to do this impedance matching.

Prahlad said:
How do I know an OpAmp is CMOS or not for TLC2274 Texas calls it LinCMOS type [ Linear CMOS ] so its clear what about other OpAmps.

If nothing special is told in datasheet, then it's a bipolar chip, all internal transistors are bipolar, like the LM741, LM324, NE5534, etc.
If it's not a plain bipolar chip then it's told in datasheet.

Let's have some examples:

1. LF356, TL084, TL052, etc: OPamp with JFET inputs. The input transistors are FET, concretely spoken JFET (junction FET, gate and source form a reverse polarized pn-junction). All other transistors are bipolar. Sometimes also some other internal circuitry is made of JFETs.

2. CA3140: BiMOS-OPamp with CMOS-transistor inputs and bipolar transistor output. Input transistors and some other internal circuitry are made of CMOS transistors (Complementary Metal Oxid Semiconductor, gate and source are isolated by a highly insulating silicon-oxid layer). All other transistors are bipolar. The OPamp is called a BiMOS-OPamp, because of the mix of bipolar and CMOS transistors.

3. CA3130: BiMOS-OPamp with CMOS-transistor inputs and CMOS-transistor output. Some other additional circuitry is also made of CMOS-transistors. All other transistors are bipolar.

4. TLC274, TLC2274, etc: LinCMOS-OPamp, all internal transistors are CMOS.

And then there are also DiFET, BiFET, CBFET, bipolar/JFET, etc., etc.


List of 157 messages in thread
Measuring capacitance very precisely.      Prahlad J. Purohit      02/13/06 09:06      
   "I want the best money can buy, but I do      Erik Malund      02/13/06 09:24      
      In this specific case I disagree.      Kalpak Dabir      02/13/06 11:22      
   Bridge?      Andy Neil      02/13/06 09:31      
      Bridge!      Oliver Sedlacek      02/13/06 09:58      
   AC bridge      Steve M. Taylor      02/13/06 14:19      
      Error Sources      Bruce Bailey      02/13/06 15:04      
         Blumlein Bridge        Steve M. Taylor      02/13/06 16:59      
            Blumlein Bridre.      Prahlad J. Purohit      02/14/06 22:11      
               Bridge excitation      Oliver Sedlacek      02/15/06 03:18      
            Blumlein      Ian Bell      02/16/06 03:21      
   Let the statistics be your friend...        Kai Klaas      02/13/06 19:19      
      frequency meter      Christian Kleiber      02/14/06 07:30      
      Product as a Transfer Standard      Bruce Bailey      02/14/06 09:31      
         Distributor of your confidence      Kai Klaas      02/22/06 07:17      
            Suppliers like that ...      Bruce Bailey      02/22/06 07:58      
               why blame the suppliers      Erik Malund      02/22/06 08:06      
                  Not at all      Kai Klaas      02/22/06 10:08      
                  Hah!      Bruce Bailey      02/22/06 10:28      
      Statistics for reference determination.      Prahlad J. Purohit      02/15/06 00:02      
         You got it!      Kai Klaas      02/15/06 07:42      
   questions      Erik Malund      02/15/06 06:24      
      he wants to select to 0.1% from 1% stock      Terry Lingle      02/15/06 07:13      
         that is a bad idea      Erik Malund      02/15/06 07:54      
            Hmm      Kai Klaas      02/15/06 08:10      
      That's a good way      Kai Klaas      02/15/06 07:48      
         posts cross timed      Erik Malund      02/15/06 08:04      
            It makes sense to select them      Kai Klaas      02/15/06 08:18      
      No sorting of Caps.      Prahlad J. Purohit      02/15/06 08:16      
         then, are you not using the wrong approa      Erik Malund      02/15/06 09:27      
            Time constant      Kai Klaas      02/15/06 10:16      
            Re: Eliminate cap sorting      Prahlad J. Purohit      02/15/06 22:06      
               Example      Steve M. Taylor      02/16/06 02:57      
               you do not need precision, you need rate      Erik Malund      02/16/06 06:36      
         Would it not be easier the other way?      Kalpak Dabir      02/15/06 10:35      
            Difficult      Kai Klaas      02/15/06 11:45      
               Yes Digital Difficult.      Prahlad J. Purohit      02/15/06 21:58      
                  I think the point has been missed again.      Jez Smith      02/16/06 07:54      
                     Demo ?      Steve M. Taylor      02/16/06 09:28      
                        Difficult to change now.      Prahlad J. Purohit      02/16/06 09:48      
                           Don't worry      Kai Klaas      02/16/06 09:59      
                              Re: Dont Worry.      Prahlad J. Purohit      02/17/06 08:07      
                                 How to calibrate an integrator?        Kai Klaas      02/19/06 13:29      
                                    Re: How to calib. an integratrator.      Prahlad J. Purohit      02/19/06 21:59      
                                       Answers      Kai Klaas      02/20/06 06:33      
                                    256Hz too fast.      Prahlad J. Purohit      02/20/06 02:51      
                                       Integrator and RMS using DSP      Kalpak Dabir      02/20/06 03:17      
                                          Whats available easily.      Prahlad J. Purohit      02/20/06 09:38      
                                             dsPIC30F2010      Kalpak Dabir      02/20/06 09:49      
                                                500 kSPS      Mahmood Elnasser      03/16/06 09:43      
                                                   This little PICcy got eaten, the SILabs      Erik Malund      03/16/06 10:33      
                                                      Is that useful?      Kalpak Dabir      03/19/06 07:09      
                                                         Planning to use dspic.      Prahlad J. Purohit      03/19/06 07:35      
                                                         Speed ?      Steve M. Taylor      03/19/06 07:55      
                                                            But for what purpose?      Kalpak Dabir      03/19/06 11:36      
                                                               MAC      Steve M. Taylor      03/19/06 12:16      
                                                                  it is amazing      Erik Malund      03/20/06 07:41      
                                                                     Here is the analysis      Kalpak Dabir      03/20/06 09:36      
                                                                        the content, not the words      Erik Malund      03/20/06 10:54      
                                                                           analysis please      Kalpak Dabir      03/20/06 11:38      
                                                                              since you, just like me, have no idea ab      Erik Malund      03/20/06 13:12      
                                                                                 Ah! The Horatio quote version 2!      Kalpak Dabir      03/20/06 19:30      
                                                                                    again, you focus on words, not content      Erik Malund      03/21/06 06:45      
                                                                                       Which is why I asked...      Kalpak Dabir      03/21/06 11:34      
                                                                                          If what you are saying is that I can not      Erik Malund      03/21/06 12:29      
                                                                                          I had to.      Steve M. Taylor      03/21/06 13:52      
                                                                                             You said it!      Kalpak Dabir      03/22/06 00:07      
                                                                              Analog integrator      Kai Klaas      03/20/06 18:48      
                                       How do you calculate the RMS?      Kai Klaas      02/20/06 06:48      
                                          RMS in detail.      Prahlad J. Purohit      02/20/06 09:35      
                                          Out of interest.      Steve M. Taylor      02/20/06 11:53      
                                             wot you want a RMS demo,steve??      Jez Smith      02/21/06 04:52      
                                                FIXED point      Steve M. Taylor      02/21/06 11:13      
                                                   fixed point is almost as bad      Jez Smith      02/21/06 14:16      
                                       Alternative        Kai Klaas      02/20/06 13:30      
                                          NCO again      Steve M. Taylor      02/20/06 16:17      
                                             Would be nice      Kai Klaas      02/22/06 06:45      
                                                Timer2 of 89C52      Kalpak Dabir      02/22/06 06:58      
                                                   clever      Steve M. Taylor      02/22/06 08:38      
                                                      Oops! Incomplete statement      Kalpak Dabir      02/22/06 10:11      
                                                         Clock feedthrough      Kai Klaas      02/22/06 10:38      
                                                            Maxim App note        Steve M. Taylor      02/22/06 10:51      
                                                               Nice links, thanks!      Kai Klaas      02/22/06 10:54      
                                                               AD9832      Steve M. Taylor      02/22/06 14:13      
                                                            sine vawe      Erik Malund      02/24/06 11:35      
                                          Re: Alternative.      Prahlad J. Purohit      02/20/06 22:34      
                                             Will work      Kai Klaas      02/21/06 05:49      
                                          Re: 64Hz Gain 1.126.      Prahlad J. Purohit      02/21/06 03:41      
                                             Maximum input voltage range      Kai Klaas      02/21/06 06:17      
                                                Re: Max. Input voltage Range.      Prahlad J. Purohit      02/22/06 04:58      
                                                   You have got it      Kai Klaas      02/22/06 06:44      
                                                      indeed, and      Erik Malund      02/22/06 07:44      
                                                         Is the resistor in gnd really needed.      Prahlad J. Purohit      02/22/06 08:11      
                                                            Negligible with CMOS Opamps      Kai Klaas      02/22/06 09:16      
                                                               Re: Negligible with CMOS Opamps.      Prahlad J. Purohit      02/24/06 03:08      
                                                                  Most opamp datasheet do specify      Kalpak Dabir      02/24/06 07:53      
                                                                  Input Bias Current      Steve M. Taylor      02/24/06 09:55      
                                                                  Impedance matching      Kai Klaas      02/24/06 20:00      
                                                                     Re: Impedence Matching.      Prahlad J. Purohit      02/24/06 21:33      
                                                         yess balancing is required and to that..      Abhishek Singh      02/22/06 19:09      
                                          4u7 Cap polarity and type.      Prahlad J. Purohit      02/23/06 03:57      
                                             Foil caps are needed      Kai Klaas      02/23/06 06:04      
                                                Available Capacitors.      Prahlad J. Purohit      03/07/06 01:31      
                                                   This one      Kai Klaas      03/07/06 07:39      
                                                      What are these MK equivalents      Prahlad J. Purohit      03/07/06 07:51      
                                                         MKT, MKP, MKC, KT, KP, KC,...      Kai Klaas      03/07/06 20:14      
                                                            Thanks Kai,      Prahlad J. Purohit      03/08/06 06:17      
                                          Ground Planes.      Prahlad J. Purohit      03/09/06 08:11      
                                             Have you had problems?      Kai Klaas      03/09/06 08:16      
                                                RE: Have you had problems.      Prahlad J. Purohit      03/09/06 08:19      
                                                   Keep the oscillator away from output      Kai Klaas      03/09/06 08:52      
                                                      Board Layouts.      Prahlad J. Purohit      03/09/06 09:53      
                                                         Recommendations      Kai Klaas      03/09/06 17:55      
                                                            Corrected Board      Prahlad J. Purohit      03/10/06 01:59      
                                                               I try to draw it for you      Kai Klaas      03/10/06 06:40      
                                                                  RE: I try to draw it.      Prahlad J. Purohit      03/11/06 08:28      
                                                                     Here it is        Kai Klaas      03/11/06 08:49      
                                                                        RE: Corrected Ver 2.0      Prahlad J. Purohit      03/12/06 01:44      
                                                                           Answers      Kai Klaas      03/13/06 06:51      
                                                                              Re: Corrected Ver. 3.0      Prahlad J. Purohit      03/14/06 03:30      
                                                                                 Some hints      Kai Klaas      03/16/06 09:02      
                                                                                    Re: I am in trouble.      Prahlad J. Purohit      03/16/06 09:35      
                                                                                       Sorry!      Kai Klaas      03/16/06 09:44      
                                                                                       Is this schematic correct.      Prahlad J. Purohit      03/16/06 09:45      
                                                                                          Corrections      Kai Klaas      03/16/06 10:04      
                                                                                             Re: Correction.      Prahlad J. Purohit      03/16/06 10:17      
                                                                                                Mylars will do      Kai Klaas      03/16/06 18:49      
                                                                                             Corrected Board and schematics Ver 4.0      Prahlad J. Purohit      03/19/06 01:44      
                                                                                                Why not doing it like this?      Kai Klaas      03/19/06 19:47      
                                                                                                   Re: Corrected boards ver 5.0      Prahlad J. Purohit      03/20/06 03:43      
                                                                                                      Some hints      Kai Klaas      03/20/06 11:50      
                                                                                                         Corrected board ver: 6.0      Prahlad J. Purohit      03/21/06 07:29      
                                                                                                            Please remove it...      Kai Klaas      03/21/06 07:49      
                                                                                                               Yes removed Gnd plane from mount hole.      Prahlad J. Purohit      03/21/06 08:00      
                                                                                                                  Looks good now      Kai Klaas      03/21/06 08:15      
                                                                                                                     Does than mean Green signal.      Prahlad J. Purohit      03/21/06 08:16      
                                                                                                                        Yes      Kai Klaas      03/21/06 08:41      
                                                                                                                           Thanks Kai.      Prahlad J. Purohit      03/21/06 19:44      
                                                                        trying to understand that      Abhishek Singh      03/13/06 21:52      
                                                                           DC impedance of ground plane      Kai Klaas      03/16/06 09:35      
                                          bipolar swings how and where?      Abhishek Singh      03/09/06 09:27      
                                             Here      Kai Klaas      03/09/06 09:30      
                                                explain a bit please      Abhishek Singh      03/09/06 10:19      
                                                   Explaination      Kai Klaas      03/09/06 10:55      
                           Me ?      Steve M. Taylor      02/16/06 10:20      
                        Exactly!!      Kai Klaas      02/16/06 09:57      
   as requested a demo.....,sort of      Jez Smith      02/16/06 13:47      
   simulation results of the demo...sort of      Jez Smith      02/16/06 23:53      
      Thanks Jez.      Prahlad J. Purohit      02/17/06 08:01      
   Changing Q7 to Q9.      Prahlad J. Purohit      03/16/06 10:23      
   AD7142/AD7745      Aubrey Kagan      04/10/06 07:26      
      Only good for a few pF      Kai Klaas      04/10/06 09:11      
         My apologies      Aubrey Kagan      04/10/06 09:26      
            No need to apologize!      Kai Klaas      04/10/06 10:43      
         Nice part      Steve M. Taylor      04/10/06 13:23      
   Built Precision Sine Wave Generator.      Prahlad J. Purohit      04/11/06 08:38      
      Good luck      Kai Klaas      04/11/06 09:10      

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