Email: Password: Remember Me | Create Account (Free)

Back to Subject List

Old thread has been locked -- no new posts accepted in this thread
???
03/05/05 16:49
Read: times


 
#89163 - it's part of your rectifier
Responding to: ???'s previous message
Dear Prahlad,
don't worry there's nothing magic. From your original post I remember the following line:

Prahlad said:
I use a Op-Amp based Inverting amplifier for signal gain control. For range selection I simply change the feedback resistor of this amplifier...

I assumed that this inverting amplifier is identical to the last OPamp of this classic rectifier circuit:



I assumed that you insert 14.67kOhm, 22kOhm or 44kOhm resistors where this 22kOhm resistor can be seen in the feedback loop of last OPamp in above circuit, and that you need an alternative to that. If so, you could use the following circuit, where you put a 14.67kOhm resistor into this last OPamp and use two additional gain stages:



So, the OPamp with the 13kOhm resistor in series to 4k7 trimmer is nothing but the last OPamp of your rectifier.

Why taking this series combination of 13kOhm resistor and 4k7 cermet trimmer instead of 14.67kOhm resistor?
It would allow you to trim certain gain error caused by the manufacturing tolerances of resistors in the integrator and rectifier. So, with the trimmer you can compensate for all these tolerances, which you still had to do elsewhere when using a fixed 14.67kOhm resistor instead.

If the trimmer is once set properly, then the two additional gain stages would not introduce further relevant errors, when low tolerance resistors are used. By this all the three lines would automatically be trimmed.

Kai

List of 37 messages in thread
TopicAuthorDate
Three Gains?            01/01/70 00:00      
   Digital Pots            01/01/70 00:00      
      Re: Digital Pots.            01/01/70 00:00      
         precision digital pots can not be found            01/01/70 00:00      
   PGA are cheap too            01/01/70 00:00      
      Easiest Method            01/01/70 00:00      
         Re: Easiest method.            01/01/70 00:00      
            Need not to be a problem            01/01/70 00:00      
            Three gains            01/01/70 00:00      
               precision            01/01/70 00:00      
                  ratio is precision            01/01/70 00:00      
      Re: PGA            01/01/70 00:00      
   could you            01/01/70 00:00      
      Re: Logarithmic Amplifier.            01/01/70 00:00      
         never used one            01/01/70 00:00      
   Use three independent lines            01/01/70 00:00      
      Re: Three Independent Lines.            01/01/70 00:00      
         Not three integrators, but...            01/01/70 00:00      
            Re: Three Amp sections for three gains            01/01/70 00:00      
               Is it unity gain buffer or I to V conv.            01/01/70 00:00      
                  it's part of your rectifier            01/01/70 00:00      
                     Thanks for clarification.            01/01/70 00:00      
               Maybe some protection scheme for ADC            01/01/70 00:00      
                  Re: Some protection for ADC.            01/01/70 00:00      
   Higher resolution avoids gain switcing            01/01/70 00:00      
      Re: Higher Resolution.            01/01/70 00:00      
   You can use better profesional method.            01/01/70 00:00      
      low voltage analog switching            01/01/70 00:00      
         Re: Low voltage analog switch.            01/01/70 00:00      
            voltage            01/01/70 00:00      
            Seebeck-effect, thermo elements            01/01/70 00:00      
               better - yes            01/01/70 00:00      
               English english            01/01/70 00:00      
                  Re: English            01/01/70 00:00      
                  Thanks!            01/01/70 00:00      
      Re: Profesional method.            01/01/70 00:00      
         Mechanical relays.            01/01/70 00:00      

Back to Subject List