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Farshid Jafari Harandi
02/14/06 11:41
Read: 2117 times

#109974 - DSO's Op-Amp adder probelm :(
Dear Kai,
Desiging my DSO I've faced a strange probelm , I have calibrated everything up to the last stage of the Analog baord, but it seems that the Adder Circuit in 'X10 Gain Buffer' stage is not working properly. This adder Circuit was intended to move the input signal from +/- 1V swing to +0.6 to +2.6 Volt to meet the ADC requirements.
Here the latest schematic (which I am refering to in this message) Could be found. I checked for a short(on the PCB) between the Positive and negative input by eye, but I didn't see any (maybe there is a short under the IC). Do you confirm the Adder Circuit is correct ?

Here are the Signal captures :

Input signal of the board (the signal that enters the baord) :
0.2V/Div - 200mS/Div

LM741 does produces a very stable voltage (-0.170 V) but after passing the R8 and right at the C11,C20 caps it gets noisy !:
0.2V/Div - 200mS/Div

Negative Input of the AD844 OpAmp (picked up right at its pin) :
0.2V/Div - 200mS/Div

Positive Input of the AD844 OpAmp (picked up right at its pin) :
0.2V/Div - 200mS/Div

Output of the AD844 OpAmp (picked up right at its pin) :
1V/Div - 200mS/Div

Input frequency is a 1Khz Square Wave.

Thanks in Advance.
Looking forward to hear from you soon,

List of 21 messages in thread
DSO's Op-Amp adder probelm :(      Farshid Jafari Harandi      02/14/06 11:41      
   Looks ok but i am missing something      Jez Smith      02/14/06 15:14      
   By the way...      Jez Smith      02/14/06 15:26      
      Answers      Farshid Jafari Harandi      02/15/06 03:41      
   I just had a thought      Jez Smith      02/14/06 15:37      
   Include R8 in the feedback loop      Kai Klaas      02/14/06 19:21      
      C6 ?      Farshid Jafari Harandi      02/14/06 20:59      
         Good idea      Kai Klaas      02/15/06 05:59      
            Pictures      Farshid Jafari Harandi      02/15/06 07:07      
               Congrets!!      Kai Klaas      02/15/06 07:54      
                  RE : Congrets!!      Farshid Jafari Harandi      02/15/06 08:26      
                     Prototyping      Kai Klaas      02/15/06 08:50      
      Corrections      Farshid Jafari Harandi      02/15/06 04:54      
         Developing      Kai Klaas      02/15/06 05:58      
            Time matters !      Farshid Jafari Harandi      02/15/06 06:56      
               some "friend"      Erik Malund      02/15/06 07:02      
                  Imagine !      Farshid Jafari Harandi      02/15/06 07:18      
               AC setting?      Kai Klaas      02/15/06 07:50      
               Obey??      Kai Klaas      02/15/06 07:52      
               Tricky      Kai Klaas      02/15/06 07:57      
   What if...      Kai Klaas      02/15/06 08:57      

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