Hi
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,
Farshid