DetectorMods

Author Topic: DC Offset in operational amplifiers in DC coupled front ends of P.I detectors  (Read 7973 times)

woody

  • Administrator
  • Sr. Member
  • *****
  • Posts: 280
    • View Profile
After testing the input stages on various detector I have found that many of the AD797 opamps with a spec sheet indication of 25mv DC offset is not the case when measured in some detectors.
Offset voltages in excess of 100mv are quite common.  Adding a potentiometer and trimming the Offset after 15 minutes run time to DC makes for a much more stable and quieter detector. As the first stages are DC coupled, reducing the Offset voltage makes for a much more behaved quieter detector. Frequency compensation can also be carried out to reduce Radio Frequency interference without degrading the low frequency slew rate or settling time of the opamp. 

Chook

  • Newbie
  • *
  • Posts: 3
    • View Profile
No wonder some of the shelf GPX are better than others,A bit of a lucky dip from the dealer? WA just had 150mm rain in Kalgoorlie so the
mud is here for at least two weeks. My 4000 is on the way for this fixit Woody while we sit it out.

barry

  • Newbie
  • *
  • Posts: 6
    • View Profile
HI WOODY & ALL ,,Reguarding the offset mentioned above I was curious as to whether this AD797 is similar to the NE5534 chip as used in many of the VFL units such as the IDX I built -up [input preamp ]..

Just curious ,,CHEERS BARRY..

woody

  • Administrator
  • Sr. Member
  • *****
  • Posts: 280
    • View Profile
Hi Barry, the AD797 is a much better chip compared to 5534  as it has a lower noise floor at low frequencies, there are even better chips around that make the 10hz corner frequency noise of an AD797 look bad.

barry

  • Newbie
  • *
  • Posts: 6
    • View Profile
HI,, WOODY ,,OK THANKS FOR THAT EXPLANATION..

CHEERS,,, BARRY..