On Wednesday, June 12, 2013 12:43:11 PM UTC-4, Julian Arnold wrote:
...
nal is easily dominating.
oscillation arises.
pletely different like reflections in front of the receiver diode.
r levels.
tion.http://imageshack.us/a/img62/1231/increasingoneszerososc1.png
into the beam) the additional oscillation appears.http://imageshack.us/a/img23/1231/increasingoneszerososc1.png
gnal calm down (Maybe some saturation ?).
image shows that the oscillation appears nearly randomly when operating nea r the 'critical' power.http://img692.imageshack.us/img692/1231/increasingon eszerososc1.png
1/7702/schematicj.png ...Hello Julian, What is the status of this issue? Did you resolve it? I am finding a very similar issue on a transimpedance amplifier design I am making with LMH662
9, and I believe that I have uncovered the solution which is that there mus t be a minimum source impedance seen by the LMH6629 input pins. For instanc e, try a 64.9 Ohm resistor (value what I found to work) between pin 3 and t he bias network (C1 etc. on your schematic). Depending on wiring length to the photodiode another series resistor may be needed there, i.e. between th e photodiode and the junction of inverting input pin and the feedback R-C. I believe the issue (which these resistors seem to mitigate) is an intern al oscillation of the LMH6629 input stage. I see the oscillation dependent significantly on the power supply voltage and even on the history of power supply voltage, and although I did not try it I believe it may well be depe ndent on signal amplitude too. In any case, I do not think it is wise to have any oscillation in your fi nal design, no oscillation is small enough to ignore. At the very least, th e unknown phase of the oscillation amounts to extra noise, and nonlineariti es must mix it into your frequency bands of interest. I hope these comments help, and I would really appreciate to hear if anyo ne else has encountered similar issues with LMH6629 in any circuit. I don't think this issue is about transimpedance applications only. By the way, th e datasheet mentions 25 Ohm minimum source impedance, but IMHO this is wron g... Also by the way, the oscillations I saw were typically at ~1.23 GHz, w ith other satellite peaks too. Sincerely,Gerard Visser Indiana University