[christianw.]

Das Scope zeigt DC, wie mein DVM, das kann auch Frequenz messen, die ist 0Hz.

Eine andere Möglichkeit wäre noch:

Zitat:Q. What other features of op amps should the user know about? A. A common problem encountered with JFET op-amps is phase inversion. If the input common-mode voltage of a JFET op-amp approaches the negative supply too closely, the inverting and non-inverting input terminals reverse functions. Negative feedback becomes positive feedback and the circuit may latch up. This latchup is unlikely to be destructive, but power may have to be switched off to correct it. This figure shows the effect of such phase inversion in a circuit where latch-up does not occur. The problem may be avoided by using bipolar amplifiers, or by restricting the common-mode range of the signal in some way.

Das kann aber eigentlich nur passieren, wenn das Amplitudenpoti voll aufgedreht ist.

Ich check das mit den 50Ohm.