Here is the shematic that all went right......
1) Make single GND
point were all GND wires met on shortest distance possible
2) Don't connect input source to the GND but to the
3) The Pre Low Pass Filter has floating GND connection
4) If removing the PreFilter, connect source to Diff-IN(-)
5) Be sure, that the source can drive ~1.3k Ohms (else
increase R10/R11, but noise will also increase !)
6) If not sure, connect a resistor (100 Ohms -1k) between
Diff-IN (-) and GND and add two vice-versa connected Diodes in parallel
7) Don't forget decoupling for U1/U2 (100nF ceramic close
to the device) and electrolytics (100µ) for the whole circuit.
8) Only use metallfilm resistors (1%) for lowest noise and
good caps (NPO or MKS/FKP for C1,C3,C4)