First test, 1,5V AA battery on DTA input. ECU reads the correct value (about 1316mV on 821 picture.
Second test, a flatline at 1 V for every AFR. In 823 picture multimeter says 1380 mV (ok) and ECU is disconnected.
In 822 picture, ECU is connected and value becomes wrong (multimeter says 5260 mV and DTA, as always, says 4737 mV).
As I supposed some posts ago, internal resistance of ECU is too low (voltmeters have a big internal resistance).
To solve, I installed a 1 Mohm resistance in series to lambda signal wire and now all perfectly works.
My last scale is near perfect, I only had little adjustments.
Thanks to all for the support! And a special thank to the Stevie's courtesy....
Luigi