Circuits
Fig 1 : Simple DI box (protection diodes omitted in schematic).
This is a common way to build a DI box. Performance in a lab environment (with short cables and test equipment with a CMRR of 100 dB or more) is very good.
n a real life environment, CMRR is much lower than in a lab, typically 40 dB to 60 dB and noiseperformance will be much poorer than in a lab. The primary source of this noise is the 24 V zener diode i n the DI box. Some improvement can be obtained by replacing the 24 V zener diode with 5 5.1 V zeners in series and/or increasing the value of C208.
The value of the resistor across the ground lift switch can not be more than around 10 Ω as the supply current flows in it.
