A DI box

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.

Fig 2 : High performance DI box.

The voltage reference is 5 5.1 V zener diodes. The noise from these are reduced by R120 and C110. Whenpower is applied, C110 is charged through Q101's base until D111 starts to conduct. After that, C110 is charged through R118 and R120.

At high signal levels, U101's current consumption will pull Vcc below the zener voltage of D111 and D114 will turn off to indicate near-clipping.

D106 from Q101's base to Vcc prevents reverse-biasing of Q101 in case the output is shorted to ground.

The 30V zener D115 does not do anything. It is on the schematic because I originally built the unit on 2 PCBs with the amplifier (and zener) on one PCB and the voltage regulator on the other.

The ground lift resistor (R115) is 100 Ω here, but in a well grounded system, this can be even higher.

Diodes D101 and D104 protects the outputs of the OP-AMPs during charge/discharge of the output capacitors