Isolation of a circuit insulates it from other circuits and earth so that noise develops across the insulation rather than circuit components. The most common noise threat to data-line circuits is voltage surges or electrical fast transients that occur after installation and the transient ratings of ISO35T are sufficient for all but the most severe installations. However, some equipment manufacturers use their ESD generators to test transient susceptibility of their equipment and can easily exceed insulation ratings. ESD generators simulate static discharges that may occur during device or equipment handling with low-energy but very high voltage transients.

Figure 9-3 models the ISO35T bus IO connected to a noise generator. C_IN and R_IN is the device and any other stray or added capacitance or resistance across the A or B pin to GND2, C_ISO and R_ISO is the capacitance and resistance between GND1 and GND2 of ISO35T plus those of any other insulation (transformer, etc.), and we assume stray inductance negligible. From this model, the voltage at the isolated bus return is shown in Equation 1 and will always be less than 16 V from V_N.

Equation 1.

If ISO35T is tested as a stand-alone device, R_IN= 6 × 10⁴Ω, C_IN= 16 × 10^-12 F, R_ISO= 10⁹Ω and C_ISO= 10^-12 F.

In Figure 9-3 the resistor ratio determines the voltage ratio at low frequency and it is the inverse capacitance ratio at high frequency. In the stand-alone case and for low frequency, use Equation 2, or essentially all noise appears across the barrier.

Equation 2.

At very high frequency, Equation 3 is true and 94% of V_N appears across the barrier.

Equation 3.

As long as R_ISO is greater than R_IN and C_ISO is less than C_IN, most of transient noise appears across the isolation barrier, as it should.

We recommend the reader not test equipment transient susceptibility with ESD generators or consider product claims of ESD ratings above the barrier transient ratings of an isolated interface. ESD is best managed through recessing or covering connector pins in a conductive connector shell and installer training.

Figure 9-3 Noise Model