ISO1176DWR datasheet(20/25 Pages) TI

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APPLICATION INFORMATION

Transient Voltages

ISO

GND

(1)

RIN

CIN

RISO

CISO

SystemGround (GND1)

BusReturn (GND2)

16V

A orB

ISO

GND

(2)

ISO

GND

ISO1176

SLLS897C – MARCH 2008 – REVISED OCTOBER 2008.................................................................................................................................................. www.ti.com

Isolating of a circuit insulates it from other circuits and earth, so that noise voltage 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. The transient ratings of the ISO1176 standard are sufficient for all but

the most severe installations. However, some equipment manufacturers use ESD generators to test equipment

transient susceptibility. This practice 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 29 models the ISO1176 bus IO connected to a noise generator. CIN and RIN is the device, and any other

stray or added capacitance or resistance across the A or B pin to GND2. CISO and RISO is the capacitance and

resistance between GND1 and GND2 of the ISO1176, plus those of any other insulation (transformer, etc.). Stray

inductance is assumed to be negligible.

From this model, the voltage at the isolated bus return

and will always be less than 16 V from VN. If the

ISO1176 is tested as a stand-alone device,

• R

IN= 6 x 10

4 Ω,

• C

IN= 16 x 10

–12 F,

• R

ISO= 10

9 Ω and

• C

ISO= 10

–12 F.

Notice from Figure 29 that the resistor ratio determines

the voltage ratio at low frequencies, and that the

inverse capacitance ratio determines the voltage ration

at high frequencies. In the stand-alone case and for

low frequencies,

Figure 29. Device Model For Static Discharge

Testing

, or essentially all of the noise appears across the

barrier.

At very high frequencies,

(3)

, and 94% of VN appears across the barrier. As long as

RISO is greater than RIN and CISO is less than CIN, most

of the transient noise appears across the isolation

barrier, as it should.

We do not recommend using ESD generators to test equipment transient susceptibility, or considering 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 by proper installer training.

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