AN614

Rev. 0.2

7

Figure 7. AC Line Current Monitor (Safety Isolation and Level Shift) Application

Figure 7 shows an ac line current monitor that demonstrates both safety isolation and level shifting. Safety isolation

galvanically isolates the 110 VAC line from the low-voltage, ground-based circuits. This circuit uses resistive shunt

R1 to sense ac current. The high-voltage interface circuit is referenced to the ac neutral (white) wire in a two-wire

(non-earth grounded) single-phase ac service.

ISOlinear input-side bias voltage is line-derived and uses a 3 V linear regulator. The low-voltage output-side

circuits are biased by a ground-referenced supply. Because there is no earth ground in this system, an ac line

perturbation can potentially cause high voltage to appear on the neutral (white) wire. This elevated neutral line

common-mode voltage is rejected by the Si86xx isolator's high CMTI of 35 kV/µs minimum, 50 kV/µs typical. For

more isolated level shifting application examples, see Silicon Labs Application Note “AN598: High-Speed Level

Shifting Using Si8xxx Isolators”.

Figure 8 shows a common ground loop in the transmission path of two linear circuits (a common scenario in test

and measurement, audio and other applications that use cable interconnects). The ground loop in the top diagram

circulates between the connector grounds while parasitic inductance (Z) causes ringing that generates output

noise. The bottom circuit of Figure 8 inserts the ISOlinear circuit between the signal source and receiver, breaking

the ground loop and dramatically reducing the local ground path lengths and associated parasitic inductance.

R1

C1

U2

3.0V

LDO

C4

C5

ISOlinear

Ref Design

INPUT

VDDA

GNDA

D1

OUTPUT

VDDB

GNDB

3.3V

Output Side

Bias Supply

1.5V

R2

R3

R5

1.5V

R12

R13

R14

R15

Single-Phase

AC Line

R4

3.3V

Mixed-Signal

Circuits

‐1,000V

+1,000V

Common Mode

Transient