17

Shunt Connections

The recommended method for

connecting the isolated

modulator to the shunt resistor

(pin 2 of the HPCL-7860/

HCPL-786J) is connected to

the positive terminal of the

3) is shorted to GND1 with the

power-supply return path

functioning as the sense line

to the negative terminal of the

current shunt. This allows a

single pair of wires or PC

board traces to connect the

isolated modulator circuit to

the shunt resistor. By

referencing the input circuit to

the negative side of the sense

resistor, any load current

induced noise transients on

the shunt are seen as a

common-mode signal and will

not interfere with the current-

sense signal. This is important

because the large load currents

flowing through the motor

drive, along with the parasitic

inductances inherent in the

wiring of the circuit, can

generate both noise spikes and

offsets that are relatively large

compared to the small voltages

that are being measured across

the current shunt.

If the same power supply is

used both for the gate drive

circuit and for the current

sensing circuit, it is very

important that the connection

from GND1 of the isolated

modulator to the sense resistor

be the only return path for

supply current to the gate

drive power supply in order to

eliminate potential ground loop

problems. The only direct

connection between the

isolated modulator circuit and

the gate drive circuit should

be the positive power supply

line.

In some applications, however,

supply currents flowing

through the power-supply

return path may cause offset

or noise problems. In this

case, better performance may

shunt resistor with two

conductors, and connecting

GND1 to the shunt resistor

with a third conductor for the

power-supply return path, as

shown in Figure 19. When

connected this way, both input

pins should be bypassed. To

minimize electromagnetic

interference of the sense

signal, all of the conductors

(whether two or three are

used) connecting the isolated

modulator to the sense resistor

should be either twisted pair

wire or closely spaced traces

on a PC board.

The 39

Ω resistor in series

with the input lead (R2) forms

a lowpass anti-aliasing filter

with the 0.01

µF input bypass

capacitor (C2) with a 400 kHz

bandwidth. The resistor

performs another important

function as well; it dampens

any ringing which might be

present in the circuit formed

by the shunt, the input bypass

capacitor, and the inductance

of wires or traces connecting

the two. Undamped ringing of

the input circuit near the

input sampling frequency can

alias into the baseband

producing what might appear

to be noise at the output of

the device.

Figure 19. Schematic for Three Conductor Shunt Connection.

+

-

MOTOR

HV-

HV+

FLOATING

POSITIVE

SUPPLY

GATE DRIVE

CIRCUIT

GND1

MCLK

MDAT

GND2

HCPL-7860/

HCPL-786J

C1

0.1 µF

C2a

0.01 µF

R2a 39

Ω

R1

D1

5.1 V

C2b

0.01 µF

R2b 39

Ω