17

Shunt Connections

The recommended method for connecting the isolated

of the ACPL-7970 is connected to the positive terminal of

power-supply return path functioning as the sense line to

the negative terminal of the current shunt. This allows a

singlepairofwiresorPCboardtracestoconnecttheisolated

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 connec-

tion 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

the 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 21. 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 resistor R2 in series with the input lead forms a low

pass anti-aliasing filter with the input bypass capacitor

C2. 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 connect-

ing 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 21. Schematic for three conductor shunt connection.

ACPL-7970

FLOATING

POSITIVE SUPPLY

RSENSE

GND1

MCLK

MDAT

GND2

NON-

ISOLATED

5 V/3.3 V

ISOLATION

BARRIER

GATE

DRIVE

CIRCUIT

HV+

HV –

C1b 0.1

µF

D1

5.1 V

C2a

20 nF

MOTOR

R2a 22

Ω

R1

R2b 22

Ω

+

–

GND2

C1a 1

µF

C3b

1

µF

C3a

0.1

µF

C2b

20 nF