Broadcom

- 14 -

ACPL-785E, HCPL-7850, HCPL-7851, 5962-97557

Data Sheet

Shunt Resistor Selections

Shunt Resistor Selections

The current-sensing shunt resistor should have low resistance

(to minimize power dissipation), low inductance (to minimize

di/dt induced voltage spikes that could adversely affect

operation), and reasonable tolerance (to maintain overall

circuit accuracy). The value of the shunt should be chosen as a

compromise between minimizing power dissipation by

making the shunt resistance smaller and improving circuit

accuracy by making it larger and utilizing the full input range of

the HCPL-785x. Broadcom recommends four different shunts

that can be used to sense average currents in motor drives up

to 35A and 35 hp. Table 1 shows the maximum current and

horsepower range for each of the LVR-series shunts from Dale.

Even higher currents can be sensed with lower value shunts

available from vendors such as Dale, IRC, and Isotek

(Isabellenhuette). When sensing currents large enough to

cause significant heating of the shunt, the temperature

coefficient of the shunt can introduce nonlinearity due to the

signal dependent temperature rise of the shunt. Using a heat

sink for the shunt or using a shunt with a lower tempco can

help minimize this effect. The Application Note 1078, Designing

with Avago Technologies Isolation Amplifiers, contains additional

information on designing with current shunts.

The recommended method for connecting the isolation

is connected to the positive terminal of the shunt resistor, while

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 isolation amplifier circuit to

the shunt resistor. 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 be obtained by connecting pin 3 to the negative terminal

of the shunt resistor separate from the power supply return

path. When connected this way, both input pins should be

bypassed. Whether two or three wires are used, it is

recommended that twisted-pair wire or very close PC board

traces be used to connect the current shunt to the isolation

amplifier circuit to minimize electromagnetic interference to

the sense signal.

The 68Ω resistor in series with the input lead forms a low-pass

anti-aliasing filter with the input bypass capacitor with a

200 kHz bandwidth. The resistor performs another important

function as well: it dampens any ringing that might be present

in the circuit formed by the shunt, the input bypass capacitor,

and the wires or traces connecting the two. Undampened

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. To be effective, the

damping resistor should be at least 39Ω.

PC Board Layout

In addition to affecting offset, the layout of the PC board can

also affect the common-mode rejection (CMR) performance of

the isolation amplifier, due primarily to stray capacitive

coupling between the input and the output circuits. To obtain

optimal CMR performance, the layout of the printed circuit

board (PCB) should minimize any stray coupling by

maintaining the maximum possible distance between the

input and output sides of the circuit and ensuring that any

ground plane on the PCB does not pass directly below the

HCPL-785x. Using surface-mount components can help

achieve many of the PCB objectives discussed in the preceding

paragraphs. An example through-hole PCB layout illustrating

some of the more important layout recommendations is

shown in Figure 26 and Figure 27. See Application Note 1078,

Designing with Avago Technologies Isolation Amplifiers, for more

information on PCB layout consideration.

Figure 28 Operating Circuit for Burn-In and Steady State Life Tests

1

2

3

4

8

7

6

5

VIN+

GND

GND

+

–

+

–

27

1 k

1 k

27

1 k

1 k

(+)

(–)

0.1 F