15

Also, multiple layers of the PC board can be used to

increase current carrying capacity. Numerous plated-

through vias should surround each non-Kelvin terminal of

the sense resistor to help distribute the current between

the layers of the PC board. The PC board should use 2 or

4 oz. copper for the layers, resulting in a current carrying

capacity in excess of 20 A. Making the current carrying

traces on the PC board fairly large can also improve the

sense resistor’s power dissipation capability by acting as a

heat sink. Liberal use of vias where the load current enters

and exits the PC board is also recommended.

Note: Please refer to Avago Technologies Application Note 1078 for

additional information on using Isolation Amplifiers.

Sense Resistor Connections

The recommended method for connecting the

ACPL-C78A/C780/C784 to the current sensing resistor is

C784) is connected to the positive terminal of the sense

the power-supply return path functioning as the sense

line to the negative terminal of the current sense resistor.

This allows a single pair of wires or PC board traces to

connect the ACPL-C78A/C780/C784 circuit to the sense

resistor. By referencing the input circuit to the negative

side of the sense resistor, any load current induced noise

transients on the resistor 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 induc-

tances 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 sensing resistor.

If the same power supply is used both for the gate drive

circuitandforthecurrentsensingcircuit,itisveryimportant

that the connection from GND1 of the ACPL-C78A/C780/

C784 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 ACPL-C78A/C780/C784 circuit

and the gate drive circuit should be the positive power

supply line.

Output Side

The op-amp used in the external post-amplifier circuit

should be of sufficiently high precision so that it does not

contribute a significant amount of offset or offset drift

relative to the contribution from the isolation amplifier.

Generally, op-amps with bipolar input stages exhibit

better offset performance than op-amps with JFET or

MOSFET input stages.

In addition, the op-amp should also have enough band-

width and slew rate so that it does not adversely affect the

response speed of the overall circuit. The post-amplifier

circuit includes a pair of capacitors (C5 and C6) that form

a single-pole low-pass filter; these capacitors allow the

bandwidth of the post-amp to be adjusted independently

of the gain and are useful for reducing the output noise

from the isola-tion amplifier. Many different op-amps

could be used in the circuit, including: TL032A, TL052A,

and TLC277 (Texas Instruments), LF412A (National Semi-

conductor).

The gain-setting resistors in the post-amp should have a

tolerance of 1% or better to ensure adequate CMRR and

adequate gain tolerance for the overall circuit. Resistor

networks can be used that have much better ratio toler-

ances than can be achieved using discrete resistors. A

resistor network also reduces the total number of compo-

nents for the circuit as well as the required board space.

Frequently asked questions about the

ACPL-C78A/C780/C784

1. The basics

1.1. Why should I use the ACPL-C78A/C780/C784 for sensing current

when Hall-effect sensors are available which don’t need an

isolated supply voltage?

Available in an auto-insertable, Stretched SO-8

package, the ACPL-C78A/C780/C784 is smaller than

and has better linearity, offset vs. temperature and

Common Mode Rejection (CMR) performance than

most Hall-effect sensors. Additionally, often the

required input-side power supply can be derived

from the same supply that powers the gate-drive

optocoupler.

2. Sense resistor and input filter

2.1. Where do I get 10 m

Ω resistors? I have never seen one that low.

Although less common than values above 10

Ω, there

are quite a few manufacturers of resistors suitable

for measuring currents up to 50 A when combined

with the ACPL-C78A/C780/C784. Example product

information may be found at Vishay's web site

(http://www.vishay.com) and Isotek’s web site

(http://www.isotekcorp.com).

2.2. Should I connect both inputs across the sense resistor instead of

This is not necessary, but it will work. If you do, be