HCPL-7860 Datasheet, PDF(26/28 Page) Agilent(Hewlett-Packard)

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HCPL-7860 Datasheet, PDF (26/28 Pages) Agilent(Hewlett-Packard) – Isolated 15-bit A/D Converter

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that any ground or power plane

on the PC board does not pass

directly below or extend much

wider than the body of the

isolated modulator.

Shunt Resistors

The current-sensing shunt

resistor should have low

resistance (to minimize power

dissipation), low inductance (to

minimize di/dt induced voltage

spikes which could adversely

affect operation), and reasonable

tolerance (to maintain overall

circuit accuracy). Choosing a

particular value for the shunt is

usually a compromise between

minimizing power dissipation and

maximizing accuracy. Smaller

shunt resistances decrease power

dissipation, while larger shunt

resistances can improve circuit

accuracy by utilizing the full

input range of the isolated

modulator.

The first step in selecting a shunt

is determining how much current

the shunt will be sensing. The

graph in Figure 22 shows the

RMS current in each phase of a

three-phase induction motor as a

function of average motor output

power (in horsepower, hp) and

motor drive supply voltage. The

440

380

220

120

0 5 10 15 20 25 30 35

MOTOR PHASE CURRENT â A (rms)

Figure 22. Motor Output Horsepower

vs. Motor Phase Current and Supply

Voltage.

maximum value of the shunt is

determined by the current being

measured and the maximum

recommended input voltage of

the isolated modulator. The

maximum shunt resistance can be

calculated by taking the maxi-

mum recommended input voltage

and dividing by the peak current

that the shunt should see during

normal operation. For example, if

a motor will have a maximum

RMS current of 10 A and can

experience up to 50% overloads

during normal operation, then the

peak current is 21.1 A

(=10x1.414x1.5). Assuming a

maximum input voltage of

200 mV, the maximum value of

shunt resistance in this case

would be about 10 mâ¦.

The maximum average power

dissipation in the shunt can also

be easily calculated by multiply-

ing the shunt resistance times the

square of the maximum RMS

current, which is about 1 W in

the previous example.

If the power dissipation in the

shunt is too high, the resistance

of the shunt can be decreased

below the maximum value to

decrease power dissipation. The

minimum value of the shunt is

limited by precision and accuracy

requirements of the design. As

the shunt value is reduced, the

output voltage across the shunt is

also reduced, which means that

the offset and noise, which are

fixed, become a larger percentage

of the signal amplitude. The

selected value of the shunt will

fall somewhere between the

minimum and maximum values,

depending on the particular

requirements of a specific design.

When sensing currents large

enough to cause significant

heating of the shunt, the

temperature coefficient (tempco)

of the shunt can introduce

nonlinearity due to the signal

dependent temperature rise of the

shunt. The effect increases as the

shunt-to-ambient thermal

resistance increases. This effect

can be minimized either by

reducing the thermal resistance

of the shunt or by using a shunt

with a lower tempco. Lowering

the thermal resistance can be

accomplished by repositioning

the shunt on the PC board, by

using larger PC board traces to

carry away more heat, or by

using a heat sink.

For a two-terminal shunt, as the

value of shunt resistance

decreases, the resistance of the

leads becomes a significant

percentage of the total shunt

resistance. This has two primary

effects on shunt accuracy. First,

the effective resistance of the

shunt can become dependent on

factors such as how long the

leads are, how they are bent, how

far they are inserted into the

board, and how far solder wicks

up the lead during assembly

(these issues will be discussed in

more detail shortly). Second, the

leads are typically made from a

material such as copper, which

has a much higher tempco than

the material from which the

resistive element itself is made,

resulting in a higher tempco for

the shunt overall.

Both of these effects are elimi-

nated when a four-terminal shunt

is used. A four-terminal shunt has

two additional terminals that are

Kelvin-connected directly across

the resistive element itself; these

two terminals are used to monitor

the voltage across the resistive

element while the other two

terminals are used to carry the

load current. Because of the

Kelvin connection, any voltage

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