HCPL-0870 datasheet(28/28 Pages) HP

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MOTOR

HV-

HV+

RSENSE

FLOATING

POSITIVE

SUPPLY

GATE DRIVE

CIRCUIT

VDD1

VIN+

VIN-

GND1

VDD2

MCLK

MDAT

GND2

HCPL-7860

0.1 µF

C2a

0.01 µF

R2a 39

Ω

5.1 V

C2b

0.01 µF

R2b 39

Ω

Figure 23. Schematic for Three Conductor Shunt Connection.

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

VIN+ and VIN- directly across the

shunt resistor with two conduc-

tors, and connecting GND1 to the

shunt resistor with a third

conductor for the power-supply

return path, as shown in Figure

23. When connected this way,

both input pins should be

bypassed. To minimize electro-

magnetic interference of the

sense signal, all of the conductors

(whether two or three are used)

connecting the isolated modula-

tor 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 low-

pass 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.

Voltage Sensing

The HCPL-7860 Isolated Modula-

tor can also be used to isolate

signals with amplitudes larger

than its recommended input

range with the use of a resistive

voltage divider at its input. The

only restrictions are that the

impedance of the divider be

relatively small (less than 1 k

Ω)

so that the input resistance

(280 k

Ω) and input bias current

µA) do not affect the accuracy

of the measurement. An input

bypass capacitor is still required,

although the 39

Ω series damping

resistor is not (the resistance of

the voltage divider provides the

same function). The low-pass

filter formed by the divider

resistance and the input bypass

capacitor may limit the achievable

bandwidth. To obtain higher

bandwidth, the input bypass

capacitor (C2) can be reduced,

but it should not be reduced

much below 1000 pF to maintain

adequate input bypassing of the

isolated modulator.

HCPL-0870 Datasheet(PDF) 28 Page - Agilent(Hewlett-Packard)

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