ACPL-7970-300E Просмотр технического описания (PDF) - Avago Technologies

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ACPL-7970-300E

Optically Isolated Sigma-Delta Modulator

Avago Technologies 

Shunt Connections

The recommended method for connecting the isolated

modulator to the shunt resistor is shown in Figure 19. VIN+

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

the shunt resistor, while VIN– is shorted to GND1, with the

power-supply 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 isolated

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

be obtained by connecting VIN+ and VIN– directly across

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.

MOTOR

HV+

+–

RSENSE

FLOATING

POSITIVE SUPPLY

GATE

DRIVE

CIRCUIT

D1

5.1 V

R2a 22 Ω

R1

C1a 1 µF

C1b 0.1 µF

R2b 22 Ω

C2b C2a

20 nF 20 nF

ISOLATION

BARRIER

VDD1

VIN+

VIN–

GND1

VDD2

MCLK

MDAT

GND2

ACPL-7970

NON-

ISOLATED

5 V/3.3 V

C3b

1 µF

C3a

0.1 µF

GND2

HV –

Figure 21. Schematic for three conductor shunt connection.

17

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