Rainbow-electronics MAX1400 Manual de usuario descargar pdf (Pagina 32)

4–20mA Loop-Powered Transmitters

Low power, single-supply operation, and easy interfac-

ing with optocouplers make the MAX1400 ideal for

loop-powered 4–20mA transmitters. Loop-powered

transmitters draw their power from the 4–20mA loop,

limiting the transmitter circuitry to a current budget of

4mA. Tolerances in the loop further limit this current

budget to 3.5mA. Since the MAX1400 consumes only

250µA, a total of 3.25mA remains to power the remain-

ing transmitter circuitry. Figure 18 shows a block dia-

gram for a loop-powered 4–20mA transmitter.

Power Supplies

No specific power sequence is required for the

MAX1400; either the V+ or the V

supply can come

up first. While the latchup performance of the MAX1400

is good, it is important that power be applied to the

MAX1400 before the analog input signals (AIN_) or the

CLKIN inputs, to avoid latchup. If this is not possible,

then the current flow into any of these pins should be

limited to 50mA. If separate supplies are used for the

MAX1400 and the system digital circuitry, then the

MAX1400 should be powered up first.

MAX1400

+5V, 18-Bit, Low-Power, Multichannel,

Oversampling (Sigma-Delta) ADC

32 ______________________________________________________________________________________

DAC

GAIN

OFST

IN+

IN-

SENSE

4–20mA LOOP

INTERFACE

FDBK

ISOLATION

BARRIER

V+

GND

V+

SPI

GND

SENSOR

VOLTAGE

REGULATOR

µP/µC

MAX1400

Figure 18. 4–20mA Transmitter

+5V

+2.5V

REFIN+

REFIN-

AGND

DGND

THERMOCOUPLE

JUNCTION

SWITCHING

NETWORK

PGA

MAX1400

BUFFER

AIN1

AIN2

Figure 17. Thermocouple Application with MAX1400

1 2 ... 27 28 29 30 31 32 33 34 35 36

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Rainbow-electronics MAX1400 Manual de usuario Pagina 32

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