Rainbow Electronics MAX1400 - Manual (Page 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
DD 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
R GAIN
R OFST R X
V IN+
V IN-
R SENSE
4–20mA LOOP
INTERFACE
R FDBK
R Y
C C
ISOLATION
BARRIER
V+
GND
V+
4
SPI
4
SPI
3
SPI
GND
SENSOR
VOLTAGE
REGULATOR
µ P/ µ C
MAX1400
Figure 18. 4–20mA Transmitter
C C
+5V
+2.5V
REFIN+
REFIN-
AGND DGND
R
R
THERMOCOUPLE
JUNCTION
SWITCHING
NETWORK
PGA
MAX1400
BUFFER
AIN1
AIN2
Figure 17. Thermocouple Application with MAX1400