MAX1400_02 Datasheet, PDF (32/34 Pages) Maxim Integrated Products – +5V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC

+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
THERMOCOUPLE
JUNCTION
R
R
C
+5V
+2.5V
MAX1400
AIN1
SWITCHING
NETWORK ^{BUFFER PGA}
AIN2
C
REFIN+
REFIN-
AGND
DGND
Figure 17. Thermocouple Application with MAX1400
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.
SENSOR
ISOLATION
V+ ^BARRIER
V+
R_GAIN
4
4
3
µP/µC
DAC
SPI
SPI
SPI
R_OFST
C_C
VOLTAGE
REGULATOR
R_Y
GND
GND
R_FDBK
V_IN+
R_X
4–20mA LOOP
INTERFACE
R_SENSE
V_IN-
Figure 18. 4–20mA Transmitter
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