Rainbow Electronics MAX1402 User Manual
Page 35
MAX1402
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
4–20mA Loop-Powered Transmitters
Low power, single-supply operation, and easy interfac-
ing with optocouplers make the MAX1402 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 MAX1402 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
MAX1402; either the V+ or the V
DD
supply can come
up first. While the latchup performance of the MAX1402
is good, it is important that power be applied to the
MAX1402 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
MAX1402 and the system digital circuitry, then the
MAX1402 should be powered up first.
3-Wire and 4-Wire
RTD Configurations
Tightly matched 200µA current sources compensate for
errors in 3-wire and 4-wire RTD configurations. In the 3-
wire configuration (Figure 19), the lead resistances
result in errors if only one current source is used. The
200
µ
A will flow through R
L1
developing a voltage error
between AIN1 and AIN2. An additional current source
compensates for this error by developing an equivalent
voltage across R
L2
ensuring the differential voltage at
AIN1 and AIN2 is not affected by lead resistance. This
assumes both leads are of the same material and of
equal length (R
L1
= R
L2
) and OUT1 and OUT2 have
matching tempcos (5ppm/°C). Both current sources will
flow through R
L3
developing a common-mode voltage
that will not affect the differential voltage at AIN1 and
AIN2. Using one of the current sources to supply the
reference voltage ensures a more accurate ratiometric
result.
Unlike the 3-wire configuration, the 4-wire configuration
(Figure 20) has no error associated with lead resis-
tances as no current flows in the measurement leads
connected to AIN1 and AIN2. Current source OUT1
provides the excitation current for the RTD and current
source OUT2 provides current to generate the refer-
ence voltage. This reference voltage developed across
R
REF
ensures that the analog input voltage span
remains ratiometric to the reference voltage. RTD temp-
co errors in the analog input voltage are due to the tem-
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
MAX1402
Figure 18. 4–20mA Transmitter
______________________________________________________________________________________
35