MAX1402_07 Datasheet, PDF(35/38 Page) Maxim Integrated Products – +5V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC

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MAX1402_07 Datasheet, PDF (35/38 Pages) Maxim Integrated Products – +5V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC

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

Oversampling (Sigma-Delta) ADC

4â20mA Loop-Powered Transmitters

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

ing with optocouplers make the MAX1402 ideal for

loop-powered 4â20mA transmitters. Loop-powered

transmitters draw their power from the 4â20mA 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

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

ing transmitter circuitry. Figure 18 shows a block dia-

gram for a loop-powered 4â20mA transmitter.

Power Supplies

No specific power sequence is required for the

MAX1402; either the V+ or the VDD 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 200ÂµA 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

200ÂµA will flow through RL1 developing a voltage error

between AIN1 and AIN2. An additional current source

compensates for this error by developing an equivalent

voltage across RL2 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 (RL1 = RL2) and OUT1 and OUT2 have

matching tempcos (5ppm/Â°C). Both current sources will

flow through RL3 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

RREF 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-

SENSOR

ISOLATION

V+ BARRIER

V+

RGAIN

ÂµP/ÂµC

DAC

SPI

ROFST

VOLTAGE

REGULATOR

VIN+

4â20mA LOOP

INTERFACE

GND

RFDBK

RSENSE

VIN-

Figure 18. 4â20mA Transmitter

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