AD7719 datasheet(39/41 Pages) AD

39 / 41 page

REV. A

AD7719

–38–

In this 3-wire configuration, the lead resistances will result in

errors if only one current source is used because the 200 µA will

flow through RL1, developing a voltage error between AIN1 and

AIN2. In the scheme outlined below, the second RTD current

source is used to compensate for the error introduced by the

200 µA flowing through RL1. The second RTD current flows

through RL2. Assuming RL1 and RL2 are equal (the leads

would normally be of the same material and of equal length),

and IOUT1 and IOUT2 match, the error voltage across RL2

equals the error voltage across RL1, and no error voltage is

developed between AIN1 and AIN2. Twice the voltage is

developed across RL3, but since this is a common-mode voltage,

it will not introduce errors. RCM is included so the current

flowing through the combination of RL3 and RCM develops enough

voltage that the analog input voltage seen by the AD7719 is

within the common-mode range of the ADC. The reference

voltage for the AD7719 is also generated using one of these

matched current sources. This reference voltage is developed

across the 12.5 k

Ω resistor as shown, and applied to the differential

reference inputs of the AD7719. This scheme ensures that the analog

input voltage span remains ratiometric to the reference voltage.

Any errors in the analog input voltage due to the temperature drift

of the RTD current source is compensated for by the variation

in the reference voltage. The typical drift matching between the

two RTD current sources is less than 1 ppm/°C. The voltage on

either IOUT pin can go to within 0.6 V of the AVDD supply.

Smart Transmitters

Smart transmitters are another key design-in area for the AD7719.

The dual

Σ-∆ converter, single-supply operation, 3-wire interface

capabilities, and small package size are all of benefit in smart

transmitters. Here, the entire smart transmitter must operate

from the 4 to 20 mA loop. Tolerances in the loop mean that the

amount of current available to power the transmitter is as low as

3.5 mA. Figure 24 shows a block diagram of a smart transmitter,

which includes the AD7719.

Not shown in Figure 24 is the isolated power source required to

power the front end. The advantages of the AD7719 in these

applications is the dual-channel operation, meaning that the

user does not have to interrupt the main channel when measur-

ing secondary variables, and therefore does not have the latency

associated with the settling times of the digital filter. The fact

that the AD7719 is factory-calibrated means that in the majority

of applications, the user will not have to perform any field cali-

bration given the excellent offset and gain drift performance of

the device as a result of the signal chain chopping employed in

the signal chain.

MICROCONTROLLER

REF OUT2

REF IN

10 F

0.1 F

AVDD

DVDD

AIN1

AIN2

DOUT

SCLK

DIN

DGND

AGND

AIN5

AIN5 COM

REFIN2

REFIN(+)

REFIN(–)

0.1 F

REF OUT1

CLOCK

LATCH

DATA

4.7 F

COM

VCC

GND

AD7719

AD421

BOOST VCC

0.01 F

1000pF

LOOP

POWER

10 F

3.3V

1.25V

DN25D

0.01 F

LOOP

RTN

COMP

DRIVE

MAIN

VARIABLES

SECONDARY

VARIABLES

AIN3

AIN4

Figure 24. Smart Transmitter Employing the AD7719

AD7719 Datasheet(PDF) 39 Page - Analog Devices

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