AD7713 datasheet(24/28 Pages) AD

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REV. D

–24–

AD7713

SYNC

RFS

TFS

SDATA

SCLK

MODE

PC0

PC1

PC2

PC3

MISO

MOSI

68HC11

AD7713

DRDY

SCK

DVDD

Figure 18. AD7713 to 68HC11 Interface

APPLICATIONS

4-Wire RTD Configurations

Figure 19 shows a 4-wire RTD application where the RTD

transducer is interfaced directly to the AD7713. In the 4-wire

configuration, there are no errors associated with lead resis-

tances as no current flows in the measurement leads connected

to AIN1(+) and AIN1(–). One of the RTD current sources is

used to provide the excitation current for the RTD. A common

nominal resistance value for the RTD is 100

Ω and, therefore, the

RTD will generate a 20 mV signal, which can be handled directly

by the analog input of the AD7713. In the circuit shown, the

second RTD excitation current is used to generate the reference

voltage for the AD7713. This reference voltage is developed

across RREF and applied to the differential reference inputs. For

the nominal reference voltage of 2.5 V, RREF is 12.5 k

Ω. 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 volt-

age. The typical matching between the two RTD current sources

is less than 3 ppm/

°C.

AIN1(+)

AIN1(–)

AVDD

AGND

DGND

A = 1 – 128

AD7713

RTD1

REF IN(+)

REF IN(–)

RTD2

200 A

RREF

INTERNAL

CIRCUITRY

DVDD

PGA

200 A

RTD

Figure 19. 4-Wire RTD Application with the AD7713

3-Wire RTD Configurations

Figure 20 shows a 3-wire RTD configuration using the AD7713.

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

errors if only one current source is used as the 200

µA will flow

through RL1 developing a voltage error between AIN1(+) and

AIN1(–). In the scheme outlined below, the second RTD cur-

rent 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

RTD1 and RTD2 match, then the error voltage across RL2 equals

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

between AIN1(+) and AIN1(–). Twice the voltage is developed

across RL3 but since this is a common-mode voltage, it will not

introduce any errors. The reference voltage is derived from one

of the current sources. This gives all the benefits of eliminating

RTD temperature coefficient errors as outlined in Figure 19.

The voltage on either RTD input can go to within 2 V of the

AVDD supply. The circuit is shown for a 2.5 V reference.

AIN(+)

AIN(–)

AVDD DVDD

AGND

DGND

A = 1 – 128

AD7713

RTD1

RTD2

12.5k

INTERNAL

CIRCUITRY

REF IN(+)

REF IN(–)

RL1

RL2

RL3

PGA

RTD

200 A

Figure 20. 3-Wire RTD Application with the AD7713

4–20 mA Loop

The AD7713’s high level input can be used to measure the

current in 4–20 mA loop applications as shown in Figure 21. In

this case, the system calibration capabilities of the AD7713 can

be used to remove the offset caused by the 4 mA flowing through

the 500

Ω resistor. The AD7713 can handle an input span as

low as 3.2

VREF (= 8 V with a VREF of 2.5 V) even though the

nominal input voltage range for the input is 10 V. Therefore,

the full span of the ADC can be used for measuring the current

between 4 and 20 mA.

REF IN(+)

AIN1(+)

AIN1(–)

AIN3

AGND

DGND

A = 1 – 128

REF IN(–)

AD7713

4–20mA

LOOP

ANALOG 5V SUPPLY

500

VOLTAGE

ATTENUATION

AVDD

DVDD

INTERNAL

CIRCUITRY

PGA

1 A

Figure 21. 4-20 mA Measurement Using the AD7713

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