AD7713 datasheet(25/28 Pages) AD

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–25–

REV. C

AD7713

APPLICATIONS

Four-Wire RTD Configurations

Figure 20 shows a four-wire RTD application where the RTD

transducer is interfaced directly to the AD7713. In the four-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 di-

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

the second RTD excitation current is used to generate the refer-

ence voltage for the AD7713. This reference voltage is devel-

oped 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 refer-

ence voltage. The typical matching between the two RTD cur-

rent sources is less than 3 ppm/

°C.

AIN1(+)

AIN1(–)

AVDD

AGND

DGND

A = 1 – 128

AD7713

RTD1

RTD

REF IN(+)

REF IN(–)

RTD2

200µA

+5V

RREF

INTERNAL

CIRCUITRY

DVDD

200µA

PGA

Figure 20. Four-Wire RTD Application with the AD7713

Three-Wire RTD Configurations

Figure 21 shows a three-wire RTD configuration using the

AD7713. In the three-wire configuration, the lead resistances

will result in errors if only one current source is used as the

200

µA will flow through R

L1 developing a voltage error between

AIN1(+) and AIN1(–). In the scheme outlined below, the sec-

ond RTD current source is used to compensate for the error in-

troduced by the 200

µA flowing through R

L1. 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 volt-

age 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 bene-

fits of eliminating RTD tempco errors as outlined in Figure 20.

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

RTD

12.5k

Ω

INTERNAL

CIRCUITRY

REF IN(+)

REF IN(–)

200µA

PGA

RL1

RL2

RL3

200µA

Figure 21. Three-Wire RTD Application with the AD7713

4–20 mA Loop

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

rent in 4–20 mA loop applications as shown in Figure 22. 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

× V

REF (= 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 A/D converter can be used for measuring the

current between 4 mA and 20 mA.

REF IN(+)

AIN1(+)

AIN1(–)

AIN3

AGND

DGND

A = 1 – 128

1µA

REF IN(–)

AD7713

4–20mA

LOOP

ANALOG +5V SUPPLY

500

Ω

VOLTAGE

ATTENUATION

AVDD

DVDD

INTERNAL

CIRCUITRY

PGA

Figure 22. 4–20 mA Measurement Using the AD7713

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