AD7709 datasheet(28/33 Pages) AD

28 / 33 page

REV. A

AD7709

–27–

Figure 21 shows a further enhancement to the circuit shown in

Figure 20. Generally, dc excitation has been accepted as the

normal method of exciting resistive based sensors like RTDs in

temperature measurement applications.

IOUT1

IOUT2

VDD

AIN2

AIN1

AIN3

AIN4

AD7709

REFIN(+)

MUX1

RREF

BUF

AND

PGA

200 A

EMF1

RESISTIVE

TRANSDUCER

EMF2

REFIN(–)

Figure 21. Low Resistance Measurement

With dc excitation, the excitation current through the sensor

must be large enough so that the smallest temperature/resis-

tance change to be measured results in a voltage change that

is larger than the system noise, offset, and drift of the system.

The purpose of switching the excitation source is to eliminate

dc-induced errors. DC errors (EMF1 and EMF2) due to para-

sitic thermocouples produced by differential metal connections

(solder and copper track) within the circuit are also eliminated

when using this switching arrangement. This excitation is a

form of synchronous detection where the sensor is excited with

an alternating excitation source and the ADC measures infor-

mation only in the same phase as the excitation source.

The switched polarity current source is developed using the

on-chip current sources and external phase control switches (A

and

A) driven by AD7709 logic outputs P1 and P2. During the

conversion process, the AD7709 takes two conversion results,

one on each phase. During Phase 1, the on-chip current source is

directed to IOUT1 and flows top to bottom through the sensor

and switch controlled by A. In Phase 2, the current source is

directed to IOUT2 and flows in the opposite direction through

the sensor and through switch controlled by

A. In all cases, the

current flows in the same direction through the reference resistor

to develop the reference voltage for the ADC. All measurements

are ratiometrically derived. The results of both conversions are

combined within the microcontroller to produce one output

measurement representing the resistance or temperature of the

transducer. For example, if the RTD output during Phase 1 is

10 mV, a 1 mV circuit-induced dc error exists due to parasitic

thermocouples, the ADC measures 11 mV. During the second

phase, the excitation current is reversed and the ADC measures

–10 mV from the RTD and again sees 1 mV dc error, giving an

ADC output of –9 mV during this phase. These measurements

are processed in the controller (11 mV – (–9 mV)/2 = 10 mV),

thus removing the dc-induced errors within the system.

In the circuit shown in Figure 20, the resistance measurement is

made using ratiometric techniques. Resistor RREF, which develops

the ADC reference, must be stable over temperature to prevent

reference-induced errors in the measurement output.

3-Wire RTD Configurations

To fully optimize a 3-wire RTD configuration, two identically

matched current sources are required. The AD7709, which

contains two well matched 200

mA current sources, is ideally

suited to these applications. One possible 3-wire configuration

using the AD7709 is shown in Figure 22.

REFIN(–)

IOUT1

GND

6.25k

AIN2

AIN1

AD7709

RL3

RCM

REFIN(+)

IOUT2

VDD

DRDY

SCLK

DIN

DOUT

XTAL1

XTAL2

RL2

RTD

200 A

RL1

CONTROLLER

Figure 22. 3-Wire RTD Configuration Using the AD7709

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

errors if only one current source is used since the 200

mA 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

flowing through RL1. The second RTD current flows through

RL2. Assuming that RL1 and RL2 are equal (the leads would

normally be of the same material and of equal length) and that

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 AD7709 is within the common-

mode range of the ADC. The reference voltage for the AD7709

is also generated using one of these matched current sources.

This reference voltage is developed across the 6.25 k

W resistor

as shown, and applied to the differential reference inputs of the

AD7709. 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 20 ppm/

∞C. The voltage on either IOUT pin

can go to within 0.6 V of the VDD supply.

The AD7709 also includes a 25

mA current source that can be used

along with the two 200

mA current sources for VBE measurement

where a 17:1 ratio is required from the current sources.

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