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
V
DD
AIN2
AIN1
AIN3
AIN4
AD7709
REFIN(+)
MUX1
R
REF
A A
BUF
AND
PGA
200A
I1
EMF1
RESISTIVE
RANSDUCER
EMF2
P1
P2
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 R
REF
, 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
5V
6.25k
AIN2
AIN1
AD7709
RL3
R
CM
REFIN(+)
IOUT2
V
DD
DRDY
SCLK
DIN
DOUT
CS
XTAL1
XTAL2
RL2
RTD
200A
200A
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 mA
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. R
CM
is included so the current flowing through
the
combination of RL3 and R
CM
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 kW 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 I
OUT
pin
can go to within 0.6 V of the V
DD
supply.
The AD7709 also includes a 25 mA current source that can be used
along with the two 200 mA current sources for V
BE
measurement
where a 17:1 ratio is required from the current sources.
