AD7713 Analog Devices, AD7713 Datasheet - Page 24

• Manufacturer Part Number: AD7713
• Description: CMOS, Low Power 24-Bit Sigma-Delta, Signal Conditioning ADC with Matched RTD Current Sources
• Manufacturer: Analog Devices

Specifications of AD7713

• Resolution (bits): 24bit
• # Chan: 3
• Sample Rate: 3.9kSPS
• Interface: Ser
• Analog Input Type: Diff-Bip,Diff-Uni,SE-Uni
• Ain Range: (2Vref/PGA Gain) p-p,Uni (Vref) x 4,Uni (Vref)/(PGA Gain)
• Adc Architecture: Sigma-Delta
• Pkg Type: DIP,SOIC

AD7713
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 R
the nominal reference voltage of 2.5 V, R
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.
Figure 19. 4-Wire RTD Application with the AD7713
68HC11
REF
RTD
Figure 18. AD7713 to 68HC11 Interface
R
and applied to the differential reference inputs. For
REF
MISO
MOSI
SCK
PC0
PC1
PC2
PC3
SS
REF IN(+)
REF IN(–)
DV
AIN1(+)
AIN1(–)
AGND
DD
RTD2
RTD1
5V
DV
200 A
200 A
DD
AV
CIRCUITRY
INTERNAL
PGA
DD
A = 1 – 128
REF
SYNC
RFS
TFS
DRDY
A0
SCLK
SDATA
MODE
DV
AD7713
DGND
is 12.5 kΩ. This
DD
AD7713
–24–
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
AV
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
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.
4–20mA
RTD
LOOP
DD
Figure 21. 4-20 mA Measurement Using the AD7713
Figure 20. 3-Wire RTD Application with the AD7713
ANALOG 5V SUPPLY
supply. The circuit is shown for a 2.5 V reference.
AIN1(+)
AIN1(–)
R
R
R
L1
L2
L3
500
AIN3
V
REF
12.5k
AIN(–)
AGND
(= 8 V with a V
AIN(+)
RTD2
RTD1
AV
ATTENUATION
DD
VOLTAGE
AV
1 A
DD
AV
DD
AGND
DV
200 A
200 A
PGA
DV
DD
DD
A = 1 – 128
REF
REF IN(–)
DGND
of 2.5 V) even though the
REF IN(–)
DGND
PGA
A = 1 – 128
REF IN(+)
AD7713
REF IN(+)
CIRCUITRY
CIRCUITRY
INTERNAL
INTERNAL
AD7713
REV. D