AD7714 Analog Devices, AD7714 Datasheet - Page 36

• Manufacturer Part Number: AD7714
• Manufacturer: Analog Devices

Specifications of AD7714

• Resolution (bits): 24bit
• # Chan: 5
• Sample Rate: 19.2kSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: (2Vref/PGA Gain) p-p,Uni (Vref)/(PGA Gain)
• Adc Architecture: Sigma-Delta
• Pkg Type: DIP,SOIC,SOP

AD7714
RTD Measurement
Figure 14 shows another temperature measurement application
for the AD7714. In this case, the transducer is an RTD (Resis-
tive Temperature Device), a PT100. The arrangement is a 4-
lead RTD configuration. There are voltage drops across the lead
resistances R
mode voltage. There is no voltage drop across lead resistances
R
lead resistances present a small source impedance so it would
not generally be necessary to turn on the buffer on the AD7714.
L2
and R
L3
as the input current to the AD7714 is very low
L1
and R
RTD
L4
but these simply shift the common-
6.25k
R
R
R
L1
L3
L4
R
L2
REF IN (+)
REF IN (–)
400 A
DGND
AGND
AIN2
AIN1
Figure 14. RTD Measurement Using the AD7714
AGND
AV
+5V
AV
DD
DD
1 A
1 A
BUFFER
BUFFER DOUT
.
DV
SERIAL INTERFACE
The
DD
A = 1–128
PGA
–36–
If the buffer is required, the common-mode voltage should be
set accordingly by inserting a small resistance between the bot-
tom end of the RTD and AGND of the AD7714. In the appli-
cation shown an external 400 A current source provides the
excitation current for the PT100 and it also generates the refer-
ence voltage for the AD7714 via the 6.25 k resistor. Variations
in the excitation current do not affect the circuit as both the
input voltage and the reference voltage vary ratiometrically with
the excitation current. However, the 6.25 k resistor must have
a low temperature coefficient to avoid errors in the reference
voltage over temperature.
REGISTER BANK
AUTO-ZEROED
CHARGE BALANCING A/D
DIN
MODULATOR
CONVERTER
CS
GENERATION
AD7714
CLOCK
SCLK
DIGITAL
FILTER
MCLK IN
MCLK OUT
STANDBY
SYNC
RESET
DRDY
POL
REV. C