AD7715 Analog Devices, AD7715 Datasheet - Page 37

• Manufacturer Part Number: AD7715
• Manufacturer: Analog Devices

Specifications of AD7715

• Resolution (bits): 16bit
• # Chan: 1
• Sample Rate: 19.2kSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: (Vref/PGA Gain) p-p,Uni (Vref)/(PGA Gain)
• Adc Architecture: Sigma-Delta
• Pkg Type: DIP,SOIC,SOP

TEMPERATURE MEASUREMENT
Another application area for the AD7715 is in temperature
measurement. Figure 15 outlines a connection from a thermo-
couple to the AD7715. In this application, the AD7715 is operated
in its buffered mode to allow large decoupling capacitors on the
front end to eliminate any noise pickup that there may have
been in the thermocouple leads. When the AD7715 is operated
in buffered mode, it has a reduced common-mode range. To place
the differential voltage from the thermocouple on a suitable
common-mode voltage, the AIN(−) input of the AD7715 is
biased up at the reference voltage, 2.5 V.
Figure 16 shows another temperature measurement application
for the AD7715. In this case, the transducer is an resistive tem-
perature device (RTD), a PT100. The arrangement is a 4-lead
RTD configuration. There are voltage drops across the lead
THERMOCOUPLE
JUNCTION
GND
+V
RTD
REF192
IN
+5V
V
OUT
R
R
R
R
L3
L4
L1
L2
6.25kΩ
400µA
R
R
REF IN(+)
REF IN(–)
DGND
AGND
C
AIN(+)
AIN(–)
REF IN(+)
REF IN(–)
Figure 15. Thermocouple Measurement Using the AD7715
AIN(+)
AIN(–)
C
AGND
Figure 16. RTD Measurement Using the AD7715
+5V
BUFFER
AV
DD
5V
BUFFER
AV
DGND
DD
A = 1 TO 128
Rev. D | Page 37 of 40
PGA
DV
A = 1 TO 128
SERIAL INTERFACE
DD
PGA
DV
DOUT
SERIAL INTERFACE
DD
DOUT
CHARGE BALANCING ADC
AUTO-ZEROED
MODULATOR
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 AD7715.
If the buffer is required, the common-mode voltage should be
set accordingly by inserting a small resistance between the bottom
end of the RTD and AGND of the AD7715. In the application
shown in Figure 16, an external 400 μA current source provides
the excitation current for the PT100 and it also generates the
reference voltage for the AD7715 via the 6.25 kΩ resistor. Varia-
tions 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 kΩ resistor must have
a low temperature coefficient to avoid errors in the reference
voltage over temperature.
REGISTER BANK
L2
CHARGE BALANCING ADC
AUTO-ZEROED
DIN
MODULATOR
Σ-∆
and R
REGISTER BANK
AD7715
DIN
Σ-∆
AD7715
L3
CS
as the input current to the AD7715 is very low. The
GENERATION
DIGITAL
L1
FILTER
CS
CLOCK
and R
GENERATION
DIGITAL
FILTER
SCLK
CLOCK
L4
SCLK
, but these simply shift the common-
MCLK IN
MCLK OUT
RESET
DRDY
MCLK IN
MCLK OUT
RESET
DRDY
AD7715