Datasheet

AD7745/AD7746
Rev. 0 | Page 24 of 28
If the capacitive sensor can be considered to be at the same
temperature as the AD7745/AD7746 chip, the internal
temperature sensor can be used as a system temperature sensor.
That means the complete system temperature drift
compensation can be based on the AD7745/AD7746 internal
temperature sensor without need for any additional external
components. See the typical performance characteristics in
Figure 18.
EXTERNAL TEMPERATURE SENSOR
DIGITAL
FILTER
AND
SCALING
24-BIT Σ-∆
MODULATOR
CLOCK
GENERATOR
EXTERNAL
TEMPERATURE
SENSOR
05468-041
VOLTAGE
REFERENCE
DATA
I
.
.
.
N
×
I
∆V
BE
V
DD
VIN (–)
VIN (+)
R
S2
R
S1
2N3906
Figure 41. Transistor as an External Temperature Sensor
The AD7745/AD7746 provide the option of using an external
transistor as a temperature sensor in the system. The ∆V
BE
method, which is similar to the internal temperature sensor
method, is used. However, it is modified to compensate for the
serial resistance of connections to the sensor. Total serial
resistance (R
S1
+ R
S2
in Figure 41) up to 100 Ω is compensated.
The VIN(–) pin must be grounded for proper external
temperature sensor operation.
The AD7745/AD7746 are factory calibrated for
Transistor 2N3906 with the ideality factor n
f
= 1.008.
See the typical performance characteristics shown
in Figure 19.
VOLTAGE INPUT
DIGITAL
FILTER
24-BIT Σ-∆
MODULATOR
CLOCK
GENERATOR
ANALOG TO DIGITAL CONVERTER
(ADC)
05468-042
VOLTAGE
REFERENCE
DATA
VIN(–)
VIN(+)
REFIN(–)
REFIN(+)
R
REF
RTD
R
T
VDD
GND
Figure 42. Resistive Temperature Sensor Connected to the Voltage Input
The AD7745/AD7746 Σ-Δ core can work as a high resolution
(up to 21 ENOB) classic ADC with a fully differential voltage
input. The ADC can be used either with the on-chip high
precision, low drift, 1.17 V voltage reference, or an external
reference connected to the fully differential reference input pins.
The voltage and reference inputs are continuously sampled by a
Σ-Δ modulator during the conversion. Therefore, the input
source impedance should be kept low. See the application
example in Figure 42.
V
DD
MONITOR
Along with converting external voltages, the AD7745/AD7746
Σ-Δ ADC can be used for monitoring the V
DD
voltage. The
voltage from the VDD pin is internally attenuated by 6.
TYPICAL APPLICATION DIAGRAM
DIGITAL
FILTER
24-BIT Σ-∆
MODULATOR
CLOCK
GENERATOR
MUX
TEMP
SENSOR
CAP DAC
CAP DAC
EXCITATION
VOLTAGE
REFERENCE
CONTROL LOGIC
CALIBRATION
AD7745
VDD
REFIN(+) REFIN(–)
GND
SDA
SCL
RDY
VIN(+)
VIN(–)
CIN1(+)
CIN1(–)
EXCA
EXCB
05468-008
HOST
SYSTEM
+
0.1µF
10µF
3V/5V
POWER SUPPL
Y
I
2
C
SERIAL
INTERFACE
10kΩ
10kΩ
Figure 43. Basic Application Diagram for a Differential Capacitive Sensor