AD7747
Rev. 0 | Page 24 of 28
CAPACITIVE SYSTEM OFFSET CALIBRATION
The capacitive offset is dominated by the parasitic offset in the
application, such as the initial capacitance of the sensor, any
parasitic capacitance of tracks on the board, and the capacitance
of any other connections between the sensor and the CDC.
Therefore, the AD7747 is not factory calibrated for capacitive
offset. It is the user’s responsibility to calibrate the system
capacitance offset in the application.
Any offset in the capacitance input larger than ±1 pF should
first be removed using the on-chip CAPDACs. The small offset
within ±1 pF can then be removed by using the capacitance
offset calibration register.
One method of adjusting the offset is to connect a zero-scale
capacitance to the input and execute the capacitance offset
calibration mode. The calibration sets the midpoint of the
±8.192 pF range (that is, Output Code 0x800000) to that
zero-scale input.
Another method is to calculate and write the offset calibration
register value; the LSB value is 31.25 aF (8.192 pF/2
17
).
The offset calibration register is reloaded by the default value at
power-on or after reset. Therefore, if the offset calibration is not
repeated after each system power-up, the calibration coefficient
value should be stored by the host controller and reloaded as
part of the AD7747 setup.
INTERNAL TEMPERATURE SENSOR
DIGITAL
FILTER
AND
SCALING
24-BIT Σ-Δ
MODULATOR
CLOCK
GENERATOR
INTERNAL TEMPERATURE SENSOR
05469-026
VOLTAGE
REFERENCE
DATA
IN × I
ΔV
BE
VDD
Figure 36. Internal Temperature Sensor
The temperature sensing method used in the AD7747 is to
measure a difference in ∆V
BE
voltage of a transistor operated at
two different currents (see
Figure 36). The ∆V
BE
change with
temperature is linear and can be expressed as
)ln()( N
q
KT
nV
f
BE
×=Δ
where:
K is Boltzmann’s constant (1.38 × 10
−23
).
T is the absolute temperature in Kelvin.
q is the charge on the electron (1.6 × 10
−19
coulombs).
N is the ratio of the two currents.
n
f
is the ideality factor of the thermal diode.
The AD7747 uses an on-chip transistor to measure the
temperature of the silicon chip inside the package. The Σ-
ADC converts the ∆V
BE
to digital; the data are scaled using
factory calibration coefficients. Thus, the output code is
proportional to temperature.
()
4096
2048
−=°
Code
CeTemperatur
The AD7747 has a low power consumption resulting in only a
small effect due to the part self-heating (less than 0.5°C at
V
DD
= 5 V).
If the capacitive sensor can be considered to be at the same
temperature as the AD7747 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 AD7747 internal temperature sensor without need
for any additional external components. See Figure 17 in the
Typical Performance Characteristics section.
EXTERNAL TEMPERATURE SENSOR
DIGITAL
FILTER
AND
SCALING
24-BIT Σ-Δ
MODULATOR
CLOCK
GENERATOR
EXTERNAL
TEMPERATURE
SENSOR
05469-027
VOLTAGE
REFERENCE
DATA
I ...
N
× I
ΔV
BE
VDD
VIN(–)
VIN(+)
R
S2
R
S1
2N3906
Figure 37. Transistor as an External Temperature Sensor
The AD7747 provides 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 37) up to 100 Ω is compensated. The VIN(−) pin must
be grounded for proper external temperature sensor operation.
The AD7747 is factory calibrated for Transistor 2N3906 with
the ideality factor n
f
= 1.008.
See
Figure 18 in the Typical Performance Characteristics section.
