XE8805 SEMTECH [Semtech Corporation], XE8805 Datasheet - Page 138

• Manufacturer Part Number: XE8805
• Description: Data Acquisition MCU
• Manufacturer: SEMTECH [Semtech Corporation]

19.6 Application examples
19.6.1
Figure 19-2 shows the basic connectivity to have a voltage controlled sensor bias. The D/A will generate a voltage
between vrep and vrefn proportional to the input code. The amplifier will copy the D/A voltage to the sensor. The
D/A code can be used to do a software temperature calibration of the sensor for instance.
Filter capacitors can be added in parallel with the sensor reference and signal, on V
The voltages V
voltages can be connected directly to VBAT and VSS for simplicity. They can also be connected to VBAT and VSS
through a low pass filter that rejects the high frequency supply noise. Finally, in most cases, the voltage range of
interest for the voltage V
equal to the limits of the voltage range of interest, the resolution of the D/A can be increased. Example: if a supply
of 5V is used and the reference voltage is equal to the supply, the D/A can generate a sensor voltage between 0V
and 5V in steps of about 5V/255≈20mV. If the sensor voltage is always to be between 3V and 4V, and by
connecting V
1V/255≈4mV.
Note that the voltage on the sensor can not be higher than VBAT-0.2V in the example of Figure 19-2 (specification
VR
19.6.2
Figure 19-3 shows the principle of a current controlled sensor bias schematic. In this case, the amplifier forces the
voltage V
© Semtech 2006
AOM
in Table 19-4).
XE8805A – DAB block
R
Voltage controlled sensor bias
Current controlled sensor bias
to be equal to the D/A output voltage V
In this configuration the bridge current and DAB_AOM
refn
refp
=3V and V
and V
D/A
sensor
increase when DAB_OUT increases.
refn
refp
can be filtered before being connected to the D/A reference inputs. The reference
=4V, the sensor voltage is adjustable between 3V and 4V with steps of about
on the sensor is only a fraction of the supply voltage. By generating V
Figure 19-2. Voltage controlled bridge bias principle.
amp
DAB_AOM
DAB_R_M
DAB_OUT
DAB_AOP
D/A
DAB_R_P
DAB_AIM
DAB_AIP
. The current I
19-4
V
V
refp
refn
VBAT
sensor
VBAT or a Reference voltage source
VSS
V
sensor
through the sensor is given by:
VSS
impedance depends on D/A code
High impedance node,
Needs a 300
Max capacitive load is 1 nF
refp
, V
refn
reference input
of the
ZoomingADC
Ω
– 100 k
and DAB_OUT.
signal input
of the
ZoomingADC
XE8805/05A
Ω
load
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refp
and V
refn