Semtech 2006
www.semtech.com
19-5
XE8805/05A
sense
refn
refp
sense
A
D
refp
sense
R
refp
sensor
R
code
V
R
V
R
V
I
)
255
1
(
)
(
/
=
=
=
The voltage Vsensor can be calculated as a function of the current Isensor and the sensor impedance.
Note that the voltage VR>VSS+2.3V and VR-Vsensor>0.2V (VRAOP and VRAOM specifications in Table 19-4) to
guarantee correct functionality of this schematic.
Choosing the Vrefp equal to the supply voltage or close to the supply voltage in order to have the highest possible
voltage on the sensor is recommended. From the equation, it can be seen that the sensor current step per LSB can
be made smaller by reducing the voltage between Vrefp and Vrefn or by increasing the sense resistor value.
As for the voltage controlled sensor bias, capacitors can be added on several nodes to filter out the noise.
Voltage must remain above VSS + 2.3 V
Needs a 300
– 100 k load
Max capacitive load is 1 nF
VSS
Voltage must remain below VBAT - 2.3 V
Voltage must remain above VSS + 2.3 V
In this configuration, the bridge current and DAB_AOM
decrease when DAB_OUT increases.
XE8805A
DAB_R_P
DAB_R_M
DAB_OUT
DAB_AIM
DAB_AOP
DAB_AOM
DAB_AIP
D
A
amp
signal
reference
Vrefp
Vrefn
Rsense
Vsensor
VD/A
VR
VSS
Figure 19-3. Current controlled bridge bias
In Figure 19-4, the sense resistor is inserted between the negative reference voltage and the sensor. This
schematic has the same principle as above, but it is easier to respect the limits on VRAOP when VBAT is low. The
sensor current is now:
sense
refn
refp
sense
refn
A
D
sense
refn
R
sensor
R
code
V
R
V
R
V
I
)
255
(
)
(
/
=
=
=
In this case, it is recommended to choose Vrefn equal to VSS or close to VSS in order to have the highest possible
voltage on the sensor. The only limit is now Vsensor<VBAT-0.2V.
Not
Recommended
for
New
Designs