Semtech 2006
www.semtech.com
19-4
XE8805/05A
19.6
Application examples
19.6.1
Voltage controlled sensor bias
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 Vrefp, Vrefn and DAB_OUT.
The voltages Vrefp and Vrefn can be filtered before being connected to the D/A reference inputs. The reference
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 Vsensor on the sensor is only a fraction of the supply voltage. By generating Vrefp and Vrefn
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 Vrefn=3V and Vrefp=4V, the sensor voltage is adjustable between 3V and 4V with steps of about
1V/255
≈4mV.
VSS
VBAT or a Reference voltage source
VSS
reference input
of the
ZoomingADC
signal input
of the
ZoomingADC
Needs a 300
– 100 k load
Max capacitive load is 1 nF
Vsensor
High impedance node,
impedance depends on D/A code
Vrefp
Vrefn
D/A
XE8805A – DAB block
DAB_R_P
DAB_R_M
DAB_OUT
DAB_AIP
DAB_AOP
DAB_AOM
DAB_AIM
In this configuration the bridge current and DAB_AOM
increase when DAB_OUT increases.
amp
VBAT
Figure 19-2. Voltage controlled bridge bias principle.
Note that the voltage on the sensor can not be higher than VBAT-0.2V in the example of Figure 19-2 (specification
VRAOM in Table 19-4).
19.6.2
Current controlled sensor bias
Figure 19-3 shows the principle of a current controlled sensor bias schematic. In this case, the amplifier forces the
voltage VR to be equal to the D/A output voltage VD/A. The current Isensor through the sensor is given by:
Not
Recommended
for
New
Designs