Datasheet

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last update
November 30, 2010
reference
smtutin
page
21/27
To find the bridge imbalance, the microcontroller calculates:
M
T T
T T
V
V
phase phase
phase phase
CD
AB
=
−
−
=
1
32
3 1
2 1
(4)
The specifications are listed in Table 17
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8.2 Mode 10. Ub1: res. bridge, ref. is V
bridge
, +/- 0.25% imbalance
In this mode, a resistive bridge can be measured where the ratio of the bridge output voltage and the
bridge supply voltage represents the physical signal. The main difference with mode Ub2 is that the
measurement range of the bridge imbalance is 0.25%. (V
CD
= 12.5 mV for V
DD
= 5V).
The connection of the bridge to the UTI is the same as in the Ub2 mode. An on-chip 15-times voltage
amplifier amplifies the small output voltage before it is processed in the same way as the divided voltage
across the bridge. Both the amplifier and divider do not need calibration. To calculate the bridge
imbalance, Equation (7) can be used, where 32 must be replaced by 480. Due to the use of the
force/sense wires, the bridge is measured in a 4-wire setup. The various voltages measured during each
phase are indicated in Table 18 . The specifications are listed in Table 19
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Parameter (V
DD
= 5 V) Typical value
K
2
56 s/V
V
0
0.54 V
Bridge excitation AC Vdd
excitation current (max E - F) 20 mA
Bridge resistance 250-10k
Bridge output Voltage <+/- 0,2V
Accuracy 11 bits (over =/- 0.2V)
Offset
<10 V
Resolution (SF=0)
< 7 V
Table 17. Specifications of the Ub2 mode
Phase Measured voltages Output periods
1 V
offset
Toffset=N*K
2
*Voffset
2 V
ab
/32+V
offset
Tab= N*K
2
*(Vab/32+Voffset)
3 15*Vcd+V
offset
Tcd=N*K
2
*(15*Vcd+Voffset)
Table 18. Measured voltages of Ub1 mode
Phase Measured voltages Output periods
1 V
offset
Toffset=N*K
2
*Voffset
2 V
ab
+V
offset
Tab= N*K
2
*(Vab+Voffset)
3 Vcd+V
offset
Tcd=N*K
2
*(Vcd+Voffset)
Table 20. Measured voltages of Ib2 mode