Datasheet

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last update
November 30, 2010
reference
smtutin
page
11/27
B
C
D
E
F
A
C
BA
C
CA
C
DA
C
EA
C
FA
UTI
Figure 6. Connection of capacitors to the UTI
6 CAPACITIVE MODES (MODE 0 – 4)
6.1 Mode 0. C25: 5 capacitors 0-2pF
In this mode, 5 capacitors in the range of 0 - 2 pF with one common electrode can be measured. In order to understand what
this means in practical terms, please refer to the note in paragraph 5.5. The connection of capacitors is depicted in Fig 6.
All measured capacitors should have a common receiver electrode, connected to node A. The signal at the transmitting
electrodes (B to F) is a square wave with amplitude V
DD
. When a capacitor is not measured, the node corresponding to this
capacitor is internally grounded. In mode C25, one cycle takes 5 measurement phases as depicted in Table 5
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In phase 1 the input capacitor C
BA
+ C
0
is measured. In this phase the output frequency is doubled, resulting in two short
periods. This enables synchronization of the microcontroller. In straight forward applications, no capacitor is connected
between B and A, as explained in the note in paragraph 5.5 the specifications for the mode C25 (mode 0) are listed in Table 6.
The remaining offset capacitance is caused by the parasitics between bonding wires, bonding pads and IC pins. When this
offset is too large, one should use the mode CMUX. In this case, an external multiplexer is used and offset can be as low as
20×10
-6
pF.
Phase
Measured
capacitor
Output periods
1 C
BA
+C
off
T
BA
=NK
1
(C
BA
+C
off
)
2 C
CA
+C
off
T
CA
=NK
1
(C
CA
+C
off
)
3 C
DA
+C
off
T
DA
=NK
1
(C
DA
+C
off
)
4 C
EA
+C
off
T
EA
=NK
1
(C
EA
+C
off
)
5 C
FA
+C
off
T
FA
=NK
1
(C
FA
+C
off
)
Table 5. Measured capacitors during each phase
Parameter Typical value
K
1
10 s/pF
C
0ff
2 pF
Max cap value (C
xa
) 2 pF
Linearity 13 bits
Resolution (SF=0) 14 bits
Remaining offset < 15 x10
-3
pF
Table 6. specifications of the C25 and C23 mode