Datasheet

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last update
November 30, 2010
reference
smtutin
page
14/27
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6.5 Mode 4. C300: 3 capacitors, range up to 300pF
In this mode, 3 capacitors in a variable range up to 300 pF with a 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 sensors and
external resistors is depicted in Figure 8 . These resistors set the voltage swing
at the transmitting electrode of C
iA
.
Figure 8. Connection of sensors to
the UTI for the C300 mode
The total capacitance at node A must be limited to 500 pF in order to keep the
nonlinearity below 10
-3
. The voltage swing at the transmitting electrodes equals
V
EF
which is set externally by means of three inaccurate resistors R
1
, R
2
and R
3
, of
which R
1
or R
3
may be zero. The DC voltage V
EF
should satisfy the following
condition: V
EF
< K
V
/ C
max
, where the constant K
V
= 60 V·pF, and C
max
is the
maximum value of C
BA
, C
CA
and C
DA
expressed in pF. The total time constant of all
resistors and capacitors should be less than 500 ns. This sets the values of the
resistors.
Example. When C
CA
= 300 pF, C
DA
= 200 pF, C
BA
= 0 and V
DD
= 5 V , practical values of the resistors are R
1
= 25 kΩ, R
2
= 1 kΩ
and R
3
= 0. The voltage swing V
EF
at the transmitting electrode amounts to 0.2 V.
The system contains two time constants C
tot
·( R
3
//( R
1
+ R
2
)) and C
tot
·( R
1
//( R
2
+ R
3
)), where C
tot
= C
BA
+ C
CA
+ C
DA
+ C
p
. Both time
constants must be smaller than 500 ns. The nonlinearity and resolution in the slow mode are depicted in Table 10. Here, the
value of C
DA
= 0 pF, C
p
= 30 pF and V
EF
has the maximum value K
V
/ C
max
, as described before. The measured capacitors during
each phase are listed in Table 11 .
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Capacitors Nonlinearity Resolution (pf)
C
BA
=C
CA
= 33 pf 1.4 x 10
-4
1.2x10
-3
C
BA
=C
CA
= 150 pf 1.9 x 10
-4
6.6x10
-3
C
BA
=C
CA
= 270 pf 9.0 x 10
-4
17x10
-3
C
BA
=C
CA
= 330 pf 2.6 x 10
-3
20x10
-3
C
BA
=C
CA
= 560 pf 6.3 x 10
-3
46x10
-3
Table 10. Nonlinearity and resolution in C300 mode for
various capacitor values
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
)
Table11. Measured capacitors during each phase of
mode C300
E
F
B
C
D
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Vdd
CBA
CCA
CDA
R
1
R
2
R
3