Datasheet
last update
November 30, 2010
reference
smtutin
page
10/27
In
figure 5
the relation between accuracy and parasitic cable capacitance Cp is depicted. From the graph it
can be seen that the linearity is still 1.0E-3 in case of a cable capacitance of about 500 pF.
When multiple capacitances are measured, each node (fi A, B, C,D,E,F) is activated for a certain time and
after that the UTI switches on to the next node. The nodes that are not active are connected to ground and
become parasitic and have therefore no influence on the measurement. Node selection is automatically
controlled by the UTI. In the CMUX mode the number of capacitors is unlimited. All the capacitors are
connected to a different voltage source and are all wired together to the receiver input of the charge amplifier
(point A). Further CMUX applications are given in the CMUX specifications. For more information about
measuring capacitors with the UTI please look into relevant application notes in the support shop of our
website.
Note:
In most capacitive applications the input which is measured during the first phase (offset phase) is left
unconnected. Under those circumstances only the internal capacitance of the device is measured
(capacitance of the chip + capacitance of the bonding wires and leads). During the second phase the
reference capacitor is measured. This is the most straight forward way to apply the three variable methods
and to eliminate the offset and gain from the equations. Using this method, in 5-capacitor mode (mode 0 or
mode 2), three unknown capacitors can be measured, while in 3-capacitor mode (mode 1 or mode 4)
one unknown capacitor can be measured,
However, under certain circumstances, it may be desirable to connect an external offset component (C
0
).
When the value of this offset component is known with the same accuracy as the value of the reference
component, the three signal method can still be applied.