Rev. 0.2 / 09 /2015
11
AN1111-2
For even better overall uncertainty, measure each PCB with a very well defined C_CAL during the
electrical test of the PCB. Correct the C_REF based on this measurement, or determine the exact
value of C_REF during the PCB in circuit test.
The deviations from reference with C_CAL = 82.79 pF are shown in Figure 9.
Figure 9: Initial accurate electronics – 1 point C_Cal; PCB corrected
4 Hints for the development engineer
Choose HCT01 version and the driving circuitry according Figure 1 depending on the accuracy and
output requirements as well as on existing electronics.
For accurate measurement it is of utmost importance to reduce as far as possible any stray
capacitance (and by this also its variation with temperature and humidity) related to the printed circuit
board. This might require several test and layout optimizing loops. These imply tests for assessing the
impact of the stay capacitance and its variations on the output signal of the device, as follows:
1. Test a sample of relevant size at defined environmental conditions (various combinations of
humidity and temperature) for determining the spread of the characteristic of the device including
HCT01 sensor.
2. Test a batch of printed circuit boards with a known, accurate capacitor instead of the humidity
sensor at defined environmental conditions (various combinations of humidity and temperature) for
determining the impact of the board on the output signal.
The electronics design shall be optimized for narrow spread of the characteristic of the entire device
and for minimum impact of the electronics layout on the output signal.
-5,0
-4,0
-3,0
-2,0
-1,0
0,0
1,0
2,0
3,0
4,0
5,0
0
20
40
60
80
100
de
vi
at
io
n
re
la
tiv
e
hu
mi
di
ty
[% rh
]
relative humidity [% rH]
Initial accurate electronics, 55 units @3V3
1 point C_CAL ; PCB corrected ; w/o hysteresis; nominal
characeristics
