```Technical note 18
Improved Output Filtering
Objective
To give recommendations on how to decrease the analogue output signal noise, prior to an A/D
conversion in a final application. This document is relevant for VTI’s SCA61T, SCA100T,
SCA103T, SCA1000, and SCA1020 series inclination and acceleration sensors.
Description
In the above products there is an internal A/D converter running constantly, causing fast
interference voltage spikes on analogue output(s). These glitches occur with an interval of 50 70µs, and last about 1 µs. They don't usually interfere with the normal analogue output signal.
However, when the output is sampled very rapidly for A/D conversion, there is a risk that the
voltage drops can be seen at the output, causing errors in the measurement information.
Figure 1. Oscilloscope picture of non-filtered output(s)
Figure 2. Oscilloscope picture of a glitch on the non-filtered
output.
Solution
The interference effect of A/D conversion can be easily filtered out with a simple RC Low Pass filter
(LPF) from the analogue output(s). The –3 dB frequency point of the filter is given by the equation:
f −3dB =
1
2 * π * R* C
where R and C are the resistance and capacitance values of the resistor and the capacitor that
create the filter (see Figure 3).
When deciding what values to use for R and C, it is important to remember that the maximum
allowed load capacitance for the mentioned products is 20 nF. Also when using high values of R,
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Improved Output Filtering
TN18
remember that you are creating a voltage divider with the measurement equipment input
impedance.
A/D
converter
Figure 3. SCA100T with RC low pass filtering R1,
C1, R2 and C2. The filter is included in the example
connection in the Data-Sheet of SCA103T.
Figure 4. Oscilloscope picture of the filtered output
during the glitch. R = 5 kΩ and C = 10 nF, f-3dB = 3.18
kHz
Results
A typical result of the filtered analogue output signal can be seen in Figure 4. Corner frequency is
set approximately to 3kHz, with C = 10nF and R = 5kΩ. Variation of the output signal has reduced
from 200mVp-p to less than 10mVp-p.
Document Change Control
Version
Date
Change Description
1.0
05.09.2005
First release
VTI Technologies reserves all rights to modify this document without prior notice.
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```