GeoSIG Ltd.
GS_GSR_GCR_UserManual_V21.doc / 18.09.2009
Page 14
GSR / GCR / AS User Manual
•
AS-12 / 16 and GSR-12 / 16 /18 recorders:
standard:
0 ± 10 VDC (20 Vpp) or 0 ± 2.5 VDC (5 Vpp) differential
optional:
0 ± 5 VDC (10 Vpp) differential
0 ± 20 VDC (40 Vpp) differential
The Digital Values (DV) for Analog Input Voltages (AIV) are summarised in the following tables:
Table 3. DV and AIV for GCR Instruments
GCR Instruments
Digital Value DV
[Bit]
Analog Input Voltage AIV
[V]
GCR-12 GCR-16
Minimum input
0.0 0 0
Zero input
2.5 2’048
32’768
Maximum input
5.0 4’095
65’535
Table 4. DV and AIV for GSR and AS Instruments
GSR and AS Instruments
Digital Value DV
[Bit]
Analog Input Voltage AIV
[V]
GSR-12
AS-12
GSR-16
AS-16
GSR-18
Minimum input
–FS –2‘048
–32‘768
–131‘072
Zero input
0.0 0 0 0
Maximum input
+FS +2’047
+32’767
+131’071
FS is the full-scale input range of the instrument and takes the following values depending on the input
configuration:
Table 5. Full Scale Input Range of the Instrument
Differential input
FS [V]
0 ± 2.5 V (5 Vpp)
2.5
0 ± 5 V (10 Vpp)
5.0
0 ± 10 V (20 Vpp)
10
0 ± 20 V (40 Vpp)
20
In GCR Instruments each signal channel has a low-pass 100 Hz 6th-order Butterworth analog
filter. The sensor input signal passes through this filter prior to being sampled and converted to
digital form. This filter removes signal energy at frequencies above one-half the input sampling
rate (1000 Hz) so that these higher frequencies are not aliased by the sampling process,
appearing as erroneous digitised data. The signal level above 400 Hz at the output of the
analog filter is at least 72 dB below the input level.
5.1.1. Digital Signal Filtering
5.1.1.1. GSR / AS Instruments
The anti-aliasing low-pass filter is automatically applied by the sigma-delta A/D converter based on the
selected data output rate. For 200 SPS the filter cut-off frequency is 50 Hz and for 100 SPS it is 25 Hz.
The transfer function with the linear phase response (FIR) has a (sin(x) / x)
3
form that provides greater
than 100 dB rejection at the notch frequencies. The relationship between the programmed cut-off
frequency and the first notch is constant (f
NOTCH
= 3.81 x f
CUTOFF
). The first notch frequency is also the
output sample rate.
Knowing exactly what happens to the data during the recording process facilitates a compensation
calculation and helps actually to get a more precise understanding about the site behaviour. Since the
