Guralp Systems CMG-5T - Manual (Page 13)

Operator's Guide
In this diagram a Güralp digitizer is being used to inject a calibration
signal into the sensor. This can be either a sine wave, a step function
or broad-band noise, depending on your requirements. As well as
going to the sensor, the calibration signal is returned to the digitizer on
a full rate channel (older digitizers used one of the 4 Hz auxiliary
(Mux) channels). The calibration signals and sensor output all travel
down the same cable from the sensor to an analogue input port on the
digitizer.
The signal injected into the sensor gives rise to an equivalent
acceleration (EA on the above diagram) which is added to the
measured acceleration to provide the sensor output. Because the
injection circuitry can be a source of noise, a Calibration enable line
from the digitizer is provided which disconnects the calibration circuit
when it is not required. Depending on the factory settings, the
Calibration enable line must be either allowed to float high (+5 to +10
V) or held low (0V, signal ground) during calibration: this is specified
on the sensor's calibration sheet.
The equivalent acceleration corresponding to 1 V of signal at the
calibration input is measured at the factory, and can be found on the
sensor calibration sheet. The calibration sheet for the digitizer
documents the number of counts corresponding to 1 V of signal at
each input port.
The sensor transmits the signal differentially, over two separate lines,
and the digitizer subtracts one from the other to improve the signal-to-
noise ratio by increasing common mode rejection. As a result of this,
the sensor output should be halved to give the true acceleration.
CMG-5T instruments are tuned at the factory to produce 1 V of output
for 1 V input on the calibration channel. For example, a sensor with
an acceleration response of 0.25 V/ms
-2 should produce 1 V output
given a 1 V calibration signal, corresponding to 1/0.25 = 4 ms -2
= 0.408 g of equivalent acceleration.
3.3 Calibrating accelerometers
Both the DM24 digitizer and Scream! software allow direct
configuration and control of any attached Güralp instruments. For full
information on how to use a DM24 series digitizer, please see its own
documentation. If you are using a third-party digitizer, you can still
calibrate the instrument as long as you activate the Calibration enable
line correctly and supply the correct voltages.
1. In Scream!'s main window, right-click on the digitizer's icon
and select Control.... Open the Calibration pane.
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