Before acquiring data using the DAQ board
the following points must be considered
•The
nature of the signal source(grounded or floating)
•The
grounding configuration of the amplifier on the DAQ board
Finding
a Common Ground
Earth
ground - potential of the earth below your feet.
Most electrical outlets have a prong that connects to the earth ground which is
usually wired into the building electrical system for safety. Many instruments
are also” grounded” to this earth ground System ground. This
type of grounding is for safety.
Ground Symbol
Referenced Signal
Reference
ground -return path or signal common.This
is usually the reference potential. The
common ground may or may not be wired to the earth ground.
Many
instruments ,devices and signal sources provide a reference (the negative terminal,common
terminal etc that gives meaning to the voltages
that we are measuring.
Signal
Source Reference Configuration
Signal sources come in two forms-(1) Referenced sources called grounded signals, (2) Non referenced sources called floating signals
Grounded signal sources have voltage signals referenced to a system ground, such as earth or a building ground.
Devices that plug into the building ground through wall outlets such as signal generators and power supplies are examples of Grounded signal sources.
Floating signal sources contain a signal that is not connected to an absolute reference such as earth or a building ground. Batteries, Thermocouple, transformers are examples of floating signal sources.
Differential Connections (DIFF configuration):
Differential connections
are those in which each analog input
signal has its own reference signal or signal return path. Each input signal is
tied to the positive input of the instrumentation amplifier, and its reference
signal, or return, is tied to the negative input of the instrumentation
amplifier.
When configuring
the DAQ for DIFF input, each signal uses two of the multiplexer inputs-one for
the signal and one for its reference signal.
Therefore, only
half the analog input channels are available when using the DIFF configuration.
Conditions for using DIFF configuration:
Use the DIFF input configuration when your DAQ system has any of the following conditions:
- Input signal levels are low (less than 1V)
- Leads connecting the signals to DAQ system are at long distance.
- Any of the input signals require a separate ground-reference point or signal-return.
- The signal travels through a noisy environment.
A differential
instrument requires two inputs where neither input to the instrumentation
amplifier is referenced to a system ground. For example, CH0+
and CH0- are wired into the positive and negative terminals of the
instrumentation amplifier respectively, but they are not connected to the
measurement system ground (AI GND). The
differential voltage across the circuit pair is the desired signal, yet an
unwanted signal that is common to both sides of a differential circuit pair can
exist. This voltage
is known as common-mode voltage. An ideal differential measurement system
completely rejects, instead of measures, the common-mode voltage for more
accurate measurements. Practical devices, however, have limitations described
by specifications such as common-mode voltage range and common-mode rejection
ratio (CMRR).
Differential input connection for Grounded sources
Differential input connection for floating sources
RSE and NRSE connections
RSE
measurement system is used to measure a floating signal, because it grounds the
signal with respect to building ground.
NRSE
measurement system, all measurements are made with respect to a common
reference, because all of the input signals are already grounded.
Use the RSE configuration for floating signal sources;
in this case, the DAQ boards provide the reference ground point for the
external signal.
Use the NRSE configuration for
ground-referenced signal sources; in this case, the external signal
supplies its own reference ground point and the DAQ boards should not supply
one.
