Omega Engineering OMB-DBK-34A, User Manual

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Differential
Amplifier Differential
Amplifier
Shield
Shield
Grounding Wire
Grounding Wire
Potential difference and no path
for current flow.
Ground loop caused by
current flow.
No Noise-Inducing Ground Loop Noise-Inducing Ground Loop
Low Input Signal
Low Input Signal
High Input Signal
High Input Signal
WRONG
CORRECT
Aside from eliminating noise-inducing ground loops, the use of bias resistors should also be considered
with isolated signal sources. Bias resistors can be used to provide bias current for the positive and negative
(high and low) input signals to the differential amplifier. The impedance value of the bias resistors
depends on the output impedance of the signal source.
Differential
Amplifier
Shield
Grounding Wire
Locate bias resistors (R and R ) as close
as possible to the differential amplifier.
1 2
Low Input Signal High Input Signal
R
1
R
2
A basic rule of thumb is:
The value of the bias resistor should be at least 10 times the output impedance of
the signal source, but less than 1 M
Ω
.. Bias resistors should be located as close as possible to the
differential amplifier. Ground only one end of the signal shield.
Unipolar and Bipolar Measurement
Unipolar signals are always zero or positive. Bipolar signals can be negative or positive and typically
range from -5 to +5 V (-10 to +10 V for the /2000 Series Devices). Using one or the other depends on the
signal from the transducer and its signal conditioning. If the DBK (or other signal conditioner) outputs a
bipolar signal, then the LogBook or Daq device should be set to bipolar. If the LogBook or Daq device
sequencer is using the wrong mode for a channel, that channel’s reading may be clipped or in error.
Reading a bipolar signal in unipolar mode misses half the signal, and the half received is not converted
with optimal resolution.
Note : The different DBKs can use either or both signal modes. Refer to the DBK documentation, and
verify that the DBK and the LogBook or Daq device are set to the proper mode for each channel.
12-Bit vs 16-Bit Resolution
An analog-to-digital converter (ADC) converts an analog voltage to a digital number. The digital number
represents the input voltage in discrete steps with finite resolution. The number of bits that represent the
digital number determines ADC resolution. An
n -bit ADC has a resolution of 1 part in 2
n
.
•
12-bit resolution is 1 part in 4096 (in binary powers, 2
12
) and corresponds to 2.44 mV for a 10 V
range.
•
16-bit resolution is 1 part in 65,536 (in binary powers, 2
16
) and corresponds to 0.153 mV in a 10 V
range.
1-4 Signal Management
886995
DBK Option Cards and Modules