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Signal Conditioning Circuits
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Bridge Circuits
– A simple common Wheatstone Bridge consists
of four two-terminal elements connected to
form a quadrilateral, a source of excitation
(voltage or current) connected along one of the
diagonals. A detector of voltage or current
comprising the other diagonal. The device was
actually first built by S. H. Christie in 1833.
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Cont.
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Bridge Circuit
Ref. DH Shingold, Transducer Interfacing Handbook
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Small Change x
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From the previous circuit, when
R = R2 = R3 = R4, while R1 has a deviation x
from the value of R, then
Vbridge = Vin · x/4 for x << 1
Null-type measurements are principally used in the
feedback systems. Such systems seek to force the
active elements (Strain gage, RTD, and thermistor)
to balance the bridge by influencing the parameter
being measured.
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Bridge Circuit
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A bridge measures an electrical property of
a circuit element indirectly, i.e., by
comparison against a similar element. It
serves as a null detector and that reads a
difference directly in voltage or current.
The resistance ratios in two branches
determine the differential voltage output.
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Review of OpAmp
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Inverting amplifier
Non-inverting amplifier
Follower
Comparator
Adder
Subtractor
Integrator
Differentiator
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Cont.
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Instrumentation Amplifier
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Consists of three opamps.
A differential input
A single ended output
A high input impedance
A simple means for adjusting gain
High common-mode rejection ratio
Gain and CMRR equations
Current Amplifier
Logarithmic amplifier
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Instrumentation Amplifier
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Schematics and equations
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Gain
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CMRR
Ref. Patrick Garrett,
Analog I/O Design ..
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Noise in Low-level Amplification
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Internal noise model
 Schematics
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Vt thermal noise
 Vc contact noise
 Rc contact resistance
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In device noise current
Vn Device noise voltage
Rs Source plus lead resistance
Noise (cont.)
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A 10 ohm carbon composition resistor produces
the same thermal noise as a 10 ohm wire wound
resistor at zero current flow. However, the carbon
composition resistor with a dc current adds
substantially more noise, due to the uneven
composition materials. The contact noise occurs at
any contact including switches and semiconductor
bonded contacts, and is referred to as 1/f or low
frequency-dependent noise.
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Noise (cont.)
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Contact noise, expressed as a noise voltage per
root Hertz of bandwidth, is applicable to a few
hundred Hertz and below.
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Vc = (10e-6)·sqrt (1/f)· (Rc + Rs) ·Idc
V/sqrt(Hz)
Above this frequency, thermal noise is the
primary circuit internal noise contribution. For
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Cont.
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An important conclusion is that for low level
measurements, especially below 100 Hz, direct current
flow through the transducer circuit should be avoided
or minimized.
 Total noise is expressed as:
 VN = sqrt( Vt2 + Vc2 + Vm2 + In2· Rs2)·sqrt(fhi) Vrms
The choice of an instrumentation amplifier is based
upon a selection of parameters that will minimize the
error budget. Criteria include CMRR, input offset,
noise and drift parameters.
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Active Filters
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An active filter can have a voltage gain, and pass
or reject a selected frequency, or frequency band.
However, a pass filter such as R L C circuit does
not have gain, but the attenuation.
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The transfer function is a ratio of second order
polynomial to another second order of polynomial.
Types of Active Filters
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Butterworth Second-order VCVS Filters
– Low pass
– High pass
– Band pass
– Band reject
– All pass (phase shifter)
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Low Pass
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High Pass
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Band Pass
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Band Reject
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