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Signal Conditioning Circuits. Bridge Circuits

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.

- Bridge Circuit
Ref. DH Shingold, Transducer Interfacing Handbook

- 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.

- 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.

- Inverting amplifier
- Non-inverting amplifier
- Follower
- Comparator
- Adder
- Subtractor
- Integrator
- Differentiator

- Instrumentation Amplifier
- 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

- Schematics and equations
- Gain
- CMRR

- Ref. Patrick Garrett,
Analog I/O Design ..

- Internal noise model
- Schematics
- Vt thermal noiseIn device noise current
- Vc contact noiseVn Device noise voltage
- Rc contact resistanceRs Source plus lead resistance

- 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.

- Contact noise, expressed as a noise voltage per root Hertz of bandwidth, is applicable to a few hundred Hertz and below.
- 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

- 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.

- 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.
- The transfer function is a ratio of second order polynomial to another second order of polynomial.

- Butterworth Second-order VCVS Filters
- Low pass
- High pass
- Band pass
- Band reject
- All pass (phase shifter)