UNIT 3

1. UNIT 3 Intermediate elements

2. Instrumentation Amplifier Instrumentation amplifier is used to enhance the signal strength of the transducers output to the desirable level by amplification and make it suitable for further processing . The instrumentation amplifier is a dedicated differential amplifier with selectable gain (with linearity and accuracy) , high stability, high CMRR, with extremely high input impedance and low output impedance.

3. Instrumentation Amplifier The instrumentation amplifier is a closed loop device with carefully set gain. Its gain can be precisely set by a single internal or external resistor. The OPAMP itself is an open loop device with some very large (but variable) gain. This allows the instrumentation amplifier to be optimized for its role as signal conditioner of low level (often d.c.) signals in large amounts of noise.The high common mode rejection makes this amplifier very useful in recovering small signals buried in large common-mode offsets and noise.

4. Instrumentation Amplifier Instrumentation amplifiers consist of two stages. The first stage offers very high input impedance to' both input signals and allows to set the gain with a single resistor. The second stage is a differential amplifier with negative feedback, produces the output as a function of the differential input voltages

5. Instrumentation Amplifier Input v1 and v2 is applied to the non-inverting input terminals of the OPAMP 1 and OPAMP2(identical to each other) respectively .It will act as an input buffers with unity gain for common mode signal and with a gain of (1+2R1/Rg) for differential signals The gain of the instrumentation amplifier can be varied by changing Rg alone . The output voltage is given by The resistors R1 are internal to integrated circuit while Rg is the gain setting resistor, may be internal or connected externally. The second stage of the instrumentation amplifier is a unity gain differential amplifier.

6. Isolation Amplifier Isolation amplifiers provides an electrical isolation to the output from input and hence protect system from common mode voltages, unexpected short circuit etc. Isolation methods . 1)A transformer -isolated amplifier ( Transformer coupling of a high-frequency carrier signal between input and output) 2) An optically-isolated amplifier (modulates current through an LED optocoupler). 3)Capacitor isolated Amplifier (capacitors are used to couple a modulated high-frequency carrier)

7. Isolation Amplifier Isolation amps typically employ transformers and high-voltage capacitors or optocouplers in the isolation barrier design Isolation amplifiers s typically modulate the linear input signal and transmit the resulting digital information across the isolation barrier to a demodulator where it is converted back to analog

8. Isolation Amplifier Transformers couple by magnetic flux. The primary and secondary windings of two transformers are not connected to each other. In addition, transformer-based Isolation amps are susceptible to signal corruption from external field interference and errors

9. Isolation Amplifier Opto couplers transfers electrical signals between two isolated circuits by using light. The sender (Light Source) and receiver (Photosensitive Device) are not electrically connected. A common type of opto-isolator consists of an LED and a Phototransistor in the same package. Opto-isolators are usually used for transmission of digital signals, but some techniques allow use with analog signals. Opto-isolators prevent high voltages from affecting the system in receiving the signal. Optocoupler-based Isolation amp versions also often suffer from poor linearity and even worse errors

10. Isolation Amplifier Capacitors allow alternating current to flow, but block direct current . They couple ac signals between circuits at different direct voltages.

11. Bridges Bridges offer an attractive alternative for measuring small resistance changes accurately. To measure parametersR, L, C, f, Q (Quality factor of a coil) and ‘D’ (Dissipation factor of a capacitor) of electronic circuits, bridge circuits are employed. The advantage with bridge-measuring circuits is that some errors which occur in measurements due to parasitic values, temperature effects, errors due to improper grounding and shielding can be eliminated.

12. Bridges DC bridges can measure resistance R accurately over wide ranges. AC bridges can be used to determine the unknown values of inductor L and capacitor C and even R and frequency f also. Even parameters like quality factor of a coil Q, dissipation factor D of a capacitor can also be measured using AC bridges in addition to frequency.

13. Bridges The bridge circuit works as a pair of two-component voltage dividers connected across the same source voltage, with a null-detector meter movement connected between them to indicate a condition of “balance” at zero volts:

14. Bridges Measurement System

15. DC Bridges

16. DC bridges DC bridges are used to determine the unknown conducting value. Wheatstone bridge and Kelvin double bridge are the two types in this category. AC bridges can also be used for resistance measurements, but they are used to determine inductance, capacitance, impedance, admittance, or the frequency of the AC input. The D.C bridges use the D.C voltages as the excitation voltage while the A.C bridges use the alternating voltage as the excitation voltage.

17. Basic DC Bridge

18. DC bridges Any one of the four resistors in the above bridge can be the resistor of unknown value, and its value can be determined by a ratio of the other three, which are “calibrated,” or whose resistances are known to a precise degree. When the bridge is in a balanced condition (zero voltage as indicated by the null detector), the ratio works out to be this:

19. Application of DC bridges

20. Wheaston’s DC Bridge DEEPAK.P

21. Wheaston’s DC bridges A bridge measures resistance indirectly by comparison with a similar resistance. All the resistances are nominally equal, but one of them (the sensor) is variable by an amount of change in R.

22. Wheaston’s DC bridges R2 = unknown value of R1 , R3 = Fixed resistor R4 = Variable resistor e= Galvanometer with high sensitivity V = Source

23. Wheaston’s bridges

24. Wheaston’s bridges

25. Wheaston’s bridges The bridge is balance when no current through the galvanometer (Ig=0) VAB = VAC or VBD=VCD VAB = VAC

26. Amplification of bridge output

27. Guarded Wheaston’s DC Bridge DEEPAK.P

28. Guarded Terminal It is normally used to measure the High resistance such as Insulation Resistance of machine or cable Resistance of circuit components like vacuum tube Leakage resistance of a capacitor

29. Guarded Wheatstone Bridge It is very difficult to measure such resistance because of the very high resistances ,small current flowing in the measuring circuits. It is very difficult to sense these small current. Another difficulty is the Presence of leakage currents This leakage currents makes errors This error can be avoid by using guard circuits

30. Guarded Terminal The function of the Guard is to act as a shunt circuit for parallel leakage paths in the test item With it, one or more parallel current paths can be removed from the measurement, thereby permitting a precise reading of the remaining path.

31. Guarded Wheatstone Bridge

32. Guarded Wheatstone bridge Major Problem in DC Wheatstones Bridge : Leakage current in high resistance measurement. Three terminal resistance is used to avoid the leakage current RL1 is parallel with R2 and RL2 is parallel with the detector. Since RL1 & RL2 is very large compared to detector & R2 ,their effect is negligible. The effect of external leakage paths are removed by this connection.

33. Guarded Wheatstone Bridge Used for high resistance measurements

34. AC Bridges DEEPAK.P

35. AC Bridges AC bridges are often used to measure accurately the value of an unknown impedance, for example, self/mutual inductance of inductors or capacitance of capacitors. Most of the AC bridges are based on a generalized Wheatstone Bridge circuit.

36. AC bridges Vs Dc Bridge

37. AC Bridges ZAZD=ZBZC

38. AC Bridges As shown in Figure, the four arms of the DC Wheatstone Bridge are replaced by impedances (ZA, ZB, ZC and ZD), the battery by an AC source and the DC galvanometer by an AC null detector (usually a pair of headphones). Using Kirchoff's Laws, it can be easily shown that the balance or null condition (i.e., when no current flows through the detector, or the potential at the point P becomes equal to that at point R) is given by

39. AC bridges The various types of A.C Bridges are, Inductance Comparison Bridge Capacitance Comparison Bridge Maxwell’s Bridge Hay’s Bridge Anderson Bridge Wien Bridge Owen's bridge Shering Bridge

40. Owens Bridges DEEPAK.P

41. Owen’s bridges This circuit, shown in Figure, is suitable for the measurement of self-inductance. R4 and c4 is variable

42. Owen’s bridges

43. Owen’s bridges

44. Owen’s bridges

45. Owen’s bridges another form

46. Owen’s bridges Advantages : 1. Owen’s bridge can be used to measure over a wide range of inductance. 2. Convergence of balance condition is much easier than other bridges due to R2 and C2; two variables are in same arm. 3. The balance equations are quit easy and do not contain any frequency terms.

47. Owen’s bridges Disadvantages : 1. The Owen’s bridge requires a variable capacitor that is expensive and makes its accuracy rather low. 2. The value of C2 tends to become rather large when measuring high Q coils.

48. Schering Bridge DEEPAK.P

49. Schering bridges The schering bridge is one of the most important ac bridge used extensively for the measurement of capacitance. In schering bridge the arm 1 contains a series combination of the resistor and the capacitor and standard arm contain only one capacitor. In the balance condition of the bridge the sum of the phase angles of the arms 1and 4 is equal the sum of the phase angle of arms 2 and 3

50. Schering bridges