ECNG3032 Instrumentation Systems. Lecture Note 9

Transcription

1 ECN303 Instrumentation Systems Lecture Note 9 Sinal Conditionin Part The Dierential Ampliier Many situations require ampliication o voltae erence Thermocouple Dierential Ampliier in in _ o in What is o?

2 Dierential Ampliier in in o ) Output due to in o in ) Output due to in Usin Superposition o in - in- o o in o in 3 The Dierential Ampliier For output to be exact erence o the two inputs: in and _ Where o in in o in in 4

3 equirement or voltae ampliication Measurement inormation contained in the potential DIFFEENCE o two terminals absolute value o potentials immaterial! Common mode voltaes should not aect measurement as no inormation in them. For example: Thermocouple outputs esistance Bride outputs 5 Di-amp common mode ain For non-zero Common Mode input, note that in in in in in U o in 6 3

4 Di-amp common mode ain in U o in Output oltae is: o in - in- 7 Di-amp common mode ain in U o in o - 8 4

5 5 9 Di-amp common mode ain That is: - L o L ain has two parts 0 Di-amp common mode ain ain has two parts Common-mode ain A v, Dierential-mode ain A v, Ideally, erence ampliier should: Ampliy input erence voltae Not respondto common mode input - o, o,

6 COMMON MODE EJECTION ATIO Extent to which device realizes this Called the COMMON MODE EJECTION ATIO A v, CM Av, CM Av, ain or erential voltae inputs Av, ain or Common-mode voltae inputs Output oltae is thus: o A v, A v, Example A eneric op amp has a CM o 0dB ( Intrinsic value). Determine the in-circuit CM or a 0 Common Mode input sinal. 0kΩ ± % 00kΩ ± % in U in o 6

7 A eneric op amp has a CM o 0dB ( Intrinsic value). Determine the in-circuit CM or a 0 Common Mode input sinal. 0kΩ ± % 00kΩ ± % in U in o Assume nominal nominal % % Example Calculate A v, o Av, Calculate A v, Av,

8 Example Calculate CM CM worst case A lo 0 lo 48.8 db A Conclusion: Opamp's CM 0 db but in circuit CM is limited to 49 db by the mismatch o resistor values! 5 COMMON MODE EJECTION ATIO Note eect o resistor tolerance on CM values! emember this in your desins esistor A v, CM Tolerance 5% 0. 0 % % % NB. I source impedance is not balanced, Why? CM is reduced urther. Source resistances balanced: CM not aected Source resistances unbalanced:cm aected 6 8

9 Precision Di amp INA45 7 Improvements Limitations o basic -amp are: Low input impedance Fixed ain These can be overcome by use o the Instrumentation Ampliier 8 9

10 Instrumentation Ampliier Precision Di Amp Used in environments with hih CM noise; Hih input Z; Low bias current; ain determined by one resistor: Standard Di amp requires 4 matched s Chane in ain requires a chane in s Commercial IA based on classic 3 op-amp desin 9 Instrumentation Ampliier in Standard Di Amp A 3 F F A3 3 o A in A and A act as input buers. A3 is the standard amp. 0

11 in Standard Di Amp A 3 F F o A3 3 o Why? in v I A Thereore v v v o o I o v v ( ) F ( ) F F F F F in Standard Di Amp A 3 F F o A3 3 o A in I F F, then v F o v v F 3 0

12 in A 3 F F A3 3 o A in Overall CM is: For ampliier A 3, A v, is A v, ' 3 3 ' ' 3 3 CM A v, A v, F 3 ' 3 3 ' ' 3 3 Instrumentation Ampliier Let s setup aain as ollows and examine what happens to the CM: in 3 3 F 00k % 0k % 00k % A 3 F F A3 3 o A in

13 in A 3 F F A3 3 o A in For ampliier A 3, A v, is Av, 0.00 CM( A3) 33 db Overall CM is: A CM A v, v, Note the improvement over the sinle op-amp 6dB Commercial IA AD60 Precision matched resistors used 6 3

14 Commercial IA INA4 Precision matched resistors used 7 4