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  • 1. ET8.017Delft University of Technology El. Instr. Electronic Instrumentation Lecturer: Kofi Makinwa K.A.A.Makinwa@TUDelft.NL 015-27 86466 Room. HB13.270 EWI building Also involved: Saleh Heidary (S.H.Shalmany@TUDelft.NL) ET8.017Delft University of Technology Introduction El. Instr. Synonyms: coherent detection, synchronous demodulation, lock-in amplification, chopping?? These are all modulation techniques that are used to improve the low frequency performance of measurement systems When square-wave modulation is employed, the technique is referred to as chopping 1
  • 2. ET8.017 Coherent detection El. Instr.Delft University of Technology Theory Why and when to use? Basic principle (time domain) Amplitude and phase measurement Signal-to-Noise Ratio (frequency domain) Switching detector (choppers) Summary Application example (DEMO 1) Strain gauges Wheatstone bridge Coherent detector: design considerations ET8.017 Coherent detection El. Instr.Delft University of Technology When to use it? When measuring low-bandwidth or quasi-static signals in the presence of high noise or disturbance levels. If a high dynamic range is required Therefore it is often used to readout sensors e.g. (MEMS) Accelerometers (fF capacitance changes) Optical (infrared) detectors (fAs of current) Magnetic sensors ( mVs ) Strain gauges ( mVs) A coherent detector behaves like a bandpass filter 2
  • 3. ET8.017 Time domain analysis El. Instr.Delft University of Technology GAMP uoM =GAmp Gmult ui sin (i t ) ur sin (r t ) cos ( (r i ) t ) cos ( (r + i ) t ) ur ui uoM =GAmp Gmult 2 Now if: r = i (Coherent signals !) then: DC ur ui uoM =GAmp Gmult 1 cos ( 2i t ) 2 ET8.017 Amplitude detection El. Instr.Delft University of Technology GAMP ur ui uoM =GAmp Gmult 1 cos ( 2i t ) r = i 2 The low-pass filter removes the second harmonic: ur ui uoF =GLPF GAmp Gmult 2 This coherent detector detects the amplitude of the input signal. Note that in this case Ui and Uref are in-phase (Synchronous detection). What if they are not in-phase ???? 3
  • 4. ET8.017 Phase detection El. Instr.Delft University of Technology Suppose Ui and Uref have a phase difference: GAMP r = i uoM =GAmp Gmult ui sin (i t + ) ur sin (r t ) cos ( (r i ) t + ) cos ( (r + i ) t + ) u r ui uoM =GAmp Gmult 2 ur ui uoF =GLPF GAmp Gmult [cos ( )] =0 2 This coherent detector is also phase sensitive. ET8.017 Signal-to-noise ratio El. Instr.Delft University of Technology What would be the S/N ratio of an oscilloscope? a coherent detector? Some numbers: What would be the detection limit of the coherent detector if the white noise spectral density = 10 nV/Hz and fLP = 1Hz ??? 4
  • 5. Coherent detection in ET8.017Delft University of Technology practice El. Instr. Linear operation of an analog multiplier circuit, working over a large range of input signals is difficult to realize in practice. A much simpler realization is a switching detector a.k.a. a chopper Chopper realization in CMOS: Easy: just 4 transistors! Accurate: does not introduce offset But: switching spikes can cause problems (residual offset) ET8.017 Summary El. Instr.Delft University of Technology GAMP REMEMBER: An ideal coherent detector is only sensitive to frequency components in a narrow band around a reference signal Uref. The low-pass filter determines the signal bandwidth So what happens if Uref is a square wave ?? 5
  • 6. DEMO: Coherent detection for ET8.017Delft University of Technology the readout of strain-gauges El. Instr. R is typically 100-2000 R is typically 10-1- 10-5 Very small changes in resistance should be measured DEMO: Using strain gauges ET8.017Delft Univers

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