Dr. Un-ki Yang Particle Physics Group - PowerPoint PPT Presentation

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  1. Amplifiers and Feedback: 3 Dr. Un-ki Yang Particle Physics Group ukyang@hep.manchester.ac.uk or Shuster 5.15

  2. Web page for Amp & Feedback

  3. Realistic OP Amplifier: review • Gain is NOT infinite • Gain is NOT constant against frequency • Output response is NOT instantaneous • Output impedance is NOT zero • Input impedance is NOT infinite Gain drops at high frequency Bandwidth: a stable range. -3dB Slew rate: response rate

  4. Positive Feedback: review • Negative feedback: stabilizes the circuit • Positive feedback: saturated output, (+/- 15 V) thus used for digital electronics.

  5. Schmitt Trigger: review • Two different thresholds V+, depending on Vout: fix a problem for noisy signal

  6. Schmitt Trigger t1 t2 to threshold V+ • Noisy problem is fixed

  7. Analogue to Digital conversion (ADC) • Why digitized signal? • Analogue signals can be distorted and attenuated • Practically impossible to analyze many analogue channels

  8. Analogue to Digital conversion (ADC) • Fast conversion (sampling rate) • High accuracy (resolution) • Linearity

  9. ADC • Sampling rate: how often do we need to digitize analogue signal? • good to have a high sampling rate but requires fast processing • Nyquist rate = use 2 x highest frequency of the signal • Resolution: digitization introduces uncertainty due to a finite step size. • Good resolution: large number of ADC bits: 2n, but requires fast processing and many chips • Resolution: LSB/2

  10. Comparator Vout=G0(V+ - V-) Vout = +15V if V+ > V- -15V if V+ < V- • 1 bit ADC: to provide a digital output indicating which of two analog input voltage is larger: the simplest ADC • Properties: very fast (1 clock cycle), very cheap but very poor resolution (~30%)

  11. Flash ADC • For n-bit, use 2n-1 comparators • Each comparator has its own threshold voltage, separated by 1 LSB • The input to all comparators in parallel (one clock cycle) • Output goes to an encoder to get binary format 3-bit ADC

  12. Flash ADC • Very fast (basically only one clock cycle): good to process high rate events (10k Hz etc) • Buy requires so many comparators for high accuracy (good resolution): very expansive. (32-bit : 4X10E9 comparators )

  13. Slope Converter • Use one integrator and one comparator Slope ~ 1/RC* Vin

  14. Slope Converter ADC • Advantage: good resolution with only two comparators • Does not require precise components: cheap, designed to average out noise • Disadvantage: slow, 2n clock cycles for n-bits

  15. Successive Approximation ADC • Use a successive approximation register • Comparator: check Vin vs DAC reference signal ( MSB --> LSB ): binary search • Advantage: faster, only n clock cycles for n-bit • Disadvantage: register for DAC need to be extremely accurate

  16. DAC (Digital-Analogue-Converter) MSB LSB