1 Dr. Un-ki Yang Particle Physics Group or Shuster 5.15 Amplifiers and Feedback: 3

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 +, d epending on Vout: fix a problem f or noisy signal

6 toto t2t2 t1t1 Schmitt Trigger  Noisy problem is fixed tV+V+ t< t o V(H) t >t o V(L) t >t 1 V(H) t >t 2 V(L) threshold V +

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: 2 n, but requires fast processing and many chips Resolution: LSB/2

10 Comparator  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%) Vout=G 0 (V + - V - ) Vout = +15V if V + > V - -15V if V + < V -

11 Flash ADC  For n-bit, use 2 n -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, 2 n 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