Paper presentation Ultra-Portable Devices

Transcription

1 Paper presentation Ultra-Portable Devices Paper: Lourans Samid, Yiannos Manoli, A Low Power and Low Voltage Continuous Time Δ Modulator, ISCAS, pp , May, Presented by: Dejan Radjen Paper Presentation - Ultra Portable Devices 1

2 Outline Introduction to Δ modulators Continuous Time Δ modulators Amplifier Requirements Measurement Results Summary and Conclusions Paper Presentation - Ultra Portable Devices 2

3 Introduction to Δ modulators Paper Presentation - Ultra Portable Devices 3

4 Introduction to Δ modulators White noise approximation is accurate if the input signal is changing rapidly and is random White noise approximation is also more accurate as the number of bits N in the quantizer increases For a one bit quantizer (a comparator) the white noise approximation is least accurate but it is used anyway with proper watchfulness Maximum theoretical SNR for an ideal ADC using white noise approximation: SNR db = 6.02N Paper Presentation - Ultra Portable Devices 4

5 Introduction to Δ modulators General Δ modulator Linear model Using the linear model the output can be written as: Y(z) = STF(z)X(z) + NTF(z)E(z) STF(z) = Signal Transfer Function NTF(z) = Noise Transfer Function Paper Presentation - Ultra Portable Devices 5

6 Introduction to Δ modulators STF = z -1 NTF = (1 z -1 ) 2 Second order noise shaping Implementation example of a 2:nd order 1 bit Δ -modulator Paper Presentation - Ultra Portable Devices 6

7 Continuous Time Δ Modulators Modulator Topology Distributed feedback nth order CTloop filter Continuous Δ modulators offer implicit anti-aliasing filter Paper Presentation - Ultra Portable Devices 7

8 Continuous Time Δ - Modulators Excess Loop Delay Excess loop delay τ d is the delay between the quantizer output signal and the DAC output signal Paper Presentation - Ultra Portable Devices 8

9 Continuous Time Δ -modulators Coefficient Mismatch In DT-Δ modulators the coefficients are given by ratios between capacitors In CT- Δ modulators the coefficients are decided by RC time constants with tolerances of ± 30 % An additional tuning circuit can be added to improve the accuracy of the RC constants Another method is to use a less aggressive noise transfer function by moving the poles Paper Presentation - Ultra Portable Devices 9

10 Continuous Time Δ - Modulators Coefficient Mismatch a. Without moving the poles b. With moving the poles Paper Presentation - Ultra Portable Devices 10

11 Continuous Time Δ -modulators Clock Jitter Error signal Exponential DAC output signals reduce sensitivity to clock jitter Paper Presentation - Ultra Portable Devices 11

12 Continuous Time Δ -modulators Integrator Implementation Active RC-integrator GmC - integrator Paper Presentation - Ultra Portable Devices 12

13 Amplifier Requirements Finite DC-gain of the amplifiers cause quantization noise leakage. A rule of thumb is A dc > OSR Finite Gain bandwidth affects the noise transfer function and can cause instability Distortion, Slew rate and noise limitations are primarily due to the amplifier in the first integrator Most power has to be spent on the amplifier in the first integrator Paper Presentation - Ultra Portable Devices 13

14 Measurement Results Measurement Results for the Third Order Continuous Time Δ modulator Paper Presentation - Ultra Portable Devices 14

15 Measurement Results Measurement Results for the Third Order Continuous Time Δ modulator Paper Presentation - Ultra Portable Devices 15

16 Summary and Conclusions The paper mainly discusses different non idealities present in CT Δ modulators Continuous time Δ modulators are more suitable for low power design than their discrete time counterparts The price paid is increased sensitivity to component mismatch, excess loop delay and clock jitter Paper Presentation - Ultra Portable Devices 16