ANALOG INTEGRATED THE JOY OF CIRCUITS Richard Schreier Jan. 9 2013 ANALOG DEVICES
Today’s Message • Analog IC design taps many areas of expertise: physics, geometry, analysis, algorithms, modeling, …. ⇒ Creativity (= Fun) is possible at multiple levels Three Examples 1 Circuit: String DAC Tricks 2 Algorithmic/Signal Processing: ∆Σ DAC 3 Physical: Layout of a current-mode DAC 2 ANALOG DEVICES
1. Resistor-String DAC 2-bit example: V ref N -bit digital input selects one of 2 N equi-spaced analog values r ( ⁄ ) V ref V o m 3 = r , , , m 0 1 2 3 = r + Simple. Ratiometric . Monotonic. – N -bit DAC requires 2 N components 3 ANALOG DEVICES
String DAC with Sub-String V ref + Number of units grows as 2 N / 2 + Monotonic + Can repeat V o sub-sub-string, etc. – Needs buffers to prevent sub-string from loading the main string Or does it? 4 ANALOG DEVICES
Unbuffered Sub-String [Dennis Dempsey US5969657] V ref r • Choose the resistor ratio r such that shifting the sub- r string up one step increases r 1 the voltage at the common point by 1 LSB V o r 1 r 1 • Note that the voltages elsewhere in the string are r unaffected r 5 ANALOG DEVICES
Required Resistor Ratio + 1 Solution: 3V r r 1 4 3 1 1 - - - = - - - + - - - r r 3 1 – + 4 3 r = + 1 1V + – + r 1 r 1 r 4V = 1V – + – • r 1 regardless of 1 1V = – the number of units in the sub-string! 6 ANALOG DEVICES
More String Cleverness V ref R R • Walk the sub-string up the main string in an alternating R R fashion rather than shift it V o R R one step at a time R R ⇒ The number of switches on the main string is halved R R 7 ANALOG DEVICES
2. Delta-Sigma ( ∆Σ ) DAC • Suppose you have a fast and accurate, but limited-resolution DAC Q1: How can you get more resolution? OR Q2: How can you get an output that is between two DAC levels? Desired Outputs DAC Output 8 DAC Time 8 ANALOG DEVICES
Switch back and forth quickly; Average with a lowpass filter 16 High-resolution 8 ? DAC LPF low-speed data High-speed clock 8-b DAC Data 16-b Input Data LPF Output Time 9 ANALOG DEVICES
16 8 One ? Digital Block 16 16 MSBs 8 8 LSBs 8 dither spanning one 8-bit LSB • The decision to quantize high/low is made independently of any previous decision • Turns the truncation noise into white noise spread uniformly over frequency 10 ANALOG DEVICES
The ∆Σ Way • Re-circulate the LSBs to bias the next decision 16 8 @50 kHz @ 5 MHz 8 z –1 8-b DAC Data 16-b Input Data Time 11 ANALOG DEVICES
Spectral Implications Desired Signal Shaped Noise Freq. 25 kHz 2.5 MHz • Truncation noise is still spread over a broad frequency range, but not uniformly • Very little noise is present in the signal band! ⇒ With the ∆Σ technique, a low-resolution DAC can be turned into a high-resolution DAC 12 ANALOG DEVICES
3. Debugging Where circuits designed in the virtual world collide with reality • One of the most important and instructive aspects of analog IC work • Consider a current-mode DAC: I in = 1 mA I out = m•I in 100 current sources Need each leg to carry the same current as the master NMOS to within, say, 1% 13 ANALOG DEVICES
Measurement Result— Disaster! Current per leg I in 20% Error! WHY? Leg 0 10 20 30 40 50 60 70 80 90 100 number 14 ANALOG DEVICES
The Circuit Layout 1 mA 0.8 mA … … 15 ANALOG DEVICES
Aha! Wiring Resistance I I I I r r r r r r 16 ANALOG DEVICES
Analysis I– ∆ I I I I I r r r r r r ∆ V – + ∆ I g m ∆ V n 2 2 ∆ V ( ⁄ ) rI = = g m 2 mA/V n 100 = = ∆ I I 1 mA 0.2 mA = = ∆ V 20 m Ω 0.1 V r = = 17 ANALOG DEVICES
How To Fix This Problem? < 1 m Ω • To get 20x better matching, need r Good luck with that. • Hopeless? 18 ANALOG DEVICES
Tree Connection 19 ANALOG DEVICES
Summary • Peeked at 3 examples of analog IC cleverness Circuit-level: clever architecture Signal-processing: clever algorithm Physical-level: clever layout geometry • Innovation exists at many more levels and in various combinations • There is still fresh ground to explore and exploit The bulldozer that is digital CMOS is blazing a trail into new territory, enabling techniques that were impractical with earlier technologies. E.g. Reliance on digital signal processing; using time intervals to represent analog quantities. ⇒ Analog IC design provides good entertainment 20 ANALOG DEVICES
Recommend
More recommend
