- Design of Analog-IC using electronic system.
- Deriving all the IC specifications using the circuit Analysis.
- A good understanding of MOS devices and Technology.
- Proficiency in analog circuit and design analysis.
- Having a good mastery in some tools--ngspice, xschem, magic.
Introduction to an electronic system design-USB-MIDI microphone.
- Microphone pre-amplifier and interface circuit design.
- Select an widely available Op-Amp for the preamplifier e.g. TI OPA 344
- Derive the important specs for the CMOS Op-Amp design.
- Derive also all the specs for the circuit Design and analysis.
Introduction to linear circuits and passive devices.
- Understanding passive devices (RLC) using basic EM principles.
- Principle of linearity and superposition
- Network analysis: KCL, KVL, Thevenin, Norton, Superposition.
- Emphasis on interfacing circuits and power transfer principle.
Introduction to all the Fundamentals
- Analysis of RC circuits.
- Understanding the BJT fundamentals.
- Understanding of MOS Transistor.
- Understanding Current Mirror and Operational Amplifier.
- Designing Differential Amplifier.
- Understanding the Semiconductor IC Devices.
- PDK & DRC Manual PDK.
- MEMS mic Datasheet.
- OP-AMP-344 Datasheet.
- Schematic Sparkfun breakout Board.
Calculate Thevenins Eqivalent of Microphone
REFERENCE
- Find the Thevenin equivalent circuit for the microphone for normal voice operation.
- Substitute the microphone's Thevenin's equivalent and find the frequency response of the analog front-end (AFE).
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Introduction to Xschem.
THEVENIN MODEL OF MICROPHONE
Key specs from the microphone datasheet and research:
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Sensitivty: -44 dBV/Pa.
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Condition: 94 dB SPL at 1 kHz which is sound pressure of 1 Pa.
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Normal voice conversation is typically 60 dB SPL.
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Vth Calculation
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Voice (Pa) = 10 ( 60 − 94 ) / 20 = 19.9 × 10 − 3 P a.
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Output (Vpk) = √2 × Vrms = 2 × 19.9 × 10 − 3 P a × 10 − 44 / 20 = 178 μVpk
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Vout - pk = 0.178 mV
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Rth (from datasheet) = 380 ohms.
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Sparkfun Schematic of the breakout Board Link.
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From Sparkfun schematic: Rin=5k, Rfb=300k, therefore Gain = 60.
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So output of the amplfier will be 60x0.178 mVpk = 10.68 mVpk.
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Sparkfun site states 100 mVpk probaby assuming 10 times higher input signal i.e. Voice is 80 dB SPL.
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Input high-pass frequency = 1 / 2πRC = 1 / 2π5k4.7uF = 6.77Hz.
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Feedback Low-pass filter frequency = 1 / 2πRC = 1 / 2π300k27pF = 19.6kHz.
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Input common-mode filter = 1 / 2π10k1uF = 15.9Hz.
Single-Pole Model of OPA 344
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Specs from the OP344 Datasheet.
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Open Loop DC Gain: 120 dB (From p-5 graph).
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Unity Gain Frequency: 1 MHz.
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Pole = 1 MHz / 10^6= 1Hz.
Microphone AFE Analysis
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Introduction and learning all the details about Xschem.
- Link for the video.
- Review of Basic Circuit Theory Link.
- Next We are doing opAmp Modeling & Creating symbols in Xschem.
- Doing all the review and ppt of the Circuits.
ASSIGNMENT WORK
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Determine the type of circuit and write the s-domain transfer function for it.
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Find the frequency of the appropriate -3 dB point (low-pass corner or high-pass corner).
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Create the schematic for the above circuit in xscheme, simulate and plot and calculate the following:
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Create a schematic and symbol for the opamp model with just a VCVS with a gain of 1000.
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Operating point
- Back annotate the operating point to the schematic and verify.
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AC simulation
- plot output voltage (in dB) and phase (in deg).
- measure the maximum gain and the frequency at the gain.
- measure the -3 dB frequency and verify with your calaculation.
- Do the above measurement for a gain of 10000 (What is it in dB?).
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Transient Simulation
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Provide an input with a sine wave of 100mV at two frequencies:
- -3 dB frequency.
- 10X -3 dB frequency.
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Measure the peak-to-peak voltage at the output and calculate the gain.
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Create the single-pole model of the OpAmp as whown in the figure.
- Write the s-domain expresion of the transfer function of the Op-Amp.
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Substitute the model in the high-pass filter.
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Repeat the high-pass filter simulation and measuement with the new OpAmp model and note the differences you observe.
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Substitute the single-pole OpAmp model in the above figure.
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Operating point
- Back annotate the operating point to the schematic and verify.
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AC simulation
- plot output voltage (in dB) and phase (in deg).
- measure the maximum gain and the frequency at the gain.
- measure the -3 dB frequency and verify with your calaculation.
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Transient Simulation
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Provide an input with a sine wave of 1mV at three frequencies:
- -3 dB frequency of the high-pass corner.
- Frequency at maximum gain.
- -3 dB frequency of the low-pass corner.
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Measure the peak-to-peak voltage at the output and verify with AC sim results.
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Acknowledgement
- I would like to thank Dr. Saroj Rout for his mentorship and guidance throughout the internship. Gratitude also to Silicon University and the open-source EDA community for enabling this work.