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 | |  | | | | | Uncovering the Mystery of Sensor Circuits' Stability
An interview with Dongjie Cheng
IC Designer, Allegro Microsystems Inc. | |
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What are the challenges faced by designers working on Sensor circuits?
Sensor's signals normally vary in a large dynamic range. When a signal is very strong, a sensor can be operating in the saturation region. During saturation a sensor cannot derive signal's transient or historical information. In this case techniques like automatic gain control (AGC) may be required for good sensor performance. Another extreme case is when signal is very weak and noisy. Then signal conditioning (through filtering and amplifying) is required to ensure signal integrity and sensor reliability. Feedback for an amplifier is generally used for this purpose. This is when a circuit designer must deal with a tough reality-instability. Then the loop gain’s Bode plots should be studied to guarantee sufficient stability margins.
What are the limitations of a traditional stability analysis approach using AC simulation? How does the methodology detailed in this paper address these limitations?
There are different frequency-domain techniques to conduct stability analysis. Some are tedious and difficult, others are over-simple and misleading, and traditional techniques are generally sensitive to analytical or application conditions. Based on Bode’s circuit feedback and stability theory, this article summarizes advantages and limitations of modeling and lab approaches for stability study from a circuit designer perspective.
Using the single AC simulation approach, the calculated loop gain is highly dependent on the type and location of the input. Also to get an accurate result, the user needs to select a favorable breakpoint location without disturbing the loop DC bias, this ideal breakpoint location however is often difficult to identify or does not exist in realistic cicruits. With the new methodology discussed in this article we can calculate the true loop gain, which is the sum of the normal (transmission) loop gain and the reverse (transmission) loop gain. Not only is this scheme the most accurate, but also it is the easiest to apply, as the breakpoint can be anywhere in the loop and the type and topology of circuit feedback do not need to be pre-identified.
How were you able to leverage Virtuoso Spectre Circuit Simulator for accurate AC stability analysis of your designs?
Cadence’s Virtuoso Spectre Circuit Simulator provides the mostly accurate and user-friendly tool to analyze a linear circuit’s stability. It provides both gain and phase margins in one simulation with a simple setup. During stability (stb) simulation Virtuoso Spectre Circuit Simulator injects both test current and test voltage and performs two AC analyses. This most advanced algorithm eliminates errors arriving from choosing a location for test signal injection and derives the true loop gain.
Other than frequency domain simulation, what other approaches would you recommend for designers concerned about circuit stability?
The AC and stb simulation approaches for stability analysis presented in the article are performed in the frequency domain. Cross-examination of the circuit stability by other means is important based on the following considerations. (1) Circuit application conditions can be very complicated (such as noisy environment). (2) A circuit usually contains parasitic components, which are not modeled. (3) The frequency domain simulation is limited to linear circuit, A combination of time-domain simulation and lab-based approaches are recommended by this article for further analysis of designs for stability. The lab approach in deriving the loop gain is effective when logic control circuits cannot be handled by AC simulation. Bench testing should be always a realistic and final check of the circuit operation as well as stability.
Read Dr. Cheng's paper, "Uncovering the Mystery of Sensor Circuits’ Stability"
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About the author
Dongjie Cheng is currently an analog and mixed-signal IC designer with Allegro Microsystems Inc. His responsibilities include designing smoke detector ICs and power ICs. Prior to his current position he worked on automotive sensors for Control Devices Inc. in Maine, and door/gate safety sensors for Miller Edge Inc. in Pennsylvania. He holds a Ph.D. from SUNY Stony Brook. |
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