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Graham Bell
Graham Bell
Graham is Sr. Director of Marketing at Real Intent. He has over 20 years experience in the design automation industry. Occasionally he writes blogs for the Dominion of Design. The views and opinions expressed in this blog are his alone and not those of his employer.

Cosmic Circuit’s Custom ASICs for Sensor Front Ends

 
January 11th, 2012 by Graham Bell
The January Cosmic Rays newsletter from Cosmic Circuits detailed the issues for creating custom ASICs for sensor front ends.  The article is shown here:

Custom ASICs are often deployed in sensor read-out electronics. These read-out systems tend to be unique and require the custom ASIC to

  • realize the unique functions at the power dissipation suitable for the system,
  • reduce cost and
  • protect intellectual property.

This article walks through the key elements of a custom solution for a Sensor Front End.

Successful Sensor Front Ends usually consist of the following key elements in realizing a complete solution

  1. Robust architectural definition
  2. Sensor interface electronics – often a preamplifier or instrumentation amplifier

ADC

  1. Sensor excitation circuits
  2. Calibration and Production support
Architecting the solution
Some of the key architectural decisions need to be made early in the architecture phase – these include AC or DC excitation of the sensor, , observation time of the sensor signal, voltage definition if battery operated and communication mechanism from sensor to digital processing engine.

Cosmic Circuits engineers work with customers through this phase to define an optimal solution after weighing feasibility of IC implementation.

 

Sensor interface electronics
Preamplifier is one of the most cricital element in sensor design as it typically defines the noise performance of the system. Depending on the sensor output choosing the appropriate preamp topology is an important aspect of preamp design. Choices available are

  1. Voltage preamp
  2. Charge preamp
  3. Trans impedance amplifier (TIA)

Other aspects critical to the design of the preamp are related to system offset and 1/f noise. Systems where signal being measured is DC as is often the case in blood pressure sensors, wheatstone bridge measurements systems, magnetic sensors, hall effect sensors, gas flow sensors, low offset is a key requirement. Depending on the system, an appropriate solution for offset and 1/f noise may be selected. Some of the solutions include continuous background offset calibration, chopper stabilization and auto-zero.

Cosmic has deployed and evaluated several of these topologies in different ASICs designed for sensor interface solutions. Examples of amplifiers developed for interfacing to sensor are presented below in Figures 1, 2, and 3.

Fig. 1 Charge amplifier for microphone sensor

Fig. 2 Projected Capacitance touch screen front end : Charge Preamp

Fig. 3 Pressure sensor (Trans-Impedance Amplifier)

Capacitive Transimpedance Amplifiers (CTIA) for focal plane arrays, PGAs for Hall sensors and amplifiers for Hall sensor read out are other applications Cosmic has explored in detail.

 

A/D converters
High resolution low noise converters in the range of 16-20 bits and few kHZ sample rate are often deployed for common sensor read out applications. Systems that need faster sampling times like capacitive touch sensors   and imaging applications use a lower resolution and faster ADC architectures like successive approximation or pipelined ADCs.

Choosing the right ADC requires understanding the system noise budget,interferers and selecting appropriate sample rate and resolution. Sigma-delta converters have much higher sample rates for a given conversion rate and thus are more tolerant to interference. However sigma-delta converters are not easily amenable to sharing them across multiple sensor channels without incurring a penalty in latency and depending on system requirements, correct ADC type must be selected. Cosmic has a wide variety of ADCs in its IP portfolio and these are leveraged for providing ASIC solutions.

Sensor Excitation Circuits
Biasing the sensor at the right point is critical for the transducer function. For instance MEMS microphone sensor needs a voltage bias to achieve right sensitivity, a MEMS accelerometer needs an AC excitation of right amplitude.

Some of the common circuits deployed in sensor excitation sub-system are :

  1. Charge pumps
  2. Amplifiers
  3. D/A circuits

In typical sensor systems the noise performance of the excitation circuitry is an important consideration. Some of the modules used in Cosmic’s sensor ASICs for sensor excitation have been

  1. High voltage charge pumps with very low power consumption – these are a key element of biasing mems based sensor. Charge pump voltages of 10-30V are common while keeping current consumption down to few tens of uA.
  2. DACs with 10-12b precision- Such D/As allow precision control of the excitation current of the sensor. Often D/As also need an amplifier to drive the sensor.
  3. Low noise amplifiers – these are usually needed to drive the excitation load presented by the sensor. For DC excitation an amplifier alone is sufficient but when excitation is AC, a DAC (for excitation wave synthesis) followed by the amplifier is usually used.

 

Calibration and Production Support
The final system will have errors like offsets, gain errors, frequency deviation etc.   Purely working out circuit solutions for these is often sub-optimal. Calibration needs to be an important tool which needs to be used appropriately to achieve the desired performance while keeping circuits reasonable calibration of the certain parameters such as gain and offset in the factory. Very often this calibration needs to be done after integration of the ASIC in the complete system. ASIC vendor must not only choose the right BIST features in the ASIC to enable this but also provide support through both hardware and software kits to enable customer to do calibration of the ASIC without adding significant cost of engineering overhead at their end.

Cosmic has built custom test and calibration hardware to facilitate calibration of ASICs supplied. The figure below shows the snapshot of calibration guide for the calibration board. Cosmic usually provides documentation, hardware and firmware to perform calibration.

Fig. 5 Calibration User Guide

Architecting and productizing sensor read out circuits needs deep understanding of the trade- offs in terms of power, cost and robustness of a solution. An ASIC partner knowledgeable in the art of optimizing such solutions can complement system companies in defining customized and differentiated solutions.

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