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The IC fabrication quantity is increasing and then to provide reliability for the system using the Integrated circuit or chip we need to use the testing methods and then different testing methods are proposed and in which off-chip are previously done. In now a day's most of the IC's are integrated with the testing equipment and the inbuilt testing methods are called as the Built-In self test Then the different testing methods in the Built in self test are used as fault methods and spectral methods and also oscillator BIST(OBIST) and also the spectral BIST and these all are done for the Mixed signal IC and the analog IC and these all are done a chip so they are called as the System On chip testing methods
Index Terms - BIST, OBIST, SOC
For any Integrated circuit the testing need to be performed because it is used in large systems and if the testing is not done then the IC cannot work properly and that can lead to the system failure so we are employing the different testing methods. In addition to this reason we need to know the specifications of the Integrated circuits. Then the testing of the devices can be done by using the different methods. In the methods previously used the most common thing is they are off chip in which the testing circuit will be separately present and the methods have a disadvantage that the circuits are tested at the board level and thus the testing of the device at the other levels cannot be done and that can leads to the errors in the output of the system. Thus the testing circuit is placed in the chip itself thus leads to the increase in the functionality of the system and then the area can also be decreased due to miniature size. In the Built in self test the adjustments are done according to the conditions and thus lead to the increase in the protection of IC and the system and also increases the controllability and observability of the IC. In these testing methods the decisions are made in the circuit itself and then the operations can be continued in two directions depending up on the decision made in the circuit.
Then there are different methods in the BIST scheme. One of the methods uses go and no-go one at the output to make decision.
The other method is using the MADBIST method which is mainly used in the analog to digital convertors and digital and analog convertors and it also uses the digital signal processor, Registers and decision making one.
Then the Built in test are used at the intelligent systems like the radio frequency front ends which uses the MMIC and the MEMS to provide programmability to some extent only and then the programmability of the devices can be done by observing the outputs and adjusting the inputs according to them.
Then the BIST schemes are used for the different applications like determining the center frequency of filters. In the BIST there can be tests which are used to test the Analog, Digital and Mixed circuits which are combination of the Analog and Digital circuits.
Mixed Signal BIST
The Mixed signal circuits are combination of the Analog and Digital components. In the previous days the two types of components are tested separately now they are modified in to single testing device.
In this the testing of the digital circuitry we need to apply to digital test vectors and then the outputs are analyzed to find if any faults exists and the test vectors used are generated from the on-chip or off-chip one. The test vectors are scanned in serially and then applied to a system in parallel.
Then the digital circuits the output is compared with the good response and then the decision can be produced to either go/no-go.
Then in Analog Components of mixed signal circuits testing signals will typically come from off-chip because the analog signal which is required for the testing of analog components need to be controllable and highly reliable.
In the Mixed signal BIST we perform the functions in four steps:
Testing of all the pure digital components
Testing of digital ones present in the ADC and DAC
Testing of Analog ones present in ADC and DAC
Test the Pure analog components.
The testing of the pure analog components using digital portion of the circuit for generating the test signals.
The mixed signal Level BIST is as shown below which has all the operations stated above:
Fig1. Mixed Signal Level BIST
In the Mixed signal level BIST there are different classes one of them is
Spectral techniques which are used for the content of analog outputs these techniques uses the relative signal levels which are used for finding S/N ratio, Gain tracking, Measuring of Frequency Response and Measuring of Distortion present in the analog components.
These methods can lead to reducing the accuracy of the source and measurement device and it can also leads to the increase in processing power and time.
In this the simulating the input to the analog circuit under test observing the frequency response of the Analog circuit under test output and the spectral BIST is modeled as shown below:
Fig2. Mixed signal BIST methods
In this to test the analog devices we need to keep the system in the test mode.
The test signal can be done by using different generation techniques including the Fast Fourier Transform and analysis can be done by using the High Speed Shift Register with multi tone signal capability. In this we can use multi signal and single signal by setting inputs.
The second technique present in the mixed signal IC BIST testing is the Oscillator Based BIST and in this the IC testing is done by transforming the circuit under test in to the oscillator.
OSCILLATOR BASED BIST
In this test the analog or mixed signal circuit is divided in to number of functional blocks like amplifier, Operational Amplifier and PLL so on and the combination of the above blocks can be used
Then the each block is converted in to oscillator with some oscillator frequency for each block.
Then the blocks can be of different types
Which in need of signal for generating the frequency at the output.
The other type is having an inherent frequency which doesn't require any signal for the input.
Then the second type doesn't have any need to rearrange and the other blocks need to be checked and then rearranged. The testing time will be less because the frequency of each block is only need to be checked.
Then the oscillation test strategy is as shown below:
Fig3. Oscillator Based BIST
The BB indicates the Building blocks from the division of the main circuit in to the separate parts and the combination is called as the circuit under test and the other components in the OBIST technique of testing are additional circuitary, Control Logic , Analog Multiplexer(AMUX). The TM indicates the Test mode and Stest is used before the testing to know the functionality of tester. Control Logic is used to control all the functions and blocks in the system and the AMUX block is used to select the particular output from different outputs.
The faults present in the analog circuits are of two types:
The hard faults are also called as the Catastrophical caused from normal variations which are called as the random defects and then the Hard faults are similar to the Stuck Open or Stuck Short occurs in the digital ones.
A stuck open occurs when there is no connection between the device and tester and then the Stuck Short is the fault occurs when there is occurrence of short in between the device and tester
The soft faults are also called as Parametric faults and these faults are caused due to the statistical Fluctuations and these faults do not affect the connectivity but can make the circuit functions out of specifications.
The oscillator Based Built In Self Test Method is as shown below:
Fig4. Oscillator BIST testing method test Strategies
Different Fault Models:
There are different Fault models present in the testing of Mixed signal IC or analog IC they are:
Analog Fault Modeling
Operational Amplifier Testing
Bipolar Amplifier Testing
Myo Electrical signal Amplifier/Filter Testing
Some of them are explanied as below:
Analog Fault Modeling:
In this the Faults need to be created and then these faults are again affected by using photolithographic mask ones which are used in IC fabrication and cause either short and open circuits causes a large deviation in the width to length ratio and the dominant faults are considered and then the short faults are dominant in the both Bipolar and CMOS Processing technology.
Operational Amplifier Testing:
In analog devices the embedded operational amplifiers test procedures will be easier and they have high fault coverage of the Operational amplifiers are tested before.
The analog device can be converted by adding feedback loop to its structure. In this the analog device specifications will also be considered.
Fig5. CMOS Opertional Amplifier
Bipolar Amplifier testing:
In this the differential amplifier is used in combination with the level shifter and the differential circuit to provide the gain and level shifter to cancel the dc component and emitter follower circuit to guarantee the low output impedance.
Fig6. Bipolar transistor
Fig7. Conversion the analog to oscillator
The Test procedure is performed in different steps:
Faults are described in Hspice Format and then they are converted in to the general circuit descriptions.
Fault Free circuit is converted to the test parameters
Fault list is derived from the Circuit Under Test
Faulty Netlist is generated
Simulation was done for Faulty Circuit Under Test
Comparison of the fault output with the fault free and then the fault coverage was calculated.
The Flowchart for the test procedure is as shown below:
Fig8. Testing Procedures
As the testing is very important in the IC testing for improving the realibilty so the testing methods which are making the system more reliable can be used to get high performance that can be provided by using system on chip which are called built in self testing methods and then the testing can be done by using the different fault models and thus the contrallability and the observability are improved.