Analog Technology FAQs

The bottleneck of parallel communication: Parallel data transmission technology has always been an important means to improve the data transmission rate, but further development has encountered obstacles. First of all, because the premise of the parallel transmission method is to use the same timing to propagate signals and receive signals at the same timing, it will be difficult to excessively increase the clock frequency to make the timing of the data transmission and the timing of the clock coincide. The wiring length is slightly different and the data will be different from the clock. The timing of delivery, in addition, increase the clock frequency is also easy to cause interference between the signal lines, resulting in transmission errors.

The advantage of serial communication: Although serial communication has only one bit width, the data transmission speed is higher than that of the parallel port because of its differential structure and strong anti-jamming capability.

What is the value of the pull-up resistor on the open drain, which cannot be large or small?

If the pull-up resistor value is too small, the VDD sink port current (Ic) will be large, which will lead to incomplete conduction of MOS transistor (triode) (Ib*β

If the pull-up resistance is too large, plus the line capacitance on the line, due to RC influence, it will increase the rise time, and the pull-up resistance is too large (the deferred delay is the transistor in the chip, is the active drive, and the speed is fast The rising delay is the passive external resistance, and the speed is slow, which causes the increase of the output impedance. When the output impedance and the load impedance can be compared, the high level of the output will be divided and reduced.

Push-pull can be driven directly, with strong drive capability, without line and function.

Open-drain pull-ups require an external pull-up resistor, weak drive capability, line and function, but it will result in extended signal rise time

For general CPUs, the GPIO power-on state is the input high-impedance state, and its state is determined by the peripheral circuit. If the external pull-up resistor, it is high. If external pull-down resistor, it is low level. Suspension is a high-impedance state.

However, some CPU power-on GPIO ports are high by default because weak pull-ups are enabled by default after CPU reset, such as F020.

1. Asynchronous transmission is character-oriented transmission while synchronous transmission is bit-oriented transmission.

2. The unit of asynchronous transmission is the character and the unit of synchronous transmission is the frame.

3, asynchronous transmission captures the opportunity for resynchronization by starting and stopping the character, and synchronous transmission extracts the synchronization information in the data.

4. Asynchronous transmissions have lower requirements on timing, and synchronous transmissions often coordinate timing through specific clock lines.

5. Asynchronous transmission is less efficient than synchronous transmission.

Synchronous transmission means that both parties have a common clock reference and can accurately send and receive data through the clock reference. Usually, this clock reference is the synchronous clock line or the same clock source. Asynchronous transmission refers to the fact that there is no common clock reference for both parties in communication. Usually, preamble synchronization is required for each data transmission and reception.

Synchronous: SPI, STM, parallel port of computer: RS-232 serial port, USB

(0) The input impedance is for the load, and the output impedance is mainly for the power supply. . .

The internal resistance of voltage and current is equivalent to the output impedance.

The load is relative to the input impedance.

For a voltage source, the larger the input impedance is, the better (for example, an actual power source consists of a power source plus internal resistance. When the load is too small (the input impedance is too small), obviously the voltage obtained by the load will be less than the power source voltage, resulting in deviation).

For a current source, the smaller the input impedance, the better (a constant current source is composed of a current source plus internal resistance, and when the load is much smaller than the internal resistance, the constant current can not be guaranteed). When the oscilloscope is connected to the optical probe, the probe's input impedance must be changed to 50 ohms

(1) What is the difference between RAIL-TO-RAIL op amp and general op amp?

The output range of the r2r op amp is almost equal to the supply voltage. The general op amp was reduced by one or two volts.

RAIL-TO-RAIL is rail-to-rail type, with output voltage close to the supply voltage

(2) The advantages of ferroelectric memory

At present, Ramtron's FRAM mainly includes two major categories: serial FRAM and parallel FRAM. Among them, serial FRAM is divided into I2C 2-wire FM24 series and SPI 3-wire FM25 series. The serial FRAM is compatible with the conventional 24-25 E2PROM pins and timing and can be directly replaced.

The advantage of ferroelectricity is that it is equivalent to an unlimited number of erasures, and writing data does not have to wait 5ms. If you do not consider these two factors can use EEPROM or FLASH. But as long as the price is 20% more expensive than EEPROM

(3) Switching power supply wiring attention?

In the switching power supply, the layout of the PCB layout is very important. The switching current is closely related to the loop inductance. The transient voltage generated by this loop inductance often causes many problems. In order to make this kind of induction the smallest, the ground wire forms the circuit, the thick line part shown in the picture should be printed on the PCB board to be a little wider, and should be as short as possible. For best results, external components should be located as close as possible to the switching IC. Ground shield or single-point grounding is preferred. It is best to use an inductor with a magnetic shield structure. If the inductor used is core open, care must be taken with its position. If the inductance flux crosses the sensitive feedback line, the connection of the ground line of the integrated circuit and the capacitance COUT of the output terminal may cause some problems. In the output adjustable solution, special attention must be paid to the position of the feedback resistor and its associated wire. Physically, on the one hand, the resistor should be close to the IC, and on the other hand, the relevant connection should be far away from the inductor. If the inductor used is open-core, this is all the more important.

(4) The op amp power supply sometimes has positive and negative dual power supply +VCC and -VCC. How do you connect it? Sometimes why is there a single power supply for the op amp?

Each system has a relative ground potential, which is GND in the circuit. Relative to this potential, determine +VCC and -VCC. The connection method in the circuit is that the ground wire of the power supply is connected to the GND of the circuit, and the power supply +VCC is connected to the circuit +VCC. For the negative voltage, the power wire of the high point is connected to the GND of the circuit, and the ground wire is connected thereto. At the -VCC position of the circuit, the relative potential requirement can be reached.

For example, two irrelevant 5V power supplies are connected together, the middle point is grounded, and both ends are +5V and -5V.

(5) External resistance value of the op amp?

Normally, the value of the matching resistor of our op amp is generally between 1K and tens of K. What are the reasons?

The external resistance cannot be made too large. It is not suitable to use MΩ class. There are two reasons for this: 1 The resistance is error, and the larger the resistance, the greater the absolute error. Such as 2MΩ resistor E1: series resistance error value: 10%, its resistance (2.2 ~ 1.8) MΩ range is allowed, even if the election E4s series resistance (error: 2%) resistance range (2.04 ~ 1.96) Within MΩ; and the resistance value will produce aging error with changes in temperature and time, making the resistance unstable, affecting the accuracy of operation; 2 the small offset current of the op amp will cause larger resistance on the external high resistance value. Error signal.

If the value of the external resistor is too small, the input resistance of the op amp will be reduced, which will increase the power consumption of the op amp, that is, increase the signal source load (here the load is the op amp).

(6) NTC thermistor calculation method?

Now NTC thermistors used in low-cost temperature measurement programs are more commonly used to obtain temperature values ​​by looking up the table, which involves the corresponding relationship between temperature and resistance. If you buy an NTC thermistor from your manufacturer, you can ask the manufacturer to check the temperature resistance. However, for ordinary lovers, the thermistor is bought from the retailer. Uncle and aunt selling components will not provide you with resistance. Value and temperature comparison table. The usual method is to use a standard thermometer, the ambient temperature does not rise once to measure the resistance of the thermistor, through this method to obtain the corresponding relationship between the resistance and temperature is relatively cumbersome, the error is relatively large, and the temperature change is not well controlled; One method is to calculate the RT table by formula. Although the temperature and resistance of the NTC thermistor are not linear, the correspondence between temperature and resistance can be calculated by the following formula:

Rt = R *EXP(B*(1/T1-1/T2))

The above formula is explained as follows:

1. Rt is the resistance of the thermistor at T1 temperature;

2. R is the nominal resistance of the thermistor at room temperature T2;

3. B value is an important parameter of the thermistor;

4. EXP is the nth power of e;

5. Here T1 and T2 refer to K degrees, ie Kelvin temperature, K degrees = 273.15 (absolute temperature) + degrees Celsius;

For example, I have a MF58502F327 thermistor in my hand

MF58 - Model Glass Package

502 - The nominal resistance at room temperature 25 degrees is 5K

F - Allowable deviation is ±1%

327 - NTC thermistor with a B value of 3270K

Then it's R=5000, T2=273.15+25, B=3270, RT=5000*EXP(3270*(1/T1-1/(273.15+25))). At this time, it can be calculated by substituting T1 temperature. The thermistor's resistance at temperature, pay attention to the temperature unit conversion, for example, we require a resistance value of 10 degrees Celsius above zero, then T1 is (273.15+10).

(7) What is the meaning of heavy load and light load? Who can talk about?

For example, if a power supply has a normal working current of 10A, then it can be said that 10A is heavy load, also called full load, 0A is no load, light load cannot be defined specifically, 0.5A is light load, 1A is also light load, 2A is also Calculate light load

The industry generally believes that 10% of the rated current is called light load, 100% of the rated current is called full load, and over rated current is overloaded.

(8) 0805 resistance knowledge

The 0805 package is in inch form 0.08 inch by 0.05 inch and is converted to metric 2.03mm*1.27mm. In China, this power specification is 1/8W (general industrial grade). 1 inch = 2.54 centimeters

(9) Triode and MOS tube understand oneself

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