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55 Analog Circuits Basics for Beginner

Author: Iggy Date: 13 Apr 2021  484


HC is COMS level and HCT is TTL level;


LS input open circuit is high level, HC input is not allowed to open circuit. HC generally requires a pull-up and pull-down resistor to determine the level when the input terminal is invalid. But LS does not have this requirement;


LS output pull-down is strong, pull-up is weak, HC pull-up and pull-down are the same;


Working voltage: LS can only use 5V, while HC is generally 2V to 6V;


CMOS can drive TTL, but not vice versa. When the TTL circuit drives the COMS circuit, a pull-up resistor needs to be added to pull up the voltage between 2.4V and 3.6V, so that the CMOS can detect the high-level input;


The driving ability is different. The driving ability of LS is generally 5mA for high level and 20mA for low level; while the high and low level of CMOS are both 5mA;


RS232 level is +12V for logic negative, -12 for logic positive;

electronic components


The 74 series are for commercial use and 54 are for military use


TTL high level> 2.4V and TTL low level <0.4V, with noise tolerance 0.4V

10. OC gate, which is an open-collector gate circuit (why is there an OC gate? Because of the realization of "wire-AND" logic), OD gate, which is an open-drain gate circuit, must be pulled up by an external resistor and power supply to switch the level Used as high and low level. Otherwise, it is generally only used to switch high voltage and high current loads, so it is also called drive gate circuit. And it can only absorb current, and only an external pull-up resistor and power supply can output current externally.


If the input current of COMS exceeds 1mA, COMS may be burnt out


When connecting a long signal transmission line, connect a matching resistor to the COMS circuit terminal


Connect a 10K resistor in series at the input of the gate circuit and then input a low level. The input end shows a high level instead of a low level.


If there is a 3.3V COMS circuit to drive a 5VCMOS circuit in the circuit, such as a 3.3V microcontroller to drive 74HC, there are several ways to solve this situation. The simplest is to directly replace the 74HC with a 74HCT chip, because 3.3VCMOS can directly drive a 5V TTL circuit; or add a voltage conversion chip; another is to set the I/O port of the microcontroller to open drain, and then add a pull-up resistor to 5V. In this case, you need to adjust the resistance according to the actual situation. Size to ensure the rising edge time of the signal.


The load current when the logic gate output is high level (sourcing current), and the load current when the logic gate output is low level (sinking current)


Since the drain stage is open, the subsequent circuit must be connected to a pull-up resistor, and the power supply voltage of the pull-up resistor can determine the output level. In this way, the open-drain form can be connected to devices of different levels for level conversion. One thing to note: The load is charged by an external pull-up passive resistor during the rising edge, so the rising edge time may not be fast enough, try to use the falling edge


Several level conversion methods:

(1) Transistor + pull-up resistor method
It is a bipolar transistor or MOSFET, the C/D pole is connected to a pull-up resistor to the positive power supply, the input level is very flexible, and the output level is roughly the positive power level.

(2) OC/OD device + pull-up resistor method
Similar to 1). It is suitable for occasions where the device output happens to be OC/OD.

(3) 74xHCT series chip boost (3.3V→5V)
Any 5VCMOS device whose input is compatible with 5VTTL level can be used for 3.3V→5V level conversion.

——This is because the level of 3.3VCMOS is just compatible with the level of 5VTTL (coincidence), while the output level of CMOS is always close to the power supply level.

Inexpensive choices such as 74xHCT (HCT/AHCT/VHCT/AHCT1G/VHCT1G/...) series (the letter T means TTL compatible).

(4) Over-limit input step-down method (5V→3.3V, 3.3V→1.8V,...)

Any logic device that allows the input level to exceed the power supply can be used to reduce the level.

The "over-limit" here refers to exceeding the power supply. Many older devices do not allow the input voltage to exceed the power supply, but more and more new devices cancel this limitation (change the input-level protection circuit).

For example, 74AHC/VHC series chips, its datasheets clearly indicate "input voltage range is 0~5.5V", if 3.3V power supply is used, 5V→3.3V level conversion can be realized.

(5) Dedicated level conversion chip
The most famous is the 164245, which can not only be used as a boost/buck, but also allows the power supplies on both sides to be asynchronous. This is the most common level conversion scheme, but it is also very expensive. Therefore, if it is not necessary, it is best to use the first two options.

(6) Resistance divider method
This is the easiest way to reduce the level. With 5V level, divided by 1.6k+3.3k resistors, it is 3.3V.

(7) Current limiting resistance method


Non-polarized capacitors and polarized capacitors: The package of the former is basically 0805,0603. The latter uses aluminum electrolytic capacitors at most, better tantalum capacitors


PQFP (Plastic Quad Flat Package), BGA (Ball Grid Array Package), PGA (Pin Grid Array Package), PLCC (Plastic Leaded Chip Carrier), SOP (Small Outline Package), TOSP (Thin Small Outline Package), SOIC (Small Outline Integrated Circuit Package)

  • Common packaging forms of integrated circuits:
  • QFP (quad flatpa ckage) has gull wing feet on all sides (package)
  • BGA (ball gridarray) ball grid array (package)
  • PLCC (plastic leaded chip carrier) has inner hook type pins on four sides (package)
  • SOJ (small out line junction) has inner hook type pins on both sides (package)
  • SOIC (small out line integrated circuit) has gull-wing pins on both sides (package)


Shielded wires have a strong inhibitory effect on static electricity, and twisted-pair wires also have a certain inhibitory effect on electromagnetic induction.


Analog signal sampling anti-jamming technology: it is possible to use a measuring amplifier with differential input, use shielded double glue lines to transmit measurement signals, or change voltage signals to current signals, and use resistance-capacitance filtering techniques


Do not leave unused IC pins floating in order to avoid interference. The positive input terminal of the unused operational amplifier is grounded, and the negative input terminal is connected to the output. The unused I/O ports of the MCU are defined as outputs. There are more than one power and ground terminals on the single-chip microcomputer, each of which must be connected, and should not be left floating


Color ring representation of resistance value: ordinary color ring resistors are represented by 4 rings, and precision resistors are represented by 5 rings.


The function of resistance is shunting, current limiting, voltage division, biasing, filtering (used in combination with capacitors) and impedance matching, etc.


The role of capacitors: DC blocking, bypass, coupling, filtering, compensation, charging and discharging, energy storage, etc.



The digital representation unit of general capacitors is pF, and electrolytic capacitors are generally uF


The main performance indicators of the capacitor:
The capacity of the capacitor (that is, the capacity of the stored charge);
Withstand voltage (refers to the maximum DC voltage or the effective value of the maximum AC voltage at which the capacitor can work reliably for a long time within the rated temperature range);
Temperature resistance value (indicating the maximum operating temperature that the capacitor can withstand.).


The role of inductors: filter, trap, oscillate, store magnetic energy, etc.


Classification of inductors: air core inductance and magnetic core inductance. Magnetic core inductance can also be called iron core inductance and copper core inductance, etc.

Classification of semiconductor diodes
Classified by material: silicon diode and germanium diode;
According to the purpose: rectifier diode, detector diode, voltage stabilizer diode, light emitting diode, photodiode, varactor diode.


The field effect tube is a voltage control element, and the transistor is a current control element. When only allowing less current to be drawn from the signal source, FETs should be used; and when the signal voltage is low and allowing more current to be drawn from the signal source, transistors should be used


Socket is a form of socket packaging, which is a rectangular socket;
Slot is a form of slot packaging, a rectangular slot


Crystal oscillator measurement method: use the multimeter RX10K file to measure the positive and negative resistance values ​​of the quartz crystal oscillator. Normally, it should be infinite. If the measured resistance of the quartz crystal oscillator is zero or zero, it means that the quartz crystal oscillator has a certain resistance value or zero. The crystal oscillator has leaked or broken down

When the IO port outputs high level, the driving ability is the lowest, and the external display is the push current;
When the IO port outputs a low level, the drive capacity is the largest, and the external display is a source of current


If the peripheral integrated digital drive circuit is driving an inductive load, a current limiting resistor or a diode must be added


9013 provides a drive current of 300mA

The output data should be latched (the peripheral speed cannot keep up, so it needs to be latched);
The input data should have a three-state buffer (a high-impedance state is added to not affect the internal data bus)


8-bit parallel output port (must have a latch function): 74LS377, 74LS27 3.8-bit parallel input port (must be a three-state gate): 74LS373, 74LS244


Serial port expansion parallel port, parallel input port: 74LS165. Parallel output port: 74LS164


There are three keyboard working modes:
1. Program scan mode
2. Timing scan mode
3. Interruption mode.
An IO port can also be specially designed for the design of dual function keys (upshift key and downshift key)


For TTL loads, DC load characteristics should be mainly considered, because TTL has a large current and small distributed capacitance. For MOS-type loads, AC load characteristics should be mainly considered, because the input current of MOS-type loads is small, and distributed capacitance is mainly considered


Pay special attention to the concept of bus load balancing!


The benefits of pull-up resistors:
1. Increase the signal level
2. Improve the anti-electromagnetic interference ability of the bus (electromagnetic signals enter the CPU through DB)
3. Suppress electrostatic interference (CMOS chip)
4. Reflected wave interference (long-distance transmission)


When voltage stabilizing, it is better to use two-stage integrated voltage stabilizing chip.


Impedance matching of transmission line:
1. Terminal parallel impedance matching (high level drops)
2. The start end is matched in series (low level is raised)
3. Terminal parallel DC isolation matching (RC series grounding)
4. Terminal connected to clamp diode

46. ​​

There are two types of grounding: shell grounding (real grounding) and working grounding (floating ground)


Types of ground in the single-chip microcomputer: digital ground, analog ground, power ground (large current, thick ground wire), signal ground, AC ground, shield ground

One point grounding: low frequency circuit (below 1MHZ)
Multi-point grounding: high frequency circuit (above 10MHZ)


The AC ground and signal ground cannot be shared. Digital ground and analog ground should be separated and connected at one point.

Vibration circuit: Mica, high-frequency ceramic capacitors can be selected;
DC blocking: paper, polyester, mica, electrolytic, ceramic and other capacitors can be selected;
Filtering: Electrolytic capacitors can be used;
Bypass: polyester, paper, ceramic, electrolytic and other capacitors can be used



Diode application circuit

  1. Limiting circuit --- It is composed of the unidirectional conductivity of the diode and the basically constant voltage at both ends after it is turned on. It can limit the signal to change in a certain range, and is divided into single-limiting and double-limiting circuits. This circuit is mostly used in signal processing circuits.
  2. Clamping circuit --- Clamp the output voltage to a certain value.
  3. Switching circuit---Using the unidirectional conductivity of a diode to make and break the circuit, which is widely used in digital circuits.
  4. Rectifier circuit---Using the unidirectional conductivity of diodes to convert AC signals into DC signals, it is widely used in DC stabilized power supplies.
  5. Low-voltage voltage stabilizing circuit---Using the characteristic that the voltage at both ends of the diode is basically unchanged after the diode is turned on, several diodes are connected in series to obtain an output voltage below 3V


The high frequency bypass capacitor is generally relatively small, and is generally 0.1u, 0.01u, etc. according to the resonance frequency. The decoupling capacitor is generally larger, 10uF or greater


Summary of pull-up resistors:

  1. When the TTL circuit drives the CMOS circuit, if the high level output by the TTL circuit is lower than the lowest high level of the CMOS circuit (usually 3.5V), a pull-up resistor needs to be connected to the output terminal of the TTL. This can achieve the purpose of increasing the value of the output high level.
  2. The OC gate circuit must be added with a pull-up resistor before it can be used.
  3. To increase the drive capability of the output pins, pull-up resistors are often used on some single-chip microcomputer pins.
  4. On the COMS chip, in order to prevent damage caused by static electricity, the unused pins cannot be left floating. Generally, a pull-up resistor is connected to reduce the input impedance and provide a leakage path.
  5. Add pull-up resistors to the pins of the chip to increase the output level, thereby increasing the noise margin of the chip's input signal and enhancing the anti-interference ability.
  6. Improve the anti-electromagnetic interference capability of the bus. If the pin is left floating, it is easier to accept external electromagnetic interference.
  7. In the long-line transmission, the resistance mismatch can easily cause the reflected wave interference, and the pull-down resistance is the resistance matching, which effectively suppresses the reflected wave interference.

It should be large enough to save power consumption and the current sink capability of the chip; the resistance is large and the current is small.

It should be small enough to ensure enough drive current; the resistance is small, the current is large.


The pull-up is to clamp the uncertain signal at a high level through a resistor.


Bypass capacitor: Generate an AC shunt to eliminate unnecessary energy that enters the susceptible area. Decoupling capacitor: Provide a local DC power supply to the active device to reduce the propagation of switching noise on the board and guide the noise to the ground (its value is about 1/100 to 1/1000 of the bypass capacitor

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