Notes on the use of COMS circuits

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Notes on the use of COMS circuits [Copy link]

1) COMS circuit is a voltage-controlled device, its input total impedance is very large, and it has a strong ability to capture interference signals. Therefore, do not leave unused pins floating, but connect pull-up resistors or pull-down resistors to give it a constant level. 2) When the input terminal is connected to a signal source with low internal resistance, a current-limiting resistor should be connected in series between the input terminal and the signal source to limit the input current to within 1mA. 3) When connecting a long signal transmission line, connect a matching resistor to the COMS circuit terminal. 4) When the input terminal is connected to a large capacitor, a protective resistor should be provided between the input terminal and the capacitor. The resistance value is R=V0/1mA. V0 is the voltage on the external capacitor. 5) If the input current of COMS exceeds 1mA, it may burn out COMS. 7. Input load characteristics in TTL gate circuit (handling of special cases with resistors at the input): 1) When it is suspended, it is equivalent to the input end being connected to a high level. Because at this time, it can be regarded as an infinite resistor at the input end. 2) After a 10K resistor is connected in series at the input end of the gate circuit and a low level is input, the input end presents a high level instead of a low level. Because from the input end load characteristics of the TTL gate circuit, only when the series resistance connected to the input end is less than 910 ohms, the low level signal input can be recognized by the gate circuit. If the series resistance is larger, the input end will always present a high level. This must be paid attention to. COMS gate circuits do not need to consider these. 8. TTL circuits have open collector OC gates, and MOS tubes also have OD gates with open drains corresponding to the collector. Its output is called open drain output. The OC gate has a leakage current output when it is cut off, which is the leakage current. Why is there a leakage current? That is because when the transistor is cut off, its base current is approximately equal to 0, but it is not really 0, and the current passing through the collector of the transistor is not really 0, but about 0. And this is the leakage current. Open drain output: The output of the OC gate is an open drain output; the output of the OD gate is also an open drain output. It can absorb a large current, but cannot output current to the outside. Therefore, in order to input and output current, it must be used together with a power supply and a pull-up resistor. The OD gate is generally used as an output buffer/driver, a level converter, and to meet the need to absorb large load current.