How to design analog circuit ?

- Oct 25, 2017-

Analog circuit design is the most troublesome, but also the most lethal part of the design of engineers! We will summarize the analog circuit design should pay attention to the problem, and share with you.

(1) in order to obtain feedback circuits with good stability, it is usually required to use a small resistor or choke outside the feedback loop to provide a buffer for capacitive loads.

(2) an integral feedback circuit usually requires a small resistor (about 560 Euro) connected in series with an integral capacitor greater than 10pF.

(3) do not use the active circuit to filter or control the RF bandwidth of the EMC, but only use the passive component (preferably RC circuit). The integral feedback method is effective only when the open loop gain of the opamp is greater than that of the closed-loop gain. At higher frequencies, the integral circuit does not control the frequency response.

(4) in order to obtain a stable linear circuit, all connections must be protected by passive filters or other suppression methods, such as photoelectric isolation.

(5) the EMC filter is used, and the IC related filter should be connected to the local 0V reference plane.

(6) the input and output filter should be placed at the junction of the external cable, and any filter at the wire connection without the shielding system needs to be filtered because of the antenna effect. In addition, filtering is required at the wire connection in the shielding system of the converter with digital signal processing or switching mode.

(7) the analog IC power supply and ground reference pins require high quality RF decoupling, which is the same as digital IC. However, analog IC usually requires low frequency power decoupling, because the power noise suppression ratio (PSRR) of analog components increases little after 1KHz. The RC or LC filter should be used in the analog power line of each op amp, comparator and data converter. The corner frequency of the power filter should compensate the PSRR corner frequency and slope of the device so as to obtain the desired PSRR in the whole operating frequency range.

(8) for high speed analog signals, transmission line technology is necessary according to the length of connection and the highest frequency of communication.  Even low frequency signals can be improved by using transmission line technology, but the transmission line without proper matching will produce antenna effect.

(9) avoid using high impedance inputs or outputs, which are very sensitive to the electric field.

(10) because most of the radiation is generated by the common mode voltage and current, and because most of the electromagnetic interference environment is common problems, so in the analog circuit using the balance of the sending and receiving (differential mode) technology will have a good EMC effect, and can reduce the crosstalk. The balance circuit (differential circuit) driver does not use the 0V reference system as the return current loop, so it can avoid the large current loop, thus reducing the RF radiation.

(11) the comparator must have hysteresis (positive feedback) to prevent the wrong output transformation due to noise and interference, and also to prevent oscillation at the break point. Don't use a comparator that needs to be faster (keep the dV/dt within the required range, as low as possible).

(12) some analog IC itself is particularly sensitive to the radio frequency field, so it is often necessary to use a small metal shielding box mounted on the PCB and connected to the PCB horizon to shield such analog components. Attention should be paid to the heat dissipation strip.

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