12 problems in designing analog circuits

- Sep 15, 2018-

(1) in order to obtain a feedback circuit with good stability, a small resistor or choke coil outside the feedback loop is usually required to provide a buffer to the capacitive load.

(2) the integral feedback circuit usually requires a small resistance (approximately 560 ohms) to be connected in series with each integral capacitance greater than 10pF.

(3) do not use active circuits outside the feedback loop to filter or control the RF bandwidth of EMC, but only use passive components (preferably RC circuits).The integral feedback method is effective only if the operating frequency of the open loop gain is larger than that of the closed loop gain.At higher frequencies, the integrator cannot 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) EMC filter is used, and all IC related filters should be connected with local 0V reference plane.

(6) an input/output filter should be placed at the connection of the external cable. Any wire connection without shielding should be filtered because of the antenna effect.Filtering is also required at the conductor connections within the shield system of a converter with digital signal processing or switching mode.

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

(8) transmission line technology is necessary for high-speed analog signals according to their connection length and the highest frequency of communication.Even for low-frequency signals, the use of transmission line technology can improve its anti-interference, but the wrong transmission line will produce antenna effect.

(9) avoid high-impedance inputs or outputs, which are very sensitive to electric fields.

(10) since most of the radiation is generated by common mode voltage and current, and because most of the electromagnetic interference in the environment is generated by common mode problems, the use of balanced sending and receiving (difference mode) technology in analog circuits will have good EMC effect and reduce crosstalk.Balanced circuit (differential circuit) drives do not use the 0V reference system as the return current loop, thus avoiding large current loop, thereby reducing RF radiation.

(11) the comparator must have a lag (positive feedback) to prevent erroneous output transformations due to noise and interference, as well as to prevent oscillations at break points.Don't use a faster comparator than you need (keep dV/dt within the required range, as low as possible).

(12) some analog IC itself is particularly sensitive to rf fields, so it is often necessary to shield such analog components by using a small metal shield box mounted on a PCB and connected to the ground surface of the PCB.Be careful to ensure the heat dissipation strip.