Design Rules For Analog Circuit PCB

Presented By:-Shovana Khan Yusufzai

M.Tech in E.D&TNIELIT, Gorakhpur

Major Factors in Designing Analog Circuit PCB

• 1 Component Placing• 2 Signal Conductors• 3 Supply and Ground Conductors

1. Component Placing• Components which need access from the front plate

have to be exactly according to the instructions of the equipment designer.

• Components for internal adjustment such as, potentiometer , trimmer, switches etc have to be arranged near the board edge and oriented in the proper direction.

• Component with metal case should not be placed very near to the access path that is needed for adjustment of screw driver other wise short circuit may occur.

• Heat producing components are regularly distributed over the entire board area, as far as it is feasible. This is to avoid a locally overheated board.

• Hot components have to be separated from the board surface by suitable surfacers.

• The locating of heat sensitive components must avoid a close proximity to hot components as well as a place in a flow of already warmed up air.

2. Signal Conductors• Signal Conductors in analog circuit PCBs have to fulfill a

variety of different tasks, e.g. input, reference level, feedback, output, etc.

• The length of any signal conductor should be made as short as possible.This is because the magnitude of the undesired inductive and capacitive coupling effects increases more or less proportionally to the length of the particular conductor.

• Apart from making of a good PCB, there are also other techniques such as shielding or guarding with metallic enclosures which have to be employed for a satisfactory operation of equipment.

The signal conductor layout has to be done particularly carefully for the following type of circuits:

2.1 High Frequency Amplifiers/oscillators:• At higher frequencies and with a long voltage supply line,

the circuit may oscillate because the voltage gain in an emitter follower can be high i.e.,

Gain= Zcollector/Zemitter• For precaution a capacitor is placed between the collector

and ground.

• And in case of oscillator similar problem occurs ,so to avoid this, the layout has to be carefully made in order to reduce the capacitive coupling.

• An improper layout of a high frequency amplifier can also result in a reduced bandwidth of the amplifier. If the input and output conductors are close to each other , there can be a feedback resulting in oscillations.

2.2 Multistage Amplifier with high power output stage:

• If the ground and voltage conductors are long , a large current I drawn by the high-power stage will flow through the conductors with their own resistivity R.

• This additional resistance can modulate the Vcc and GND of the input stage resulting in low frequency oscillations which is called motorboating effect.

• To overcome this effect decoupling of the power supply conductors with sufficiently large capacitor must be done.

• Also separate Vcc and GND conductors can be provided for two different stages.

2.3 Feedback Amplifiers

• In designing high-wattage power supplies considerable care in PCB layout design as well as in planning of the remote sensing line to the front panel is necessary to meet the regulation factor.

• A star grounding technique is used for the feedback amplifier PCBs. The rectifier/capacitor circuit must be connected to the star point for good cicuit performance.

2.4 High Gain DC Amplifiers(Thermal Effects):

• When a device(transistor, DC amplifier) is soldered onto the PCB, a thermocouple junction is formed between copper( i.e. PCB conductor) and the kovar lead of the transistor.

• If a temperature gradient exists, then different junctions will create different voltages, thus effecting a noisy signal to the amplifier.

2.5 Amplifier handling low level signals

a) Low level, high impedance:Due to high impedance the capacitive coupling occurs . For

reducing the coupling , it is necessary to provide sufficient distance between the high impedance conductors and other interfering signal lines.

b) Low level, low Impedance:If the impedance is low one has to be careful about induced

voltage due to magnetic fields or inductive coupling.

3. Supply and Ground Conductors

3.1 Ground Lines• Analog and digital circuits on the

same PCB should strictly have independent ground conductor networks.

• There is one more type of circuit part which should be provided with separate ground conductors: Reference voltage circuits. These are sensitive to ground potential fluctuations because the difference voltage between an input and reference is mostly amplified and \or used to drive a switching device.

3.2 Supply Lines

• Supply lines have to be provided with sufficient width to keep resistance and inductivity low.

• Supply lines are connceted to the circuit end with the highest signal power level.

• Reference voltage circuits should tap with the supply lines directly at the input to the PCB.

Discussion? Questions?