Designing rules for Analog Circuit PCB.

1. Design Rules For Analog Circuit PCB
Presented By:-
Shovana Khan Yusufzai
M.Tech in E.D&T
NIELIT, Gorakhpur

2. Major Factors in Designing Analog Circuit PCB
• 1 Component Placing
• 2 Signal Conductors
• 3 Supply and Ground Conductors

3. 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.

4. • 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.

5. 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.

6. 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.

7. • 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.

8. • 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.

9. 2.2 Multistage Amplifier with high power
output stage:

10. • 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.

11. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. Discussion? Questions?