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Analog Considerations

Power Supply Decoupling

Care should be taken with the design of the printed circuit board when using 12-bit devices such as the TLC2543. The

power supply terminal of each analog integrated circuit should be separately decoupled to the analog ground using a

0.1 µF ceramic capacitor. The inclusion of a 10 µF tantalum capacitor in parallel with the ceramic capacitor at each

device supply terminal is also recommended, particularly in noisy environments.

Grounding

Separate ground return paths for analog and digital components back to the power supply should be used to prevent any

noise currents induced by digital components from passing through the analog ground return path. These noise currents

can induce noise voltages to occur in the analog ground return and thus corrupt the analog signal. Remember that, for

a 5-V full scale signal, only 600 microvolts represents approximately half an LSB for a 12-bit ADC.

The important point to remember is that all ground return paths have a finite impedance. This impedance should be kept

to a minimum by the use of wide printed circuit board tracks or ground planes where possible. A separate star connected

ground topology is recommended for the analog components. This involves connecting each analog component’s

ground terminal to a central star point, which can then be connected via a wide printed circuit track to the power supply

ground connection.

Board Layout

Digital devices and power switching elements should be kept as far away physically from analog components, such as

the TLC2543, as possible. Particular attention should be paid to the use of switching power supplies. The high frequency

switching currents which flow in the ground return paths of these space saving power blocks can introduce several LSBs

of noise into 12-bit analog circuits. Linear regulated power supplies should be used or, if essential, switching regulators

should be as far as possible from the analog circuitry with their outputs decouple.

Judicious use of ground planes can help to reduce analog ground impedances.

Figure 14 illlustrates a typical bypassing scheme for the TLC2543-to-TMS370 microcontroller interface.

I/O CLOCK

DATA INPUT

DATA OUT

TLC2543

SPICLK

SPISIMO

SPISOMI

GND

AIN0

AIN1

AIN2

AIN3

AIN4

AIN5

AIN6

AIN7

AIN8

AIN9

AIN10

TMS370C010

GND

10 µF

Tantalum

10 µF

Tantalum

0.1 µF

Creamic

0.1 µF

Creamic

10 µF

Tantalum

0.1 µF

Creamic

10 µF

Tantalum

0.1 µF

Creamic

Connection To Output

Of Power Supply +

Direction Of

Current Flow

Direction Of

Current Flow

Ground Return

To Power Supply

External

Data/Address

Bus (If Used)

Connection to

Output of

Power Supply –

Figure 14. TLC2543 to Microcontroller Interface: Grounding and Decoupling Scheme

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