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AD652

Rev. C | Page 16 of 28

DECOUPLING AND GROUNDING

It is good engineering practice to use bypass capacitors on the

supply-voltage pins, and to insert small valued resistors (10 Ω to

100 Ω) in the supply lines to provide a measure of decoupling

between the various circuits in a system. Ceramic capacitors of

0.1 µF to 1.0 µF should be applied between the supply voltage

pins and analog signal ground for proper bypassing on the

AD652.

Additionally, a larger board-level decoupling capacitor of 1 µF

to 10 µF should be located relatively close to the AD652 on each

power supply line. Such precautions are imperative in high

resolution data acquisition applications where one expects to

exploit the full linearity and dynamic range of the AD652.

Separate digital and analog grounds are provided on the AD652.

Only the emitter of the open-collector frequency output

transistor and the clock input threshold are returned to the

digital ground. Only the 5 V reference is connected to analog

ground. The purpose of the two separate grounds is to allow

isolation between the high precision analog signals and the

digital section of the circuitry. Much noise can be tolerated on

the digital ground without affecting the accuracy of the VFC.

Such ground noise is inevitable when switching the large

currents associated with the frequency output signal.

At high full-scale frequencies, it is necessary to use a pull-up

resistor of about 500 Ω in order to get the rise time fast enough

to provide well-defined output pulses. This means that from a

5 V logic supply, for example, the open collector output draws

10 mA. This much current being switched causes ringing on

long ground runs due to the self-inductance of the wires. For

instance, 20-gauge wire has an inductance of about 20 nH per

inch; a current of 10 mA being switched in 50 ns at the end of

12 inches of 20-gauge wire produces a voltage spike of 50 mV.

The separate digital ground of the AD652 easily handles these

types of switching transients.

A problem remains from interference caused by radiation of

electromagnetic energy from these fast transients. Typically, a

voltage spike is produced by inductive switching transients;

these spikes can capacitively couple into other sections of the

circuit. Another problem is ringing of ground lines and power

supply lines due to the distributed capacitance and inductance

of the wires. Such ringing can also couple interference into

sensitive analog circuits. The best solution to these problems is

proper bypassing of the logic supply at the AD652 package. A

1 µF to 10 µF tantalum capacitor should be connected directly

to the supply side of the pull-up resistor and to the digital

ground, Pin 12. The pull-up resistor should be connected

directly to the frequency output, Pin 11. The lead lengths on the

bypass capacitor and the pull-up resistor should be as short as

possible. The capacitor supplies (or absorbs) the current

transients, and large ac signals flow in a physically small loop

through the capacitor, pull-up resistor, and frequency output

transistor. It is important that the loop be physically small for

two reasons: first, there is less inductance if the wires are short,

and second, the loop does not radiate RFI efficiently.

The digital ground (Pin 12) should be separately connected to

the power supply ground. Note that the leads to the digital

power supply are only carrying dc current. There may be a dc

ground drop due to the difference in currents returned on the

analog and digital grounds. This does not cause a problem;

these features greatly ease power distribution and ground

manage-ment in large systems. The proper technique for

grounding requires separate digital and analog ground returns

to the power supply. Also, the signal ground must be referred

directly to the analog ground (Pin 6) at the package. More

information on proper grounding and reduction of interference

can be found in Noise Reduction Techniques in Electronic

Systems

, by H.W. Ort, (John Wiley, 1976).

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