AD8033/AD8034

Rev. D | Page 18 of 24

LAYOUT, GROUNDING, AND BYPASSING CONSIDERATIONS

BYPASSING

Power supply pins are actually inputs, and care must be taken

so that a noise-free stable dc voltage is applied. The purpose of

bypass capacitors is to create low impedances from the supply

to ground at all frequencies, thereby shunting or filtering a

majority of the noise. Decoupling schemes are designed to

minimize the bypassing impedance at all frequencies with a

parallel combination of capacitors. The chip capacitors, 0.01 μF

or 0.001 μF (X7R or NPO), are critical and should be placed as

close as possible to the amplifier package. Larger chip capacitors,

such as the 0.1 μF capacitor, can be shared among a few closely

spaced active components in the same signal path. The 10 μF

tantalum capacitor is less critical for high frequency bypassing, and

in most cases, only one per board is needed at the supply inputs.

GROUNDING

A ground plane layer is important in densely packed PCBs to

spread the current, thereby minimizing parasitic inductances.

However, an understanding of where the current flows in a

circuit is critical to implementing effective high speed circuit

design. The length of the current path is directly proportional

to the magnitude of the parasitic inductances and, thus, the

high frequency impedance of the path. High speed currents

in an inductive ground return create unwanted voltage noise.

The length of the high frequency bypass capacitor leads is most

critical. A parasitic inductance in the bypass grounding works

against the low impedance created by the bypass capacitor.

Place the ground leads of the bypass capacitors at the same

physical location.

Because load currents flow from the supplies as well, the ground

for the load impedance should be at the same physical location

as the bypass capacitor grounds. For the larger value capacitors

that are intended to be effective at lower frequencies, the current

return path distance is less critical.

LEAKAGE CURRENTS

Poor PCB layout, contaminants, and the board insulator material

can create leakage currents that are much larger than the input

bias currents of the AD8033/AD8034. Any voltage differential

between the inputs and nearby runs set up leakage currents

through the PCB insulator, for example, 1 V/100 GΩ = 10 pA.

Similarly, any contaminants on the board can create significant

leakage (skin oils are a common problem). To significantly reduce

leakages, put a guard ring (shield) around the inputs and input

leads that is driven to the same voltage potential as the inputs.

This way there is no voltage potential between the inputs and

surrounding area to set up any leakage currents. For the guard

ring to be completely effective, it must be driven by a relatively

low impedance source and should completely surround the input

leads on all sides, above, and below using a multilayer board.

Another effect that can cause leakage currents is the charge

absorption of the insulator material itself. Minimizing the amount

of material between the input leads and the guard ring helps to

reduce the absorption. In addition, low absorption materials

such as Teflon® or ceramic may be necessary in some instances.

INPUT CAPACITANCE

Along with bypassing and ground, high speed amplifiers can be

sensitive to parasitic capacitance between the inputs and

ground. A few pF of capacitance reduces the input impedance at

high frequencies, in turn it increases the gain of the amplifier

and can cause peaking of the overall frequency response or even

oscillations if severe enough. It is recommended that the external

passive components that are connected to the input pins be placed

as close as possible to the inputs to avoid parasitic capacitance.

The ground and power planes must be kept at a distance of at

least 0.05 mm from the input pins on all layers of the board.