AD8033 datasheet(15/20 Pages) AD

15 / 20 page

REV. B

AD8033/AD8034

–15–

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. 0.01

µF or 0.001 µF (X7R or NPO) chip capacitors

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 PC boards in

order to spread the current, thereby minimizing parasitic induc-

tances. 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 will create unwanted voltage noise. The

length of the high frequency bypass capacitor leads is most critical.

A parasitic inductance in the bypass grounding will work 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 PC board 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 will set up leakage currents

through the PC board insulator, for example, 1 V/100 G

Ω = 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 are 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 will help

to reduce the absorption. Also, 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 will reduce the input impedance at high

frequencies, in turn increasing the amplifiers’ gain and causing

peaking of the overall 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.

APPLICATIONS

High Speed Peak Detector

The low input bias current and high bandwidth of the AD8033/

AD8034 make the parts ideal for a fast settling, low leakage peak

detector. The classic fast-low leakage topology with a diode in

the output is limited to 1.4 V p-p max in the case of the

AD8033/AD8034 because of the protection diodes across the

inputs, as depicted in Figure 4.

AD8033/

AD8034

VIN

~1.4V p-p MAX

VOUT

Figure 4. High Speed Peak Detector with Limited Input Range

AD8033 Datasheet(PDF) 15 Page - Analog Devices

AD8033 Datasheet(HTML) 15 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL