Data Sheet

AD8000

Rev. C | Page 15 of 17

LOW DISTORTION PINOUT

The AD8000 LFCSP features Analog Devices low distortion

pinout. The new pinout lowers the second harmonic distortion

and simplifies the circuit layout. The close proximity of the

non-inverting input and the negative supply pin creates a source

of second harmonic distortion. Physical separation of the non-

inverting input pin and the negative power supply pin reduces

this distortion significantly, as seen in Figure 22.

By providing an additional output pin, the feedback resistor can

be connected directly across Pin 2 and Pin 3. This greatly simplifies

the routing of the feedback resistor and allows a more compact

circuit layout, which reduces its size and helps to minimize

parasitics and increase stability.

The SOIC also features a dedicated feedback pin. The feedback

pin is brought out on Pin 1, which is typically a no connect on

standard SOIC pinouts.

Existing applications that use the standard SOIC pinout can

take full advantage of the performance offered by the AD8000.

For drop-in replacements, ensure that Pin 1 is not connected to

ground or to any other potential because this pin is connected

internally to the output of the amplifier. For existing designs,

Pin 6 can still be used for the feedback resistor.

EXPOSED PADDLE

The AD8000 features an exposed paddle, which can lower the

thermal resistance by 25% compared to a standard SOIC plastic

package. The paddle can be soldered directly to the ground plane

of the board. Figure 53 shows a typical pad geometry for the LFCSP,

the same type of pad geometry can be applied to the SOIC package.

Thermal vias or heat pipes can also be incorporated into the design

of the mounting pad for the exposed paddle. These additional

vias improve the thermal transfer from the package to the PCB.

Using a heavier weight copper on the surface to which the exposed

paddle of the amplifier is soldered also reduces the overall

thermal resistance seen by the AD8000.

Figure 53. LFCSP Exposed Paddle Layout

PRINTED CIRCUIT BOARD LAYOUT

Laying out the printed circuit board (PCB) is usually the last

step in the design process and often proves to be one of the

most critical. A brilliant design can be rendered useless because

of a poor or sloppy layout. Because the AD8000 can operate

into the RF frequency spectrum, high frequency board layout

considerations must be taken into account. The PCB layout,

signal routing, power supply bypassing, and grounding all must

be addressed to ensure optimal performance.

SIGNAL ROUTING

The AD8000 LFCSP features the new low distortion pinout with a

dedicated feedback pin and allows a compact layout. The dedicated

feedback pin reduces the distance from the output to the inverting

input, which greatly simplifies the routing of the feedback network.

To minimize parasitic inductances, use ground planes under

high frequency signal traces. However, remove the ground

plane from under the input and output pins to minimize the

formation of parasitic capacitors, which degrades phase margin.

Run signals that are susceptible to noise pickup on the internal

layers of the PCB, which can provide maximum shielding.

POWER SUPPLY BYPASSING

Power supply bypassing is a critical aspect of the PCB design

process. For best performance, the AD8000 power supply pins

need to be properly bypassed.

A parallel connection of capacitors from each of the power supply

pins to ground works best. Paralleling different values and sizes

of capacitors helps to ensure that the power supply pins see a

low ac impedance across a wide band of frequencies. This is

important for minimizing the coupling of noise into the amplifier.

Starting directly at the power supply pins, place the smallest

value and sized component on the same side of the board as the

amplifier, and as close as possible to the amplifier, and connected

to the ground plane. Repeat this process for the next larger value

capacitor. It is recommended for the AD8000 that a 0.1 μF ceramic

0508 case be used. The 0508 offers low series inductance and

excellent high frequency performance. The 0.1 μF case provides

low impedance at high frequencies. Place a 10 μF electrolytic

capacitor in parallel with the 0.1 μF. The 10 μF capacitor provides

low ac impedance at low frequencies. Smaller values of electrolytic

capacitors can be used, depending on the circuit requirements.

Additional smaller value capacitors help to provide a low

impedance path for unwanted noise out to higher frequencies

but are not always necessary.

Placement of the capacitor returns (grounds), where the capacitors

enter into the ground plane, is also important. Returning the

capacitors grounds close to the amplifier load is critical for

distortion performance. Keeping the capacitors distance short,

but equal from the load, is optimal for performance.