AD8007/AD8008

Rev. E | Page 17 of

20

LAYOUT AND GROUNDING CONSIDERATIONS

GROUNDING

A ground plane layer is important in densely packed printed

circuit boards (PCB) to minimize 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

parasitic inductances and thus the high frequency impedance of

the path. High speed currents in an inductive ground return

create unwanted voltage noise. Broad ground plane areas reduce

parasitic inductance.

INPUT CAPACITANCE

Along with bypassing and ground, high speed amplifiers can be

sensitive to parasitic capacitance between the inputs and ground.

Even 1 pF or 2 pF of capacitance reduces the input impedance at

high frequencies, in turn increasing the gain of the amplifier, which

causes peaking of the frequency response or even oscillations if

severe enough. Place the external passive components that are

connected to the input pins 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.

OUTPUT CAPACITANCE

To a lesser extent, parasitic capacitances on the output can cause

peaking of the frequency response. The following two methods

minimize its effect:

•

Put a small value resistor in series with the output to isolate

the load capacitance from the output stage of the amplifier

(see Figure 12).

•

Increase the phase margin by increasing the gain of the

amplifier or by increasing the value of the feedback resistor.

INPUT-TO-OUTPUT COUPLING

To minimize capacitive coupling, the input and output signal

traces should not be parallel. When they are not parallel, they

help reduce unwanted positive feedback.

EXTERNAL COMPONENTS AND STABILITY

The AD8007/AD8008 are current feedback amplifiers and, to a

first order, the feedback resistor determines the bandwidth and

stability. The gain, load impedance, supply voltage, and input

impedances also have an effect.

Figure 11 shows the effect of changing RF on the bandwidth and

peaking for a gain of 2. Increasing RF reduces peaking but also

reduces bandwidth. Figure 6 shows that for a given RF increasing

the gain also reduces peaking and bandwidth. Table 4 shows the

recommended RF and RG values that optimize bandwidth with

minimal peaking.

Table 4. Recommended Component Values

Gain

RF (Ω)

RG (Ω)

RS (Ω)

−1

499

499

200

+1

499

Not applicable

200

+2

499

499

200

+5

499

124

200

+10

499

54.9

200

The load resistor also affects bandwidth, as shown in Figure 7 and

Figure 10. A comparison between Figure 7 and Figure 10 also

demonstrates the effect of gain and supply voltage.

When driving loads with a capacitive component, stability

improves by using a series snub resistor, RSNUB, at the output.

The frequency and pulse responses for various capacitive

loads are illustrated in Figure 12 and Figure 47, respectively.

For noninverting configurations, a resistor in series with the

input, RS, is needed to optimize stability for a gain of 1, as

illustrated in Figure 8. For larger noninverting gains, the effect

of a series resistor is reduced.