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AD8027/AD8028

Rev. B | Page 19 of 24

Circuit Considerations

BALANCED INPUT IMPEDANCES

Balanced input impedances can help improve distortion per-

formance. When the amplifier transitions from PNP pair to

NPN pair operation, a change in both the magnitude and direc-

tion of the input bias current will occur. When multiplied times

imbalanced input impedances, a change in offset will result. The

key to minimizing this distortion is to keep the input imped-

ances balanced on both inputs. Figure 59 shows the effect of the

imbalance and degradation in distortion performance for a

50 Ω source impedance, with and without a 50 Ω balanced feed-

back path.

FREQUENCY (MHz)

0.1

–100

–90

–80

–70

–60

–50

–40

–30

–20

G = +1

VOUT = 2V p-p

RL = 1kΩ

VS = +3V

RF = 24.9Ω

RF = 49.9Ω

RF = 0Ω

03327-A-058

Figure 59. SFDR vs. Frequency and Various RF

PCB LAYOUT

As with all high speed op amps, achieving optimum perform-

ance from the AD8027/AD8028 requires careful attention to

PCB layout. Particular care must be exercised to minimize lead

lengths of the bypass capacitors. Excess lead inductance can

influence the frequency response and even cause high fre-

quency oscillations. The use of a multilayer board, with an

internal ground plane, will reduce ground noise and enable a

tighter layout.

To achieve the shortest possible lead length at the inverting

input, the feedback resistor, RF, should be located beneath the

board and span the distance from the output, Pin 6, to the input,

Pin 2. The return node of the resistor RG should be situated as

closely as possible to the return node of the negative supply

bypass capacitor connected to Pin 4.

On multilayer boards, all layers underneath the op amp should

be cleared of metal to avoid creating parasitic capacitive

elements. This is especially true at the summing junction (i.e.,

the –input). Extra capacitance at the summing junction can

cause increased peaking in the frequency response and lower

phase margin.

GROUNDING

To minimize parasitic inductances and ground loops in high

speed, densely populated boards, a ground plane layer is critical.

Understanding where the current flows in a circuit is critical in

the implementation of 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. Fast current changes in an inductive ground return

will create unwanted noise and ringing.

The length of the high frequency bypass capacitor pads and

traces is critical. A parasitic inductance in the bypass grounding

will work against the low impedance created by the bypass

capacitor. Because load currents flow from supplies as well as

ground, the load should be placed at the same physical location

as the bypass capacitor ground. For large values of capacitors,

which are intended to be effective at lower frequencies, the cur-

rent return path length is less critical.

POWER SUPPLY BYPASSING

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

provide a clean, low noise dc voltage source to these inputs. The

bypass capacitors have two functions:

Provide a low impedance path for unwanted frequencies

from the supply inputs to ground, thereby reducing the

effect of noise on the supply lines.

Provide sufficient localized charge storage, for fast

switching conditions and minimizing the voltage drop at

the supply pins and the output of the amplifier. This is

usually accomplished with larger electrolytic capacitors.

Decoupling methods are designed to minimize the bypassing

impedance at all frequencies. This can be accomplished with a

combination of capacitors in parallel to ground.

Good quality ceramic chip capacitors should be used and

always kept as close to the amplifier package as possible. A par-

allel combination of a 0.01 µF ceramic and a 10 µF electrolytic

covers a wide range of rejection for unwanted noise. The 10 µF

capacitor is less critical for high frequency bypassing, and in

most cases, one per supply line is sufficient.

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