AD8132ARM datasheet(16/20 Pages) AD

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REV. 0

AD8132

–16–

Driving a Capacitive Load

A purely capacitive load can react with the pin and bondwire

inductance of the AD8132 resulting in high frequency ringing in

the pulse response. One way to minimize this effect is to place a

small capacitor across each of the feedback resistors. The added

capacitance should be small to avoid destabilizing the ampliﬁer.

An alternative technique is to place a small resistor in series with

the ampliﬁer’s outputs as shown in TPC 47.

LAYOUT, GROUNDING AND BYPASSING

As a high-speed part, the AD8132 is sensitive to the PCB

environment in which it has to operate. Realizing its superior

speciﬁcations requires attention to various details of good high-

speed PCB design.

The ﬁrst requirement is a good solid ground plane that covers as

much of the board area around the AD8132 as possible. The

only exception to this is that the two input pins (Pins 1 and 8)

should be kept a few mm from the ground plane, and ground

should be removed from inner layers and the opposite side of

the board under the input pins. This will minimize the stray

capacitance on these nodes and help preserve the gain flatness

vs. frequency.

The power supply pins should be bypassed as close as possible

to the device to the nearby ground plane. Good high-frequency

ceramic chip capacitors should be used. This bypassing should

be done with a capacitance value of 0.01

µF to 0.1 µF for each

supply. Further away, low frequency bypassing should be provided

with 10

µF tantalum capacitors from each supply to ground.

The signal routing should be short and direct in order to avoid

parasitic effects. Wherever there are complementary signals, a

symmetrical layout should be provided to the extent possible to

maximize the balance performance. When running differential

signals over a long distance, the traces on PCB should be close

together or any differential wiring should be twisted together to

minimize the area of the loop that is formed. This will reduce

the radiated energy and make the circuit less susceptible to

interference.

CIRCUITS

RF1

RF2

RG1

RG2

Figure 4. Typical Four-Resistor Feedback Circuit

RF2

RG2

VIN

Figure 5. Typical Circuit with

β1 = 0

RF1

RG1

Figure 6. Typical Circuit with

β2 = 1

VIN

Figure 7. Resistorless G = 2 Circuit with

β1 = 0

RF1

RG1

VIN

Figure 8. Typical Circuit with

β2 = 0

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