ADA4841-1/ADA4841-2

Rev. E | Page 18 of 20

Table 6. Recommended Values and Typical Performance

Gain

−3 dB BW (MHz)

Slew Rate (V/μs)

Peaking

(dB)

Output Noise ADA4841-1/

ADA4841-2 Only (nV/√Hz)

Total Output Noise

Including Resistors (nV/√Hz)

+1

0

N/A

77

12.5

0.9

2

2

+2

499

499

34

12.5

0.3

4

5.73

−1

499

499

38

12.5

0.4

4

5.73

+5

499

124

11

12

0

10

11.9

+10

499

54.9

5

12

0

20

21.1

+20

499

26.1

2.3

11.2

0

40

42.2

Capacitor selection is critical for optimal filter performance.

Capacitors with low temperature coefficients, such as NPO

ceramic capacitors, are good choices for filter elements. Figure 51

shows the filter response.

0.03

10

FREQUENCY (MHz)

1

0.1

–40

–35

–30

–25

–20

–15

–10

–5

0

5

Figure 51. Filter Frequency Response

LAYOUT CONSIDERATIONS

To ensure optimal performance, careful and deliberate attention

must be paid to the board layout, signal routing, power supply

bypassing, and grounding.

GROUND PLANE

It is important to avoid ground in the areas under and around

the input and output of the ADA4841-1/ADA4841-2. Stray

capacitance created between the ground plane and the input

and output pads of a device are detrimental to high speed

amplifier performance. Stray capacitance at the inverting input,

along with the amplifier input capacitance, lowers the phase

margin and can cause instability. Stray capacitance at the output

creates a pole in the feedback loop. This can reduce phase

margin and can cause the circuit to become unstable.

POWER SUPPLY BYPASSING

Power supply bypassing is a critical aspect in the performance

of the ADA4841-1/ADA4841-2. A parallel connection of

capacitors from each of the power supply pins to ground works

best. A typical connection is shown in Figure 49. Smaller value

capacitors offer better high frequency response where larger

value electrolytics offer better low frequency performance.

Paralleling different values and sizes of capacitors helps to

ensure that the power supply pins are provided a low ac impedance

across a wide band of frequencies. This is important for minimizing

the coupling of noise into the amplifier. This can be especially

important when the amplifier PSR is starting to roll off—the

bypass capacitors can help lessen the degradation in PSR

performance.

Starting directly at the ADA4841-1/ADA4841-2 power supply

pins, the smallest value capacitor should be placed on the same

side of the board as the amplifier, and as close as possible to the

amplifier power supply pin. The ground end of the capacitor

should be connected directly to the ground plane. Keeping the

capacitors’ distance short but equal from the load is important

and can improve distortion performance. This process should

be repeated for the next largest value capacitor.

It is recommended that a 0.1 μF ceramic 0508 case be used. The

0508 case size offers low series inductance and excellent high

frequency performance. A 10 μF electrolytic capacitor should be

placed in parallel with the 0.1 μF capacitor. Depending on the

circuit parameters, some enhancement to performance can be

realized by adding additional capacitors. Each circuit is different

and should be individually analyzed for optimal performance.