ADA4841-2_15 Datasheet, PDF (18/20 Pages) Analog Devices – Low Power, Low Noise and Distortion, Rail-to-Rail Output Amplifiers

ADA4841-1/ADA4841-2
Data Sheet
Table 6. Recommended Values and Typical Performance
Gain
+1
+2
−1
+5
+10
+20
RF (Ω)
0
499
499
499
499
499
RG (Ω)
N/A
499
499
124
54.9
26.1
−3 dB BW (MHz)
77
34
38
11
5
2.3
Slew Rate (V/µs)
12.5
12.5
12.5
12
12
11.2
Peaking
(dB)
0.9
0.3
0.4
0
0
0
Output Noise ADA4841-1/
ADA4841-2 Only (nV/√Hz)
2
4
4
10
20
40
Total Output Noise
Including Resistors (nV/√Hz)
2
5.73
5.73
11.9
21.1
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.
5
0
–5
–10
–15
–20
–25
–30
–35
–40
0.03
0.1
1
10
FREQUENCY (MHz)
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.
Rev. F | Page 18 of 20