THS3001IDR Datasheet, PDF (23/39 Pages) Texas Instruments – 420-MHz HIGH-SPEED CURRENT-FEEDBACK AMPLIFIER

THS3001
www.ti.com................................................................................................................................................. SLOS217H – JULY 1998 – REVISED SEPTEMBER 2009
PCB DESIGN CONSIDERATIONS
Proper PCB design techniques in two areas are important to ensure proper operation of the THS3001. These
areas are high-speed layout techniques and thermal-management techniques. Because the THS3001 is a
high-speed part, the following guidelines are recommended.
• Ground plane - It is essential that a ground plane be used on the board to provide all components with a low
inductive ground connection, but should be removed from below the output and negative input pins as noted
below.
• The DGN package option includes a thermal pad for increased thermal performance. When using this
package, it is recommended to distribute the negative supply as a power plane, and tie the thermal pad to this
supply with multiple vias for proper power dissipation. It is not recommended to tie the thermal pad to ground
when using split supply (±V) as this will cause worse distortion performance than shown in this data sheet.
• Input stray capacitance - To minimize potential problems with amplifier oscillation, the capacitance at the
inverting input of the amplifiers must be kept to a minimum. To do this, PCB trace runs to the inverting input
must be as short as possible, the ground plane must be removed under any etch runs connected to the
inverting input, and external components should be placed as close as possible to the inverting input. This is
especially true in the noninverting configuration. An example of this can be seen in Figure 53, which shows
what happens when a 1-pF capacitor is added to the inverting input terminal. The bandwidth increases at the
expense of peaking. This is because some of the error current is flowing through the stray capacitor instead
of the inverting node of the amplifier. Although, while the device is in the inverting mode, stray capacitance at
the inverting input has a minimal effect. This is because the inverting node is at a virtual ground and the
voltage does not fluctuate nearly as much as in the noninverting configuration. This can be seen in Figure 54,
where a 10-pF capacitor adds only 0.35 dB of peaking. In general, as the gain of the system increases, the
output peaking due to this capacitor decreases. While this can initially look like a faster and better system,
overshoot and ringing are more likely to occur under fast transient conditions. So proper analysis of adding a
capacitor to the inverting input node should be performed for stable operation.
OUTPUT AMPLITUDE
vs
FREQUENCY
OUTPUT AMPLITUDE
vs
FREQUENCY
7
1 kΩ
6
C_I = 1 pF
C_in
5
V_in
−
V_out
4
+
R_L =
50 Ω 150 Ω
3
2
1
0
1
C_I = 10 pF
0
−1
C_I = Stray C Only
−2
V_in
−3
C_in
1 kΩ
50 Ω
−4
−5
1 kΩ
−
V_out
+
R_L =
150 Ω
−1
−2 _{Gain = 1}
−3
V_CC = ±15 V
V_O = 200 mV RMS
C_I = 0 pF
(Stray C Only)
−4
100k
1M
10M
100M
1G
f − Frequency − Hz
−6
Gain = −1
−7 V_CC = ±15 V
V_O = 200 mV RMS
−8
100k
1M
10M
100M
1G
f − Frequency − Hz
Figure 53.
Figure 54.
• Proper power-supply decoupling - Use a minimum 6.8-μF tantalum capacitor in parallel with a 0.1-μF ceramic
capacitor on each supply terminal. It may be possible to share the tantalum among several amplifiers
depending on the application, but a 0.1-μF ceramic capacitor should always be used on the supply terminal of
every amplifier. In addition, the 0.1-μF capacitor should be placed as close as possible to the supply terminal.
As this distance increases, the inductance in the connecting etch makes the capacitor less effective. The
designer should strive for distances of less than 0.1 inch between the device power terminal and the ceramic
capacitors.
Copyright © 1998–2009, Texas Instruments Incorporated
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