−2

−4

−6

−8

1M

f − Frequency − Hz

−1

−3

−5

−7

100k

10M

100M

0

1

1G

Gain = −1

+

−

1 k

Ω

150

Ω

1 k

Ω

50

Ω

2

0

−2

−4

1M

f − Frequency − Hz

3

1

−1

−3

100k

10M

100M

4

6

5

7

1G

Gain = 1

(Stray C Only)

+

−

1 k

Ω

50

Ω

150

Ω

THS3001

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

OUTPUT AMPLITUDE

vs

vs

FREQUENCY

FREQUENCY

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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