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OPTIMIZING FREQUENCY RESPONSE

_

+

150 Ω

50 Ω

300 Ω

7

6

2

0

100k

1M

10M

8

9

f − Frequency − Hz

10

100M

500M

5

4

3

1

(StrayCOnly)

Gain=2

300 Ω

_

+

360 Ω

150 Ω

56 Ω

360 Ω

1

0

−4

−6

100k

1M

10M

2

3

f − Frequency − Hz

4

100M

500M

−1

−2

−3

−5

(StrayCOnly)

Gain= −1

THS6062

SLOS228D – JANUARY 1999 – REVISED OCTOBER 2007

Internal frequency compensation of the THS6062 was selected to provide very wide bandwidth performance and

still maintain a very low noise floor. In order to meet these performance requirements, the THS6062 must have a

minimum gain of 2 (–1). Because everything is referred to the noninverting terminal of an operational amplifier,

the noise gain in a G = –1 configuration is the same as in a G = 2 configuration.

One of the keys to maintaining a smooth frequency response, and hence, a stable pulse response, is to pay

particular attention to the inverting terminal. Any stray capacitance at this node causes peaking in the frequency

response (see Figure 43 and Figure 44). There are two things that can be done to help minimize this effect. The

first is to simply remove any ground planes under the inverting terminal of the amplifier. This also includes the

trace that connects to this terminal. Additionally, the length of this trace should be minimized. The capacitance at

this node causes a lag in the voltage being fed back due to the charging and discharging of the stray

capacitance. If this lag becomes too long, the amplifier will not be able to correctly keep the noninverting terminal

voltage at the same potential as the inverting terminal's voltage. Peaking and possibly oscillations will then occur.

Figure 43.

Figure 44.

response. This is why in Figure 30, a feedback resistor of 3.9 k

Ω with a gain resistor of 1 kΩ only shows a small

peaking in the frequency response. The parallel resistance is only 800

Ω. This value, in conjunction with a very

small stray capacitance test PCB, forms a zero on the edge of the amplifier's natural frequency response. To

eliminate this peaking, all that needs to be done is to reduce the feedback and gain resistances. One other way

to compensate for this stray capacitance is to add a small capacitor in parallel with the feedback resistor. This

helps to neutralize the effects of the stray capacitance. To keep this zero out of the operating range, the stray

capacitance and resistor value's time constant must be kept low. But, as can be seen in Figure 23 to Figure 28, a

value too low starts to reduce the bandwidth of the amplifier. Table 1 shows some recommended feedback

resistors to be used with the THS6062.

Table 1. Recommended Feedback Resistors

GAIN

2

300

Ω

–1

360

Ω

5

3.3 k

Ω (low stray-c PCB only)

Copyright © 1999–2007, Texas Instruments Incorporated

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Product Folder Link(s): THS6062