THS6062IDR Datasheet, PDF (21/36 Pages) Texas Instruments – LOW-NOISE ADSL DUAL DIFFERENTIAL RECEIVER

THS6062
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SLOS228D – JANUARY 1999 – REVISED OCTOBER 2007
OPTIMIZING FREQUENCY RESPONSE
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.
10
V_CC = ± 15 V
9 Gain = 2
R_F = 300 Ω
8 R_L = 150 Ω
V_O(PP) = 0.4 V
7
C_IN− = 10 pF
4
V_CC = ± 15 V
3 ^{Gain = −1}
R_F = 360 Ω
2 ^RL ^{= 150 Ω}
V_O(PP) = 0.4 V
1
C_IN− = 10 pF
6
No C_IN−
5
(Stray C Only)
0
No C_IN−
−1
(Stray C Only)
4
3
2
1
0
100 k
C_IN−
300 Ω
V_I
300 Ω
_
+
50 Ω
V_O
150 Ω
1M
10 M
f − Frequency − Hz
Figure 43.
100 M
500 M
−2
−3 _VI
360 Ω
−4 ^{56 Ω CIN−}
360 Ω
_
+
V_O
150 Ω
−5
−6
100 k
1M
10 M
f − Frequency − Hz
Figure 44.
100 M
500 M
The next thing that helps to maintain a smooth frequency response is to keep the feedback resistor _f) and the
gain resistor _g) values fairly low. These two resistors are effectively in parallel when looking at the ac small-signal
response. This is why in Figure 30, a feedback resistor of 3.9 kΩ 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
–1
5
R_f for V_CC = ±15 V, ±5 V, 5 V
300 Ω
360 Ω
3.3 kΩ (low stray-c PCB only)
Copyright © 1999–2007, Texas Instruments Incorporated
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