THS4031_16 Datasheet, PDF (22/47 Pages) Texas Instruments – THS403x 100-MHz Low-Noise High-Speed Amplifiers

THS4031, THS4032
SLOS224H – JULY 1999 – REVISED JUNE 2016
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Feature Description (continued)
e_no
= e_ni A_V
=
e_ni
æç₁₊
è
R_F
R_G
ö
÷
ø
(Noninverting Case)
(2)
As the previous equations show, to keep noise at a minimum, small-value resistors should be used. As the
closed-loop gain is increased (by reducing R_G), the input noise is reduced considerably because of the parallel
resistance term. This leads to the general conclusion that the most dominant noise sources are the source
resistor (R_S) and the internal amplifier noise voltage (e_n). Because noise is summed in a root-mean-squares
method, noise sources smaller than 25% of the largest noise source can be effectively ignored. This advantage
can greatly simplify the formula and make noise calculations much easier to calculate.
For more information on noise analysis, see the application note, Noise Analysis for High-Speed Op Amps.
9.3.2 Optimizing Frequency Response
Internal frequency compensation of the THS403x was selected to provide very wide bandwidth performance and
still maintain a very low noise floor. To meet these performance requirements, the THS403x 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 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 53 and Figure 54). Two things can be done to help minimize this effect. The first is to
simply remove any ground planes under the inverting terminal of the amplifier, including 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 possible oscillations will then occur if this happens.
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_i− = 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_i−= 10 pF
6
No C_i−
5
(Stray C Only)
0
No C_i−
−1
(Stray C Only)
4
C_i−
300 Ω
3
300 Ω
_
2
V_I
+
V_O
50 Ω
150 Ω
1
0
100 k
1M
10 M
100 M 500 M
f − Frequency − Hz
Figure 53. Output Amplitude vs Frequency
−2
−3 _VI
360 Ω
−4 ^{56 W Ci−}
360 Ω
_
+
V_O
150 Ω
−5
−6
100 k
1M
10 M
100 M 500 M
f − Frequency − Hz
Figure 54. Output Amplitude vs Frequency
The second precaution to help maintain a smooth frequency response is to keep the feedback resistor (R_f) and
the gain resistor (R_g) values fairly low. These two resistors are effectively in parallel when looking at the AC
small-signal response. But, as can be seen in Figure 21 through Figure 32, a value too low starts to reduce the
bandwidth of the amplifier. Table 3 shows some recommended feedback resistors to be used with the THS403x.
22
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