LMH6629_1011 Datasheet, PDF (21/28 Pages) National Semiconductor (TI) – Ultra-Low Noise, High-Speed Operational Amplifier with Shutdown

• Choose the Optimum R_S (R_OPT)
R_OPT is the point at which the NF curve reaches a minimum
and is approximated by:
R_OPT ≈ e_n/ i_n
Figure 12 is a plot of NF vs R_S with the circuit of Figure 6 (R_f
= 240Ω, A_V = +10V/V). The NF curves for both Unterminated
(R_T = open) and Terminated systems (R_T = R_S) are shown.
Table 1 indicates NF for various source resistances including
R_S = R_OPT.
LOW-NOISE TRANSIMPEDANCE AMPLIFIER
Figure 14 implements a high-speed, single-supply, low-noise
Transimpedance amplifier commonly used with photo-
diodes. The transimpedance gain is set by R_F.
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FIGURE 12. Noise Figure vs. Source Resistance
TABLE 1. Noise Figure for Various R_s
R_S (Ω)
NF (Terminated)
(dB)
NF (Unterminated)
(dB)
50
8
3.2
R_OPT
4.1
(R_OPT = 750Ω)
1.1
(R_OPT = 350Ω)
SINGLE-SUPPLY OPERATION
The LMH6629 can be operated with single power supply as
shown in Figure 13. Both the input and output are capacitively
coupled to set the DC operating point.
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FIGURE 14. 200MHz Transimpedance Amplifier
Configuration
Figure 15 shows the Noise Gain (NG) and transfer function
(I-V Gain). As with most Transimpedance amplifiers, it is re-
quired to compensate for the additional phase lag (Noise Gain
zero at f_Z) created by the total input capacitance ( C_D (diode
capacitance) + C_CM (LMH6629 input capacitance) ) looking
into R_F; this is accomplished by placing C_F across R_F to create
enough phase lead (Noise Gain pole at f_P) to stabilize the
loop.
FIGURE 13. Single Supply Operation
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FIGURE 15. Transimpedance Amplifier Noise Gain &
Transfer Function
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