LMH6642Q-Q1_15 Datasheet, PDF (24/34 Pages) Texas Instruments – Low Power, 130 MHz, 75 mA Rail-to-Rail Output Amplifiers

LMH6642Q-Q1, LMH6643Q-Q1
SNOSC61C – JANUARY 2012 – REVISED SEPTEMBER 2014
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Single Supply, Low Power Photodiode Amplifier (continued)
For this example, optimum C_f was empirically determined to be around 5pF. This time domain response is shown
in Figure 58 below showing about 9 ns rise/fall times, corresponding to about 39 MHz for f_p. The overall supply
current from the +5 V supply is around 5 mA with no load.
200 mV/DIV
20 ns/DIV
Figure 58. Converter Step Response (1V_PP, 20 ns/DIV)
8.3 Printed Circuit Board Layout and Component Values Section
Generally, a good high frequency layout will keep power supply and ground traces away from the inverting input
and output pins. Parasitic capacitances on these nodes to ground will cause frequency response peaking and
possible circuit oscillations (see Application Note OA-15 (SNOA367) for more information). Texas Instruments
suggests the following evaluation boards as a guide for high frequency layout and as an aid in device testing and
characterization:
LMH6642QMF
LMH6643QMM
DEVICE
5-Pin SOT-23
8-Pin VSSOP
PACKAGE
EVALUATION BOARD PN
LMH730216
LMH730123
Another important parameter in working with high speed/high performance amplifiers, is the component values
selection. Choosing external resistors that are large in value will effect the closed loop behavior of the stage
because of the interaction of these resistors with parasitic capacitances. These capacitors could be inherent to
the device or a by-product of the board layout and component placement. Either way, keeping the resistor values
lower, will diminish this interaction to a large extent. On the other hand, choosing very low value resistors could
load down nodes and will contribute to higher overall power dissipation.
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