LMH6624_16 Datasheet, PDF (26/38 Pages) Texas Instruments – LMH6624 and LMH6626 Single/Dual Ultra Low Noise Wideband Operational Amplifier

LMH6624, LMH6626
SNOSA42G – NOVEMBER 2002 – REVISED DECEMBER 2014
www.ti.com
9 Power Supply Recommendations
The LMH6624 and LMH6626 devices can operate off a single supply or with dual supplies as long as the input
CM voltage range (CMIR) has the required headroom to either supply rail. Supplies should be decoupled with
low inductance, often ceramic, capacitors to ground less than 0.5 inches from the device pins. The use of ground
plane is recommended, and as in most high speed devices, it is advisable to remove ground plane close to
device sensitive pins such as the inputs.
10 Layout
10.1 Layout Guidelines
TI suggests the copper patterns on the evaluation boards shown in Figure 62 and Figure 63 as a guide for high
frequency layout. These boards are also useful as an aid in device testing and characterization. As is the case
with all high-speed amplifiers, accepted-practice RF design technique on the PCB layout is mandatory.
Generally, a good high frequency layout exhibits a separation of power supply and ground traces from the
inverting input and output pins as shown in Figure 62. Parasitic capacitances between these nodes and ground
may cause frequency response peaking and possible circuit oscillations. See Application Note OA-15, Frequent
Faux Pas in Applying Wideband Current Feedback Amplifiers (SNOA367) for more information. Use high quality
chip capacitors with values in the range of 1000 pF to 0.1 µF for power supply bypassing as shown in Figure 62.
One terminal of each chip capacitor is connected to the ground plane and the other terminal is connected to a
point that is as close as possible to each supply pin as allowed by the manufacturer’s design rules. In addition,
connect a tantalum capacitor with a value between 4.7 μF and 10 μF in parallel with the chip capacitor. Signal
lines connecting the feedback and gain resistors should be as short as possible to minimize inductance and
microstrip line effect as shown in Figure 63. Place input and output termination resistors as close as possible to
the input/output pins. Traces greater than 1 inch in length should be impedance matched to the corresponding
load termination.
Symmetry between the positive and negative paths in the layout of differential circuitry should be maintained to
minimize the imbalance of amplitude and phase of the differential signal.
Component value selection is another important parameter in working with high speed and high performance
amplifiers. Choosing external resistors that are large in value compared to the value of other critical components
will affect the closed loop behavior of the stage because of the interaction of these resistors with parasitic
capacitances. These parasitic capacitors could either be inherent to the device or be a by-product of the board
layout and component placement. Moreover, a large resistor will also add more thermal noise to the signal path.
Either way, keeping the resistor values low will diminish this interaction. On the other hand, choosing very low
value resistors could load down nodes and will contribute to higher overall power dissipation and high distortion.
DEVICE
LMH6624MF
LMH6624MA
LMH6626MA
LMH6626MM
PACKAGE
SOT-23–5
SOIC-8
SOIC-8
VSSOP-8
EVALUATION BOARD PART NUMBER
LMH730216
LMH730227
LMH730036
LMH730123
26
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