LMH6657_16 Datasheet, PDF (20/35 Pages) Texas Instruments – 270-MHz Single Supply, Single and Dual Amplifiers

LMH6657, LMH6658
SNOSA35G – AUGUST 2002 – REVISED JULY 2015
Application Information (continued)
OUTPUT
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INPUT
V_S = ±5V, A_V = +6, R_F = 1k
R_G = 200:R_L = OPEN
20 ns/DIV
Figure 53. Output Overload Recovery
8.1.1.2 Driving Capacitive Loads
The LMH6657 and LMH6658 can drive moderate values of capacitance by utilizing a series isolation resistor
between the output and the capacitive load. Typical Characteristics shows the settling time behavior for various
capacitive loads and 20 Ω of isolation resistance. Capacitive load tolerance will improve with higher closed loop
gain values. Applications such as ADC buffers, among others, present complex and varying capacitive loads to
the operational amplifier; best value for this isolation resistance is often found by experimentation and actual trial
and error for each application.
8.1.1.3 Distortion
Applications with demanding distortion performance requirements are best served with the device operating in
the inverting mode. The reason for this is that in the inverting configuration, the input common-mode voltage
does not vary with the signal and there is no subsequent ill effects due to this shift in operating point and the
possibility of additional non-linearity. Moreover, under low closed loop gain settings (most suited to low
distortion), the noninverting configuration is at a further disadvantage of having to contend with the input common
voltage range. There is also a strong relationship between output loading and distortion performance (that is, 1
kΩ vs. 100 Ω distortion improves by about 20 dB at 100 KHz) especially at the lower frequency end where the
distortion tends to be lower. At higher frequency, this dependence diminishes greatly such that this difference is
only about 4 dB at 10 MHz. But, in general, lighter output load leads to reduced HD3 term and thus improves
THD.
9 Power Supply Recommendations
The LMH665x can operate off a single-supply or with dual supplies. The input CM capability of the parts (CMVR)
extends all the way down to the V- rail to simplify single-supply applications. Supplies should be decoupled with
low-inductance, often ceramic, capacitors to ground less than 0.5 inches from the device pins. TI recommends
the use of ground plane, 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
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, Frequent Faux Pas in Applying Wideband Current
Feedback Amplifiers (SNOA367) for more information. TI suggests the following evaluation boards as a guide for
high frequency layout and as an aid in device testing and characterization:
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