LMH6611_14 Datasheet, PDF (23/42 Pages) National Semiconductor (TI) – LMH6611/LMH6612 Single Supply 345 MHz Rail-to-Rail Output Amplifiers

LMH6611, LMH6612
www.ti.com
R_F = R_G
665
1000
f −3 dB (MHz)
110
113
SNOSB00K – NOVEMBER 2007 – REVISED OCTOBER 2013
Peaking (dB)
0
0.6
MINIMIZING NOISE
With a low input voltage noise of 10 nV/√Hz and an input current noise of 2 pA√Hz the LMH6611 and LMH6612
are suitable for high accuracy applications. Still being able to reduce the frequency band of operation of the
various noise sources (that is, op amp noise voltage, resistor thermal noise, input noise current) can further
improve the noise performance of a system. In a non-inverting amplifier configuration inserting a capacitor, C_G, in
series with the gain setting resistor, R_G, will reduce the gain of the circuit below frequency, f = 1/2πR_GC_G. This
can be set to reduce the contribution of noise from the 1/f region. Alternatively applying a feedback capacitor, C_F,
in parallel with the feedback resistor, R_F, will introduce a pole into your system at f = 1/2πR_FC_F and create a low
pass filter. This filter can be set to reduce high frequency noise and harmonics. Finally remember to keep resistor
values as small as possible for a given application in order to reduce resistor thermal noise.
POWER SUPPLY BYPASS
Since the LMH6611 and LMH6612 are wide bandwidth amplifiers, proper power supply bypassing is critical for
optimum performance. Improper power supply bypassing can result in large overshoot, ringing or oscillation. 0.1
μF capacitors should be connected from the supply pins, V⁺ and V^−, to ground, as close to the device as is
practical. Additionally, a 10 μF electrolytic capacitor should be connected from both supply pins to ground
reasonably close to the device. Finally, near the device a 0.1 μF ceramic capacitor between the supplies will
provide the best harmonic distortion performance.
INTERFACING HIGH PERFORMANCE OP AMPS WITH ADCs
These amplifiers are designed for ease of use in a wide range of applications requiring high speed, low supply
current, low noise, and the ability to drive complex ADC and video loads.
The source that drives the modern high resolution analog-to-digital converters (ADCs) sees a high frequency AC
load and a DC load of a few hundred ohms or more. Thus, a high performance op amp with high input
impedance of a few mega ohms and low output impedance would be an ideal choice as an input ADC driver.
The LMH6611/LMH6612 have the low output impedance of 0.07Ω at f = 1 MHz. The ADC driver acts as a buffer
and a low pass filter to reduce the overall system noise. To utilize the full dynamic range of the ADC, the ADC
input has to be driven to full scale input voltage.
As signals travel through the traces of a printed circuit board (PCB) and long cables, system noise accumulates
in the signals and a differential ADC rejects any signals noise that appears as a common mode voltage. There
are a couple of advantages to using differential signals rather than single-ended signals. First, differential signals
double the dynamic range of the ADC and second, they offer better harmonic distortion performance. There are
several ways to produce differential signals from a dual op amp configuration. One method is to utilize the single-
ended to differential conversion technique and the other is the differential to differential conversion technique.
The first method requires a single input source and the second method requires differential input source.
A real world input source can have non-ideal impedance thus the buffer amplifier, with very low output
impedance, is required to drive the input of the ADC. To minimize the droop in the input voltage, external shunt
capacitance (C_L) should be about ten times larger than the internal input capacitance of the ADC and external
series resistance (R_L) should be large enough to maintain the phase delay at the output of the op amp and
hence maintain the stability (See Figure 69). Most applications benefit from the inclusion of a series isolation
resistor connected between the op amp output and ADC input. This series resistor helps to limit the output
current of the op amp. The value chosen for this series resistor is very important, as a higher value will increase
the load impedance seen by the op amp and improve the total harmonic distortion (THD) performance of the op
amp; however, the ADC prefers a low impedance source driving it. Thus, the optimum value for this series
resistor must be found so that it will offer the best performance in terms of THD, SNR and SFDR of the combined
op amp and ADC.
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