LMH6518_15 Datasheet, PDF (20/41 Pages) Texas Instruments – LMH6518 900 MHz, Digitally Controlled, Variable Gain Amplifier

LMH6518
SNOSB21C – MAY 2008 – REVISED JULY 2013
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Assuming a nominal 0.7V_PP output, the LMH6518’s minimum FS input swing is limited by the maximum signal
path gain possible and vice versa:
0.7 V_PP
Minimum LMH6518 FS Input = _{(38.8 + 1.41) dB} = 6.8 mV_PP
10
20
(4)
(or 8 mV_PP with no ADC fine adjust)
0.7 V_PP
Maximum LMH6518 FS Input = _{(-1.16 ± 1.21) dB} = 920 mV_PP
20
10
(5)
(or 800 mV_PP with no ADC FS adjust)
To accommodate a higher FS input, an additional attenuator is needed before the LMH6518. This front-end
attenuator is shown in the Figure 62 with its details shown in Figure 71. The highest minimum attenuation level is
determined by the largest FS input signal (FS_max):
FS_MAX (V_PP)
Attenuation (dB) = 20 x log
800 mV_PP
(6)
So, to accommodate 80 V_PP, 40 dB minimum attenuation is needed before the LMH6518.
In a typical oscilloscope application, the voltage range encountered is from 1 mV/DIV to 10 V/DIV with 8 vertical
divisions visible on the screen. One of the primary concerns in a digital oscilloscope is SNR which translates to
display trace width/ thickness. Typically, oscilloscope manufacturers need the noise level to be low enough so
that the “no-input” visible trace width is less than 1% of FS. Experience has shown that this corresponds to a
minimum SNR of 52 dB.
The factors that influence SNR are:
• Scope front end noise (Front-end attenuator + scope probe Hi-Z buffer which is discussed later in this
document and shown in Figure 62)
• LMH6518
• ADC
LMH6518 related SNR factors are:
• Bandwidth
• Preamp used (Preamp High Gain or Low Gain)
• Ladder Attenuation
• Signal level
SNR increases with the inverse square root of the bandwidth. So, reducing bandwidth from 450 MHz to 200
MHz, for example, improves SNR by 3.5 dB
450 MHz
(20 x log
= 3.5 dB)
200 MHz
(7)
The other factors listed above, preamp and ladder attenuation, depend on the signal level and also impact SNR.
The combined effect of these factors is summarized in Figure 58 where SNR is plotted as a function of the
LMH6518 FS input voltage (assuming scope bandwidth of 200 MHz) and not including the ADC and the front end
noise:
20
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