LMH6518SQEVK National Semiconductor, LMH6518SQEVK Datasheet - Page 18

• Manufacturer Part Number: LMH6518SQEVK
• Description: KIT EVAL FOR LMH6518 VGA
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LMH6518SQEVK

• Channels Per Ic: 1 - Single
• Amplifier Type: Variable Gain
• Output Type: Differential
• -3db Bandwidth: 900MHz
• Operating Temperature: -40°C ~ 85°C
• Current - Supply (main Ic): 210mA
• Voltage - Supply, Single/dual (±): 4.75 V ~ 5.25 V
• Board Type: Fully Populated
• Utilized Ic / Part: LMH6518
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output / Channel: -
• Slew Rate: -
• Lead Free Status / Rohs Status: Supplier Unconfirmed

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modate any FS signal from 6.8 mV
LMH6518 input, with 0.62 dB (= 2.62-2) overlap.
Assuming a nominal 0.7V
FS input swing is limited by the maximum signal path gain
possible and vice versa:
(or 8 mV
(or 800 mV
To accommodate a higher FS input, an additional attenuator
is needed before the LMH6518. This front-end attenuator is
shown in the Figure 6 block diagram with its details shown in
Figure 15. The highest minimum attenuation level is deter-
mined by the largest FS input signal (FS
So, to accommodate 80 V
needed before the LMH6518.
In a typical oscilloscope application, the voltage range en-
countered is from 1 mV/DIV to 10 V/DIV with 8 vertical divi-
sions 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:
•
•
•
LMH6518 related SNR factors are:
•
•
•
•
SNR increases with the inverse square root of the bandwidth.
So, reducing bandwidth from 450 MHz to 200 MHz, for ex-
ample, improves SNR by 3.5 dB
Scope front end noise (Front-end attenuator + scope
probe Hi-Z buffer which is discussed later in this document
and shown in Figure 6)
LMH6518
ADC
Bandwidth
Preamp used (Preamp High Gain or Low Gain)
Ladder Attenuation
Signal level
PP
PP
with no ADC fine adjust)
with no ADC FS adjust)
PP
PP
output, the LMH6518’s minimum
, 40 dB minimum attenuation is
PP
to 920 mV
max
):
PP
, at the
18
The other factors listed above, preamp and ladder attenua-
tion, depend on the signal level and also impact SNR. The
combined effect of these factors is summarized in Figure 2
where SNR is plotted as a function of the LMH6518 FS input
voltage (assuming scope bandwidth of 200 MHz) and not in-
cluding the ADC and the front end noise:
FIGURE 2. LMH6518 SNR & Ladder Attenuation used vs.
As can be seen from Figure 2, SNR of at least 52 dB is main-
tained for FS inputs above 24 mV
assuming the LMH6518’s internal 200 MHz filter is enabled.
Most oscilloscope manufacturers relax the SNR specifica-
tions to 40 dB for the highest gain (lowest scope voltage
setting) . From Figure 2, LMH6518’s minimum SNR is 43.5
dB, thereby meeting the relaxed SNR specification for the
lower range of scope front panel voltages.
In Figure 2, the step-change in SNR near Input FS of
90 mV
HG with a subsequent 3 dB difference due to the Preamp HG/
20 dB ladder attenuation’s lower output noise compared to
Preamp LG/ 2 dB ladder attenuation’s noise. Judicious choice
of front end attenuators can ensure that the 52 dB SNR spec-
ification is maintained for scope FS inputs
confining the LMH6518 gain range to the lower 30.5 dB
from the total range of 40 dB (= 38.8 - (−1.16)) possible.
Here is an example:
To cover the range of 1 mV/DIV to 10 V/DIV (80 dB range),
here is a configuration which affords good SNR:
PP
is the transition point from Preamp LG to Preamp
Input
PP
(3 mV/DIV on a scope)
≥
24 mV
30068815
PP
by