LMH6518SQEVK National Semiconductor, LMH6518SQEVK Datasheet - Page 21

• 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

With the scheme of Figure 5, Vx is kept at 1.2V, by proper
selection of external resistor values, so that the LMH6518
outputs are not CM-loaded. As was the case with input level
shifting, this output level shifting also consumes additional
power (0.58W).
Output Swing, Clamping, and Operation Beyond Full
Scale
One of the major concerns in interfacing to low voltage ADC’s
(such as the Gsample/s ADC’s that the LMH6518 is intended
to drive) is ensuring that the ADC input is not violated with
excessive drive. For this reason, plus the very important re-
FIGURE 5. Output CM Shift Scheme
FIGURE 6. Digital Oscilloscope Front-End
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quirement of an oscilloscope to recover quickly and gracefully
from an overdrive condition, the LMH6518 is fitted with three
overvoltage clamps; one at the Preamp output and one at
Main and Auxiliary outputs each. The Preamp clamp is re-
sponsible for preventing the Preamp from saturation (to min-
imize recovery time) with large ladder attenuation when
Preamp output swing is at its highest. On the other hand, the
output clamps, perform this function when the Ladder atten-
uation is lower and hence the output amplifier is closer to
saturation, and prolonged recovery, if not properly clamped.
The combination of these clamps results in the Typical Per-
formance Characteristic plots of “Output vs. Input” where it is
possible to observe where output limiting starts due to the
clamp action. LMH6518 owes its fast recovery time (< 5 ns)
from 50% overdrive to the said clamps.
“Output vs. Input” plots, in the Typical Performance Charac-
teristics section, can be used to determine the LMH6518
linear swing beyond full scale. This information sets the over-
drive limit for both oscilloscope waveform capture and for
signal triggering. The Preamp clamp is set tighter than the
output clamp, evidenced by lower output swing with 20 dB
Ladder attenuation than with 0 dB. With high ladder attenua-
tion (20 dB) defining the limit, the graphs show that the “+Out”
and “−Out” difference of 0.4V is well inside the clamp range,
thereby ensuring 0.8 V
corresponds to an overdrive capability of approximately ±7%
beyond full scale.
Here is a block diagram for how the LMH6518 is used in an
oscilloscope:
PP
of unhindered output swing. This
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