LMH6518SQEVK National Semiconductor, LMH6518SQEVK Datasheet - Page 25

• 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

OSCILLOSCOPE TRIGGER APPLICATIONS
With the Auxiliary output of the LMH6518 offering a second
output that follows the Main one (except for a slightly reduced
distortion performance), the oscilloscope trigger function can
be implemented by tapping this output. The “V
the LMH6518 allows the Auxiliary common mode to be set.
The trigger function can be physically located at a distance
from the main signal path, if need be, by taking advantage of
the differential Auxiliary output and rejecting any board relat-
ed common mode interference pick-up at the receive end.
If Trigger circuitry is physically close to the LMH6518, the cir-
cuit diagram shown in Figure 12 allows operation using only
one of two Auxiliary outputs. The unused output does need to
be terminated properly using R
(DAC101C085) generates a 0- 2.5V trigger level, with 2.4 mV
resolution
or 0.7% (= 2.4 mV x 100/0.35 V
to the LMH6518 “+Out Aux” by using an ultra-fast comparator,
U2’s minimum Toggle Rate specification of 750 Mb/s with ±50
mV overdrive allow the oscilloscope to trigger on repetitive
waveforms well above the 500 MHz oscilloscope bandwidth
applications, when the input signal is at least 14.3% of FS
swing
FIGURE 12. Single-Ended Trigger from LMH6518 Auxiliary Output
PP
) of FS, which is compared
1
, R
11
combination. U3
CM_Aux
” input of
25
U2 (LMH7220). U2’s complimentary LVDS output is termi-
nated in the required 100Ω load (R
where the LVDS Trigger output is available. The LMH7220’s
offset voltage (±9.5 mV) and offset voltage drift (±50 µV/°C)
error will be 5.9 LSB
of the Trigger DAC (U3). The offset voltage related portion of
this error can be nulled-out, if necessary, during the oscillo-
scope initial calibration. To do so, the LMH6518 input is
terminated properly with no input applied and U3 output is
adjusted around V
for U2’s output transition. U3’s output, relative to V
transition corresponds to U2’s offset error which can be fac-
tored into the Trigger readings and thus eliminated, leaving
only the Offset voltage temperature drift component (= 2
LSB).
The worst case single event minimum discernable pulse width
is set by the LMH7220’s propagation delay specification of
3.63 ns (20 mV overdrive).
Both the Main and the Auxiliary outputs can recover gracefully
and quickly from a 50% overdrive condition as tabulated in
the Electrical Characteristics table under overdrive Recovery
Time. Overdrive conditions beyond 50%, however, could re-
sult in longer recovery times due to the interaction between
an internal clamp and the common mode feedback loop that
sets the output common mode voltage. This may have an im-
pact on both the displayed waveform and the oscilloscope
CM_Aux
voltage (1.2V ±10 mV) while looking
10
), for best performance,
30068830
www.national.com
CM_Aux
at