LMH6555EVAL National Semiconductor, LMH6555EVAL Datasheet - Page 18

• Manufacturer Part Number: LMH6555EVAL
• Manufacturer: National Semiconductor

Specifications of LMH6555EVAL

• Lead Free Status / Rohs Status: Not Compliant

www.national.com
In the schematic of Figure 17, the LMH6555 converts a single
ended input into a differential output for direct interface to the
ADC's 100Ω differential input. An alternative approach to us-
ing the LMH6555 for this purpose, would have been to use a
balun transformer, as shown in Figure 18.
In the circuit of Figure 18, the ADC will see a 100Ω differential
driver which will swing the required 800 mV
V
200Ω for the frequency range that the transformer is specified
to operate. Note that with this scheme, the signal to the ADC
must be AC coupled, because of the transformer’s minimum
operating frequency which would prevent DC coupling. For
the transformer specified, the lower operating frequency is
around 4.5 MHz and the input high pass filter’s −3 dB band-
width is around 340 kHz for the values shown (or (1/
Hz where R
Table 1 compares the LMH6555 solution (Figure 17) vs. that
of the balun transformer coupling (Figure 18) for various cat-
egories.
GAIN FLATNESS
In applications where the full 1.2 GHz bandwidth of the
LMH6555 is not necessary, it is possible to improve the gain
flatness frequency at the expense of bandwidth. Figure 19
Lower Power Consumption
Lower Distortion
Wider Dynamic Range
DC Coupling & Broadband
Applications
Highest Gain & Phase
Balance
Input/ Output Broadband
Impedance Matching
(Highest Return Loss)
Additional Gain
ADC Input Protection against
Overdrive
Highest SNR
Ability to Control Gain
Flatness
PP
FIGURE 18. Single Ended to Differential Conversion
TABLE 1. ADC Input Coupling Schemes Compared
. The source (V
Category
(AC only) with a Balun Transformer
EQ
= 200Ω).
IN
) will see an overall impedance of
(see below)
LMH6555
Preferred Solution
✓
✓
✓
✓
✓
✓
✓
✓
PP
when V
Transformer
Balun
✓
✓
20127777
π
IN
R
is 1.6
EQ
C)
18
shows C
reduce the frequency response gain peaking and thereby to
increase the ±0.5 dB gain flatness frequency.
Figures 20, 21 and Figure 22 show the FFT analysis results
with the setup shown in Figure 17.
FIGURE 19. Increasing ±0.5 dB Gain Flatness using
FIGURE 20. LMH6555 FFT Result When Used as the
FIGURE 21. LMH6555 FFT Result When Used as the
O
placed across the LMH6555 output terminals to
External Output Capacitance, C
Differential Driver to ADC081500
Differential Driver to ADC081500
(Lower Fs/2 Region Magnified)
O
20127743
20127744
20127776