LMH6555 Datasheet, PDF (24/37 Pages) Intersil Corporation – Low Distortion 1.2 GHz Differential Driver

LMH6555
SNOSAJ1D – NOVEMBER 2006 – REVISED MARCH 2013
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Table 1 compares the LMH6555 solution (Figure 39) vs. that of the balun transformer coupling (Figure 40) for
various categories.
Table 1. ADC Input Coupling Schemes Compared
Category
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
LMH6555
✓
✓
✓
✓
✓
✓
✓
✓
(see below)
Preferred Solution
Balun Transformer
✓
✓
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 41 shows C_O placed across the LMH6555 output
terminals to reduce the frequency response gain peaking and thereby to increase the ±0.5 dB gain flatness
frequency.
R_S1
340 mV_PP 50:
a ^VIN
R_S2
50:
R_F1
V_IN+ ^RG1
+
-
V_IN- _RG2
LMH6555 ^RF2
R_T2
50:
R_T1
50:
V_OUT = 0.8 V_PP
C_O
ADC081000/
ADC081500
V_{CMO SPI}
V_{CM_REF}
3.3V
OPT
+
LMV321 _OPT
-
Figure 41. Increasing ±0.5 dB Gain Flatness using External Output Capacitance, C_O
Figure 42, Figure 43, and and Figure 44 show the FFT analysis results with the setup shown in Figure 39.
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