LMH6555 Datasheet, PDF (22/37 Pages) Intersil Corporation – Low Distortion 1.2 GHz Differential Driver

LMH6555
SNOSAJ1D – NOVEMBER 2006 – REVISED MARCH 2013
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For the component values shown in Figure 37 use:
1. V_C_High = V_CC – R_L (I_cQ + I_PP / 2 - I_N (differential) /2)
V_C_Low = V_CC – R_L (I_cQ - I_PP / 2 + I_N (differential) /2)
(31)
whitespace
I_N (differential) = I_PP * R_L / (R_L + R_S + R_G) = 1.88 mA (based on the values used.)
→V_C_High = 10 – 80.6 (10 + 15 / 2 − 1.88 /2) mA = 8.67V
→V_C_Low = 10 – 80.6 (10 − 15 / 2 + 1.88 /2) mA = 9.72V
(32)
whitespace
V_IN = V_X ± R_G. I_N (differential) /2
→V_IN_High = 0.3150 + 39 * 1.88 mA /2 = 0.3517V
→V_IN_Low = 0.3150 - 39 * 1.88 mA /2 = 0.2783V
(33)
Figure 38 shows the AC coupled implementation of the Figure 37 schematic along with the node voltages
marked to demonstrate the reduced V_{I_CM} of the LMH6555 and the increase in the U1 collector voltage
minimum.
V_CC
10V
R_L1
80.6:
1%
8.67V to 9.72V
R_L2
80.6:
1%
C_S1
0.01 PF
R_S1
523:
1%
10 mA + 15 mA_PP
U1
C_S2 R_S2
0.01 PF ^523:
1%
0.278V to 0.352V
V_IN+
LMH6555
V_IN-
V_OUT = 800 mV_PP
ADC081000/
ADC081500
Figure 38. AC Coupled Version of Figure 37
Note that the lower cut-off frequency is:
f_cut-off = 1 / (πReqC_S) where Req = R_S1+ R_S2 + R_{IN_DIFF} where R_{IN_DIFF} ≈ 78Ω
(34)
So, for the component values shown (C_S = 0.01 μF and R_S1 = R_S2 = 523Ω):
f_cut-off = 28.2 kHz
(35)
22
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