LMH6555 Datasheet, PDF (19/37 Pages) Intersil Corporation – Low Distortion 1.2 GHz Differential Driver

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V_CC
LMH6555
SNOSAJ1D – NOVEMBER 2006 – REVISED MARCH 2013
R_L2
U1
R_L1
R_F1
R_S1
R_G1
+
-
R_S2
R_G2
R_F2
LMH6555
R_T2
50:
R_T1
50:
V_OUT-
ADC081000/
ADC081500
V_OUT+
V_CMO
V_{CM_REF}
V_{CM_REF} buffer not shown
Figure 34. Differential Amplification and ADC Drive
For applications similar to the one shown in Figure 34, the following conditions should be maintained:
1. The LMH6555 differential output voltage has to comply with the ADC full scale voltage (800 mV_PP in this
case).
2. The LMH6555 input Common Mode Voltage Range is observed. “CMVR”, as specified in Electrical
Characteristics, is to be between −0.3V and 2.0V for the specified CMRR.
3. U1 collector voltage swing must to be observed so that the U1 output transistors do not saturate. The
expected operating range of these output transistors is defined by the specifications and operating conditions
of U1.
Consider a numerical example (R_L refers to R_L1 & R_L2, R_S refers to R_S1 & R_S2).
Assume:
V_CC = 10V, U1 peak-to-peak collector current (I_PP) = 15 mA_PP with 10 mA quiescent (I_cQ), and minimum
operational U1 collector voltage = 6V.
Here are the series of steps to take in order to carry out this design:
a. Select the R_L value which allows compliance with the U1 collector voltage (6V in this case) with 1V extra as
margin because of LMH6555 loading.
R_L = [10 - (6+1)] V / (10+ 7.5) mA = 171Ω
Choose 169Ω, 1% resistors for R_L
b. Find the value of R_S to get the proper swing at the output (800 mV_PP). To do so, convert the input stage into
its Norton equivalent as shown in Figure 35
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