LMH6555_14 Datasheet, PDF (13/37 Pages) Texas Instruments – LMH6555 Low Distortion 1.2 GHz Differential Driver

LMH6555
www.ti.com
SNOSAJ1C – NOVEMBER 2006 – REVISED MARCH 2011
SINGLE-ENDED INPUT
The following is the procedure for determining the device operating conditions for single ended input applications.
This example will use the schematic shown in Figure 25.
V_IN
0.3 V_PP
R_S1
50:
R_S2
50:
V_IN+
LMH6555
V_IN-
V_OUT-
R_L
100:
V_OUT+
Figure 25. Single-Ended Input Drive
1. Determine the driven input’s (V_IN+ or V_IN−) swing knowing that each input common mode impedance to
ground (R_IN) is 50Ω:
V_IN+ (or V_IN−) = V_IN · R_IN/(R_IN + R_S)
(1)
whitespace
For Figure 25:
VIN+ = 0.3 VPP · 50/(50+50) = 0.15 VPP
(2)
whitespace
2. Calculate V_OUT knowing the Insertion Gain (A_{V_DIFF}):
V_OUT = (V_IN/2) · A_{V_DIFF}
A_{V_DIFF} = 2 · R_F/ (2R_S + R_{IN_DIFF})
where
• R_F = 430Ω
• R_{IN_DIFF} = 78Ω
(3)
whitespace
For Figure 25:
R_S = 50Ω → A_{V_DIFF} = 4.83 V/V
V_OUT = (0.3 V_PP/2) · 4.83 V/V= 724.5 mV_PP
(4)
whitespace
3. Determine the peak-to-peak differential current (I_{IN_DIFF}) through the device’s differential input impedance
(R_{IN_DIFF}) which would result in the V_OUT calculated in step 2:
I_{IN_DIFF} = V_OUT/ R_F
(5)
whitespace
For Figure 25:
I_{IN_DIFF} = 724.5 mV_PP/ 430Ω = 1.685 mA_PP
(6)
whitespace
4. Determine the swing across the input terminals (V_{IN_DIFF}) which would give rise to the I_{IN_DIFF} calculated in
step 3 above.
V_{IN_DIFF} = I_{IN_DIFF} · R_{IN_DIFF}
(7)
whitespace
For Figure 25:
V_{IN_DIFF} = 1.685 mA_PP · 78Ω = 131.4 mV_PP
(8)
whitespace
5. Calculate the undriven input’s swing, based on V_{IN_DIFF} determined in step 4 and V_IN+ calculated in step 1:
V_IN− = V_IN+ - V_{IN_DIFF}
(9)
whitespace
Copyright © 2006–2011, Texas Instruments Incorporated
Submit Documentation Feedback
13
Product Folder Links: LMH6555