LMH6555_14 Datasheet, PDF (21/37 Pages) Texas Instruments – LMH6555 Low Distortion 1.2 GHz Differential Driver

LMH6555
www.ti.com
SNOSAJ1C – NOVEMBER 2006 – REVISED MARCH 2011
V_IN = V_X (R_N – R_G) / R_N + I_N R_G
→V_IN_High = 0.4595 * (1358- 39) / 1358 + 7.05 mA * 39 = 0.721V
→V_IN_Low = 0.4595 * (1358- 39) / 1358 + 5.19 mA * 39 = 0.649V
(29)
whitespace
Figure 36 shows the complete solution using the values derived above, with the node voltages marked on the
schematic for reference.
V_CC
10V
R_L1
169:
1%
6.22V to 8.44V
10 mA + 15 mA_PP
U1
R_L2
169:
1% R_S1
1.15 k:
1%
R_S2
1.15 k:
1%
0.65V to 0.72V
V_IN+
LMH6555
V_IN-
V_OUT = 800 mV_PP
ADC081000/
ADC081500
Figure 36. Implementation #1 of Figure 34
Design Example
It is important to note that the matching of the resistors on either input side of the LMH6555 (R_S1 to R_S2 and R_L1
to R_L2) is very important for output offset voltage and gain balance. This is particularly true with values of R_S
higher than the nominal 50Ω. Therefore, in this example, 1% or better resistor values are specified.
If the U1 collector voltage turns out to be too low due to the loading of the LMH6555, lower R_L. Lower values of
R_L result in lower R_S which in turn increases the LMH6555's V_{I_CM} because of increased pull up action towards
V_CC. The upper limit on V_{I_CM} is 2V. Figure 37 shows the 2^nd implementation of this same application with
lowered values of R_L and R_S. Notice that the lower end of U1’s collector voltage and the upper end of LMH6555’s
V_{I_CM} have both increased compared to the 1^st implementation.
V_CC
10V
R_L
80.6:
1%
7.6V to 8.7V
10 mA + 15 mA_PP
U1
R_L
80.6:
1% RS
523:
1%
R_S
523:
1%
1.13V to 1.20V
V_OUT = 800 mV_PP
V_IN+
LMH6555
V_IN-
ADC081000/
ADC081500
Figure 37. Implementation #2 of Figure 34 Design Example
An alternative would be to AC couple the LMH6555 inputs. With this approach, the design steps would be very
similar to the ones outlined except that there would be no common mode interaction between the LMH6555 and
U1 and this results in fewer design constraints:
V_x / R_E = 12.6 mA → V_x = 0.3150V
(30)
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