LMH6555 Datasheet, PDF (18/37 Pages) Intersil Corporation – Low Distortion 1.2 GHz Differential Driver

LMH6555
SNOSAJ1D – NOVEMBER 2006 – REVISED MARCH 2013
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To shift the LMH6555 differential output negative by about 100 mV, referring to the plot in Figure 32, R_X would be
chosen to be around 3.9 kΩ in the schematic of Figure 31(b) (using V_X = V_S = 3.3V).
In applications where V_IN has a built-in non-zero offset voltage, or when R_S1 and R_S2 are not 50Ω, the Figure 32
plot cannot be used to estimate the required value for R_X.
Consider the case of a more general offset correction application, shown in Figure 33(a), where R_S1 = R_S2 = 75Ω
and V_IN has a built-in offset of −50 mV. It is necessary to shift the differential output offset voltage of the
LMH6555 to 0 mV. Figure 33(b) is the Thevenin equivalent of the circuit in Figure 33(a) assuming R_X >> R_S2.
V_S = 3.3V
R_X
R_S1
75:
V_IN+
V_OUT-
R_S2
LMH6555
R_L
75:
100:
V_IN
V_IN-
with -50 mV
OFFSET
V_OUT+
(b)
R_S1
V_IN+
LMH6555
R_S2
V_IN-
R_S2 || R_X # 75:
V_TH # -50 mV +
75:
R_X
3.3V
V_OUT-
R_L
100:
V_OUT+
(a)
Figure 33. Offset Correction Example (R_S = 75Ω)
From the gain expression in Equation 44 (see Appendix) (but with opposite polarity because V_TH is applied to
V_IN^− instead):
V_{OUT =} -R_{F Ÿ}
V_TH 2R_S + 78
V_OUT
=
-430:
(150 + 78):
x
¨¨©§^-50
mV
+
75
R_X
3.3V
(20)
The expression derived for V_OUT in Equation 20 can be set equal to zero to solve for R_X resulting in R_X = 4.95
kΩ. If the differential output offset voltage, V_OOS, is also known, V_OUT could be set to a value equal to –V_OOS. For
example, if the V_OOS for the particular LMH6555 is +30 mV, then the following nulls the differential output:
V_OUT
=
-30
mV
=
(-1.89) _{¨¨©§}-50
mV
+
248
R_X
Ÿ R_X = 3.76 k:
(21)
R_X >> R_S2 confirming the assumption made in the derivation. Note that Equation 21, which is derived based on
the configuration in Figure 31(b), will yield a real solution for R_X if and only if:
V_OOS t (V_{IN_OFFSET} x 1.89)
For Figure 31(b) and with R_s = 75Ω
where
• V_{IN_OFFSET} is the source offset shown as −50 mV in Figure 33(a)
(22)
If Equation 22 were not satisfied, then Figure 31(a) offset correction, where R_X is tied to the V_IN+ side, should be
employed instead.
Alternatively, replace the V_X and R_X combination with a discrete current source or current sink. Because of a
current source’s high output impedance, there will be less gain imbalance. However, a current source might have
a relatively large output capacitance which could degrade high frequency performance.
INTERFACE DESIGN EXAMPLE
As shown in Figure 34 below, the LMH6555 can be used to interface an open collector output device (U1) to a
high speed ADC. In this application, the LMH6555 performs the task of amplifying and driving the 100Ω
differential input impedance of the ADC.
18
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