LMH6555EVAL National Semiconductor, LMH6555EVAL Datasheet - Page 15

• Manufacturer Part Number: LMH6555EVAL
• Manufacturer: National Semiconductor

Specifications of LMH6555EVAL

• Lead Free Status / Rohs Status: Not Compliant

FIGURE 10. LMH6555 Differential Output Shift Due to R
To shift the LMH6555 differential output negative by about
100 mV, referring to the plot in Figure 10, R
to be around 3.9 kΩ in the schematic of Figure 9(b) (using
V
In applications where V
age, or when R
cannot be used to estimate the required value for R
Consider the case of a more general offset correction appli-
cation, shown in Figure 11(a), where R
V
differential output offset voltage of the LMH6555 to 0 mV.
Figure 11(b) is the Thevenin equivalent of the circuit in Figure
11(a) assuming R
X
IN
= V
FIGURE 11. Offset Correction Example (R
has a built-in offset of −50 mV. It is necessary to shift the
S
= 3.3V).
S1
X
and R
>> R
IN
in Figure 9
S2
S2
has a built-in non-zero offset volt-
.
are not 50Ω, the Figure 10 plot
(b)
(a)
S1
X
= R
would be chosen
S2
20127753
20127735
20127734
S
= 75Ω and
= 75Ω)
X
.
X
15
From the gain expression in Equation 4 (see Appendix) (but
with opposite polarity because V
The expression derived for V
equal to zero to solve for R
differential output offset voltage, V
could be set to a value equal to –V
V
nulls the differential output:
R
Note that Equation 2, which is derived based on the configu-
ration in Figure 9(b), will yield a real solution for R
if:
where V
Figure 11(a).
If Equation 3 were not satisfied, then Figure 9(a) offset cor-
rection, where R
instead.
Alternatively, replace the V
crete current source or current sink. Because of a current
source’s high output impedance, there will be less gain im-
balance. However, a current source might have a relatively
large output capacitance which could degrade high frequency
performance.
INTERFACE DESIGN EXAMPLE
As shown in Figure 12 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.
V
CM_REF
OOS
X
FIGURE 12. Differential Amplification and ADC Drive
>> R
for the particular LMH6555 is +30 mV, then the following
buffer not shown
S2
IN_OFFSET
confirming the assumption made in the derivation.
X
is tied to the V
is the source offset shown as −50 mV in
X
X
resulting in R
and R
OUT
IN
TH
+
in Equation 1 can be set
is applied to V
OOS
side, should be employed
X
OOS
combination with a dis-
, is also known, V
. For example, if the
X
= 4.95 kΩ. If the
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X
IN
if and only
−
instead):
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OUT
(1)
(2)
(3)