LMH6555EVAL National Semiconductor, LMH6555EVAL Datasheet - Page 10

• Manufacturer Part Number: LMH6555EVAL
• Manufacturer: National Semiconductor

Specifications of LMH6555EVAL

• Lead Free Status / Rohs Status: Not Compliant

www.national.com
Application Information
See the Definition of Terms and Conditions section for defi-
nition of terms used.
GENERAL
The LMH6555 consists of three individual amplifiers: The
V
Being a differential amplifier, the LMH6555 will not respond
to the common mode input (as long as it is within its input
common mode range) and instead the output common mode
The single ended input and output impedances of the
LMH6555 I/O pins are close to 50Ω as specified in the Elec-
trical Characteristics table (R
drive, the differential input impedance (R
78Ω.
The device nominal input common mode voltage (V
close to 0.3V when R
the input source will experience a DC current with 0V input.
Because of this, the differential output offset voltage is influ-
enced by the matching between R
ended input condition, if the signal source is AC coupled to
one input, the undriven input needs to also be AC coupled in
order to cancel the output offset voltage (V
In applications where low output offset is required, it is pos-
sible to inject some current to the appropriate input
OUT
+
driver, V
OUT
−
driver, and the common mode amplifier.
S1
and R
IN
S2
and R
of Figure 1 are open. Thus,
S1
O
and R
FIGURE 1. Single Ended to Differential Conversion
). With differential input
IN_DIFF
OOS
S2
. So, in a single
).
) is close to
I_CM
) is
10
is forced by the built-in common mode amplifier with
V
output of most differential high speed ADC’s is tied to the
V
mode control. In some cases, the output drive capability of the
ADC V
increase its current capability in order to drive the V
The LMH6555 Electrical Characteristics table shows the gain
(Gain_V
the device output common mode.
(V
offset voltage of the LMH6555. This is explained later in this
document. The nominal value of R
the insertion gain (A
with the input AC coupled through equal valued DC blocking
capacitors (C) in series with V
coupling capacitors need to be large enough to not block the
low frequency content. The lower cutoff frequency will be
1/(
≈
The single ended output impedance of the LMH6555 is 50Ω.
The LMH6555 Electrical Characteristics shows the device
performance with 100Ω differential output load, as would be
the case if a device such as the ADC081000/ ADC081500
(single/ dual ADC) were being driven.
CM_REF
CM_REF
IN
78Ω.
π
+
R
or V
EQ
CMO
CM_REF
C)Hz with R
as its input. As shown, in Figure 1 below, the V
input of the LMH6555 for direct output common
IN
output may need an external buffer, as shown, to
−
) as an effective method of trimming the output
) and the offset (V
V_DIFF
EQ
= R
). The LMH6555 can also be used
S1
+ R
20127704
IN
S2
+
OS_CM
and V
S1
+ R
and R
IN_DIFF
) from the V
IN
−
. In this case, the
S2
where R
will also affect
CM_REF
CM_REF
IN_DIFF
CMO
pin.
to