LMH6555EVAL National Semiconductor, LMH6555EVAL Datasheet - Page 17

• Manufacturer Part Number: LMH6555EVAL
• Manufacturer: National Semiconductor

Specifications of LMH6555EVAL

• Lead Free Status / Rohs Status: Not Compliant

It is important to note that the matching of the resistors on
either input side of the LMH6555 (R
is very important for output offset voltage and gain balance.
This is particularly true with values of R
inal 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
in lower R
cause of increased pull up action towards V
on V
same application with lowered values of R
that the lower end of U1’s collector voltage and the upper end
of LMH6555’s V
1
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 re-
sults in fewer design constraints:
V
For the component values shown in Figure 15 use:
V
V
I
on the values used.)
N
st
x
C
C
FIGURE 15. Implementation #2 of Figure 12 Design
(differential) = I
/ R
implementation.
_High = V
_Low = V
I_CM
E
FIGURE 14. Implementation #1 of Figure 12
= 12.6 mA
is 2V. Figure 15 shows the 2
S
which in turn increases the LMH6555's V
CC
CC
– R
– R
I_CM
PP
→
L
L
(I
* R
V
(I
Design Example
have both increased compared to the
cQ
x
cQ
L
= 0.3150V
- I
+ I
Example
/ (R
PP
PP
L
/ 2 + I
+ R
/ 2 - I
L
. Lower values of R
S
N
S1
+ R
N
nd
(differential) /2)
(differential) /2)
to R
S
implementation of this
G
higher than the nom-
) = 1.88 mA (based
CC
S2
L
. The upper limit
and R
and R
L1
S
. Notice
I_CM
L
to R
result
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be-
L2
)
17
Figure 16 shows the AC coupled implementation of the
Figure 15 schematic along with the node voltages marked to
demonstrate the reduced V
crease in the U1 collector voltage minimum.
Note that the lower cut-off frequency is:
f_cut-off = 1 / (
where R
So, for the component values shown (C
= R
f_cut-off = 28.2 kHz
DATA ACQUISITION APPLICATIONS
Figure 17 shows the LMH6555 used as the differential driver
to the National Semiconductor ADC081500 running at 1.5G
samples/second.
FIGURE 17. Schematic of the LMH6555 Interfaced to the
→
→
V
→
→
IN
V
V
V
V
S2
C
C
IN
IN
= V
_High = 10 – 80.6 (10 + 15 / 2 − 1.88 /2) mA = 8.67V
_Low = 10 – 80.6 (10 − 15 / 2 + 1.88 /2) mA = 9.72V
_High = 0.3150 + 39 * 1.88 mA /2 = 0.3517V
_Low = 0.3150 - 39 * 1.88 mA /2 = 0.2783V
FIGURE 16. AC Coupled Version of Figure 15
= 523Ω):
X
IN_DIFF
± R
G
. I
≈
π
N
ReqC
78Ω
(differential) /2
S
ADC081500
) where Req = R
I_CM
of the LMH6555 and the in-
S
S1
= 0.01 μF and R
+ R
S2
www.national.com
+ R
IN_DIFF
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S1