Datasheet

R
G
R
G
R
F
R
F
R
O
+
-
R
O
LMH6553
IN-
IN+
ADC
V
+
V
-
V
OUT
+
-
R
T
R
S
R
M
V
IN
A
V
, R
IN
a
A
V
=
2(1 - E
1
)
E
1
+
E
2
¨
¨
©
§
¨
¨
©
§
R
IN
=
2R
G
+ R
M
(1-E
2
)
1 + E
2
¨
¨
©
§
¨
¨
©
§
E
2
=
R
G
+ R
M
R
G
+ R
F
+ R
M
¨
¨
©
§
¨
¨
©
§
¨
¨
©
§
E
1
=
R
G
R
G
+ R
F
¨
¨
©
§
R
S
=
R
T
|| R
IN
R
M
=
R
T
|| R
S
+
-
V
CM
+
-
V
CLAMP
V
IN
V
CM
R
L
275:
50:
V
CLAMP
a
+
-
275:
275:
275:
50:
61:
61:
R
S
= 50:
R
S
= 50:
LMH6553
SNOSB07H –SEPTEMBER 2008–REVISED MARCH 2013 www.ti.com
When driven from a differential source, the LMH6553 provides low distortion, excellent balance, and common
mode rejection. This is true provided the resistors R
F
, R
G
and R
O
are well matched and strict symmetry is
observed in board layout.
Figure 55. Differential S-Parameter Test Circuit
The circuit configuration shown in Figure 55 was used to measure differential S parameters in a 50Ω
environment at a gain of 1 V/V. Refer to Figure 45 and Figure 46 in Typical Performance Characteristics for
measurement results.
SINGLE-ENDED INPUT TO DIFFERENTIAL OUTPUT OPERATION
In many applications, it is required to drive a differential input ADC from a single-ended source. Traditionally,
transformers have been used to provide single to differential conversion, but these are inherently bandpass by
nature and cannot be used for DC coupled applications. The LMH6553 provides excellent performance as a
single-to-differential converter down to DC. Figure 56 shows a typical application circuit where an LMH6553 is
used to produce a differential signal from a single ended source.
Figure 56. Single-Ended Input with Differential Output
When using the LMH6553 in single-to-differential mode, the complementary output is forced to a phase inverted
replica of the driven output by the common mode feedback circuit as opposed to being driven by its own
complementary input. Consequently, as the driven input changes, the common mode feedback action results in a
varying common mode voltage at the amplifier's inputs, proportional to the driving signal. Due to the non-ideal
common mode rejection of the amplifier's input stage, a small common mode signal appears at the outputs which
is superimposed on the differential output signal. The ratio of the change in output common mode voltage to
output differential voltage is commonly referred to as output balance error. The output balance error response of
the LMH6553 over frequency is shown in the Typical Performance Characteristics.
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