Datasheet

R
G
R
G
R
F
R
F
R
O
+
-
R
O
LMH6554
IN-
IN+
ADC
V
+
V
-
V
O
+
-
R
T
R
S
R
M
V
S
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
S
V
CM
R
L
=100:
200:
50:
V
EN
a
+
-
200:
200:
200:
50:
67:
67:
R
S
50:
R
S
50:
LMH6554
+
V
IN
-
-
V
OUT
+
R
G
R
G
R
F
R
F
R
IN
LMH6554
SNOSB30O –OCTOBER 2008–REVISED MARCH 2013 www.ti.com
Here is the expression for the input impedance, R
IN
, as defined in Figure 30:
R
IN
= 2R
G
When driven from a differential source, the LMH6554 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. With an intrinsic device CMRR of greater than 70 dB, using 0.1% resistors will give a
worst case CMRR of around 50 dB for most circuits.
The circuit configuration shown in Figure 30 was used to measure differential S-parameters in a 100Ω
environment at a gain of 1 V/V. Refer to Figure 28 in Typical Performance Characteristics V
S
= ±2.5V for
measurement results.
Figure 30. Differential S-Parameter Test Circuit
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 LMH6554 provides excellent performance as a
single-ended input to differential output converter down to DC. Figure 31 shows a typical application circuit where
an LMH6554 is used to produce a balanced differential output signal from a single ended source.
Figure 31. Single Ended Input with Differential Output
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