LMH6554_14 Datasheet, PDF (12/24 Pages) National Semiconductor (TI) – LMH6554 2.8 GHz Ultra Linear Fully Differential Amplifier

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.
200:
R_S
67:
50:
V_S
a
+
V_IN
R_IN
-
R_S
50: ^67:
200:
R_{G VCM}
R_G
200:
R_F
+
LMH6554
-
V_EN
R_F
50:
-
VOUT
+
R_L=100:
50:
200:
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.
A_V, R_IN
R_S
R_G
a VS
R_T
V_CM
R_M
R_G
+-
R_F
V⁺
+
LMH6554
-
V^-
R_F
R_O
-
V_O
+
R_O
IN-
ADC
IN+
A_V =
¨©§¨
2(1 - E1)
E₁ + E₂
R_IN
=
¨¨©§^2RG
+
1
R_M (1-E₂)
+ E₂
E₁ = _{¨¨©§RG}^R+G_RF
E₂ = _{¨¨©§RG}^R+G_R+F^R+M_RM
R_S = R_T || R_IN
R_M = R_T || R_S
Figure 31. Single Ended Input with Differential Output
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