LMH6552 Datasheet, PDF (18/38 Pages) Texas Instruments – 1.5-GHz Fully Differential Amplifier

LMH6552
SNOSAX9I – APRIL 2007 – REVISED JANUARY 2015
Typical Applications (continued)
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R_F
R_O
R_G
V_S
=
+
V_CM
-
R_G
C_L
R_L V_O
R_O
R_F
ENABLE
Figure 39. Typical Fully Differential Application Schematic
9.2.1.1 Design Requirements
One typical application for the LMH6552 is to drive an ADC. The following design is a single ended to differential
circuit with an input impedance of 50 Ω and an output impedance of 100 Ω. The VCM voltage of the amplifier
needs to be set to the same voltage as the ADC reference voltage which is typically 1.2 V. Figure 45 shows the
design equations required to set the external resistor values. This design also requires a gain of 1 and -74 dBc
THD at 70 MHz.
9.2.1.2 Detailed Design Procedure
To match the input impedance of the circuit in Figure 45 to a specified source resistance, RS, requires that RT ||
RIN = RS. The equations governing RIN and AV for single-to-differential operation are also provided in
Figure 45. These equations, along with the source matching condition, must be solved iteratively to achieve the
desired gain with the proper input termination. Component values for several common gain configurations in a
50-Ω environment are given in Table 1. Gain Component Values for 50-Ω System WSON Package. Typically
RS=50 Ω while RM=RS||RT.
9.2.1.2.1 WSON Package
Due to its size and lower parasitics, the WSON requires the lower optimum value of 275 Ω for R_F. This will give a
flat frequency response with minimal peaking. With a lower R_F value the WSON package will have a reduction in
noise compared to the SOIC with its optimum R_F = 360 Ω.
9.2.1.2.2 Fully Differential Operation
The LMH6552 will perform best in a fully differential configuration. The circuit shown in Figure 39 is a typical fully
differential application circuit as might be used to drive an analog to digital converter (ADC). In this circuit the
closed loop gain A_V = V_OUT/ V_IN = R_F/R_G, where the feedback is symmetric. The series output resistors, R_O, are
optional and help keep the amplifier stable when presented with a capacitive load. Refer to Driving Capacitive
Loads for details.
When driven from a differential source, the LMH6552 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 80 dB, using 0.1% resistors will give a worst case
CMRR of around 60 dB for most circuits.
The circuit configuration shown in Figure 40 was used to measure differential S parameters in a 50-Ω
environment at a gain of 1 V/V. Refer to Figure 34 and Figure 35 in Typical Characteristics V⁺ = +5 V, V^− = −5 V
for measurement results.
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