LMH6552SDEVAL Eval Board AppNote Datasheet by Texas Instruments

l TEXAS INSTRUMENTS (SNOSAXQ) (SNOSAXQ)
User's Guide
SNOA496BFebruary 2008Revised May 2013
AN-1743 LMH6552SDEVAL High Speed Differential
Amplifier Evaluation Board
1 General Description
The LMH6552SDEVAL evaluation board is designed to aid in the characterization of Texas Instruments
LMH6552 fully differential amplifier in an 8-pin LLP package. The LMH6552 is part of the LMH™ high-
speed amplifier family.
Use the evaluation board as a guide for high frequency layout and as a tool to aid in device testing and
characterization.
The evaluation board schematic is shown in Figure 1. For recommended for component values, see the
device-specific data sheets.
2 Basic Operation
The LMH6552SDEVAL evaluation board has been set up to provide maximum flexibility for evaluating TI’s
differential LMH6552 operational amplifier. The board supports fully differential operation as well as single-
ended to differential and single-ended to single-ended operation. For fully differential operation, use
resistors R5and R6to set the input impedance of the amplifier. Input resistance will be equal to 2*R5||
2*R1. Where R5= R6and R1= R2. In this mode resistors R1, R2, R3and R4set the gain of the amplifier.
Amplifier gain = RF/RG= R4/R2where R1= R2and R3= R4. For single-ended input mode of operation, the
input and termination resistance must be properly configured to give the correct gain and RIN. For
example, if a gain of 2 V/V is desired, R5= 28.7 , R1= R2= 127 , R3= R4= 275 , and R6= 68.1 ,
which will make RIN = 50 at the most positive node of R5looking into R2. For more details of single-
ended input mode calculations, see the LMH6552 1.5 GHz Fully Differential Amplifier Data Sheet
(SNOSAX9). Components R7, R8, C6, and C7should be used for AC-couple applications otherwise they
can be left empty.
For differential output applications, load R12 and R13 with the desired values to match the output load and
leave R10 and R11 empty.
If single-ended output is desired leave R12 and R13 empty and load R10, R11 and an output transformer
such as the ADT4-1WT from mini circuits. The ADT4-1WT has a 4:1 impedance ratio (2:1 turns/voltage
ratio). This is particularly useful for interfacing to 50 test equipment. When referencing the transformer
data sheet, the evaluation board has the primary windings on the output side of the evaluation board and
the amplifier is driving the secondary windings. This provides a step down transformation from the
differential amplifier output to the test equipment. The center-tapped secondary winding also allows a
differential to single ended conversion (Balun). The impedance seen by the differential amplifier = (R10 +
R11 + RL*4), where RLis the impedance from pin 3 of the transformer to the load.
Pin 7 on the LMH6552 device is the enable (EN) pin that can be used to disable the device with an
external signal. For more details, see the LMH6552 1.5 GHz Fully Differential Amplifier Data Sheet
(SNOSAX9).
LMH is a trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
1
SNOA496BFebruary 2008Revised May 2013 AN-1743 LMH6552SDEVAL High Speed Differential Amplifier Evaluation
Board
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Layout Considerations
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3 Layout Considerations
Printed circuit board (PCB) layout and supply bypassing play major roles in determining high frequency
performance. When designing your own board use these evaluation boards as a guide and follow these
steps to optimize high frequency performance:
1. Symmetry is of the utmost importance.
2. Use precision resistors 0.1% or 0.01%.
3. Use a ground plane.
4. Include large ( ~ 10 μF) capacitors on both supplies (C1and C3).
5. Near the device use 0.01 μF ceramic capacitors from both supplies to ground (C2, C4).
6. A capacitor between V+and V(C5) is optional, but will help lower distortion.
7. Remove the ground and power planes from under and around the part, especially the input and output
pins.
8. Minimize all trace lengths.
9. Use terminated transmission lines for long traces.
Sample artwork for the LMH6552SDEVAL evaluation board is included in Figure 2.
4 Measurement Hints
Balance, CMRR and HD2 are highly dependent on resistor matching. Use 0.1 or 0.01% resistors.
The LMH6552SDEVAL evaluation board is designed for differential or single-ended output measurements,
but not both at the same time. When not using the transformer make sure to leave R10 and R11 empty.
Likewise, when making single-ended output measurements leave R12 and R13 empty.
Many differential amplifiers are optimized for the higher impedances represented by most ADCs.
On a differential amplifier both inputs are inverting, keep parasitic capacitance to a minimum on both
inputs. Also, using probes of any kind on a differential circuit is not recommended.
T1 = Mini Circuits ADT4-1WT
2AN-1743 LMH6552SDEVAL High Speed Differential Amplifier Evaluation SNOA496BFebruary 2008Revised May 2013
Board Submit Documentation Feedback
Copyright © 2008–2013, Texas Instruments Incorporated
POWER SUPPLY CONNECTIONS
GND
A
V+
V+V-
V-
A
A
R14
R6
R5
R15
R3
R1
V+
1 2 3 4
5
678
R2
R4R13
R11 T1
4
6
1
3
A
R12
R10
U1
IN-
VCM
V+
OUT+
IN+
V-
OUT-
V-
ADT4-1WT+
C1C2
C5
C4C3
A
SMA
EN/VCLAMP
R9OUT
OUT+
OUT-
VCM
IN-
IN+
A
SMA
A
SMA
A
SMA
A
SMA
A
SMA
A
C8
C9
C6
R7
A
C7
R8
A
A
AA
A
SMA
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Measurement Hints
Figure 1. Board Schematic
3
SNOA496BFebruary 2008Revised May 2013 AN-1743 LMH6552SDEVAL High Speed Differential Amplifier Evaluation
Board
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Measurement Hints
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Figure 2. Board Layout
4AN-1743 LMH6552SDEVAL High Speed Differential Amplifier Evaluation SNOA496BFebruary 2008Revised May 2013
Board Submit Documentation Feedback
Copyright © 2008–2013, Texas Instruments Incorporated
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