LMH730154 Datasheet, PDF (1/5 Pages) National Semiconductor (TI) – High Speed Differential Amplifier Evaluation Board

December 2004
LMH730154 High Speed Differential Amplifier Evaluation
Board
General Description
The LMH™730154 evaluation board is designed to aid in the
characterization of National Semiconductor’s High Speed
fully differential amplifiers.
Use the evaluation board as a guide for high frequency
layout and as a tool to aid in device testing and character-
ization.
The evaluation board schematic is shown below in Figure 1.
Refer to the product data sheets for recommendations for
component values.
Basic Operation
The LMH730154 evaluation board has been set up to pro-
vide maximum flexibility for evaluating National’s differential
operational amplifiers. The board supports fully differential
operation as well as single ended to differential and single
ended to single ended operation. For fully differential opera-
tion, use resistors RIN and R4 to set the input impedance of
the amplifier. Input resistance will be equal to 2*RIN || 2*R17.
Where RIN = R4 and R17= R18. For single ended mode
input resistance = RIN || R17, and R4 should be equal to
RIN/2. In all modes resistors R17, R18, R19 and R20 set the
gain of the amplifier. Amplifier gain = RG/RG = R19/R17,
where R19 = R20 and R17 = R18.
For differential output, load R15 and R16 with the desired
values to match the output load and leave R22 and R23
empty.
If single ended output is desired leave R15 and R16 empty
and load R22, R23 and an output transformer such as the
TC 4-1W from Mini Circuits. The TC 4-1W has a 4:1 imped-
ance ratio (2:1 turns/voltage ratio). This is particularly useful
for interfacing to 50Ω test equipment. When referencing the
transformer datasheet, the LMH730154 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 differen-
tial 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 = (R22 + R23 + RL*4), where RL is the impedance
from pin 5 of the transformer to the load. The component
marked R12 is not normally used.
Layout Considerations
Printed circuit board layout and supply bypassing play major
roles in determining high frequency performance. When de-
signing 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 ( ^~6.8 µF) capacitors on both supplies (C1
& C2).
5. Near the device use 0.01 µF ceramic capacitors from
both supplies to ground (C10, C12).
6. A capacitor between V⁺ and V^− (C13) 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 LMH730154 Evaluation board is
included on the next page in Figure 2.
Measurement Hints
Balance, CMRR and HD2 are highly dependent on resistor
matching. Use 0.1 or 0.01% resistors.
The LMH730154 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 R15 and R16 empty. Likewise, when making single
ended output measurements leave R22 & R23 empty.
Many differential amplifiers are optimized for the higher im-
pedances 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 rec-
ommended.
LMH™ is a trademark of National Semiconductor Corporation.
© 2004 National Semiconductor Corporation MS201319
www.national.com