LMH6702MF datasheet(11/13 Pages) NSC

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Application Section (Continued)

DC ACCURACY AND NOISE

Example below shows the output offset computation equa-

tion for the non-inverting configuration using the typical bias

current and offset specifications for A

V =+2:

Output Offset : V

O =(±IBN ·RIN ± VIO)(1+RF/RG) ± IBI ·RF

Where R

IN is the equivalent input impedance on the non-

inverting input.

Example computation for A

V = +2, RF = 237

Ω,R

IN =25

Ω:

O =(±6µA · 25

Ω ± 1mV) (1 + 237/237) ± 8µA · 237 =

±4.20mV

A good design, however, should include a worst case calcu-

lation using Min/Max numbers in the data sheet tables, in

order to ensure "worst case" operation.

Further improvement in the output offset voltage and drift is

possible using the composite amplifiers described in Appli-

cation Note OA-7. The two input bias currents are physically

unrelated in both magnitude and polarity for the current

feedback topology. It is not possible, therefore, to cancel

their effects by matching the source impedance for the two

inputs (as is commonly done for matched input bias current

devices).

The total output noise is computed in a similar fashion to the

output offset voltage. Using the input noise voltage and the

two input noise currents, the output noise is developed

through the same gain equations for each term but com-

bined as the square root of the sum of squared contributing

elements. See Application Note OA-12 for a full discussion of

noise calculations for current feedback amplifiers.

PRINTED CIRCUIT LAYOUT

Generally, a good high frequency layout will keep power

supply and ground traces away from the inverting input and

output pins. Parasitic capacitances on these nodes to

ground will cause frequency response peaking and possible

circuit oscillations (see Application Note OA-15 for more

information). National Semiconductor suggests the following

evaluation boards as a guide for high frequency layout and

as an aid in device testing and characterization:

Device

Package

Evaluation Board

Part Number

LMH6702MF

SOT23-5

CLC730216

LMH6702MA

SOIC

CLC730227

These free evaluation boards are shipped when a device

sample request is placed with National Semiconductor.

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