Datasheet

LMH6702
SNOSA03F NOVEMBER 2002REVISED MARCH 2013
www.ti.com
DC ACCURACY AND NOISE
Example below shows the output offset computation equation for the non-inverting configuration using the typical
bias current and offset specifications for A
V
= + 2:
Output Offset :
V
O
= (±I
BN
· R
IN
± V
IO
) (1 + R
F
/R
G
) ± I
BI
· R
F
(1)
Where R
IN
is the equivalent input impedance on the non-inverting input.
Example computation for A
V
= +2, R
F
= 237, R
IN
= 25:
V
O
= (±6μA · 25 ± 1mV) (1 + 237/237) ± 8μA · 237 = ±4.20mV (2)
A good design, however, should include a worst case calculation 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
Application 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 combined 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). Texas Instruments 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 SOT-23 CLC730216
LMH6702MA SOIC CLC730227
These free evaluation boards are shipped when a device sample request is placed with Texas Instruments.
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