Datasheet

4kTR
Seq
e
n
2
+ i
n
2
(R
Seq
+ (R
f
|| R
g
))
2
+ 4KTR
Seq
+ 4kt (R
f
|| R
g
)
NF = 10LOG
S
i
/N
i
S
o
/N
o
= 10LOG
e
ni
2
e
t
2
e
ni
=
e
n
2
+ 2 (i
n
· R
Seq
)
2
+ 4kT (2R
Seq
)
e
ni
=
e
n
2
+ (i
n+
· R
Seq
)
2
+ 4kTR
Seq
+ (i
n-
· (R
f
|| R
g
))
2
+ 4kT(R
f
|| R
g
)
LMH6654, LMH6655
www.ti.com SNOS956D –JUNE 2001–REVISED MARCH 2013
Figure 47. Non-Inverting Amplifier Noise Model
(3)
Figure 48. Noise Model with R
f
|| R
g
= R
seq
(4)
If bias current cancellation is not a requirement, then R
f
|| R
g
does not need to equal R
seq
. In this case, according
to Equation 3, R
f
and R
g
should be as low as possible in order to minimize noise. Results similar to Equation 3
are obtained for the inverting configuration on Figure 46 if R
seq
is replaced by R
b
|| R
g
is replaced by R
g
+ R
s
.
With these substitutions, Equation 3 will yield an e
ni
referred to the non-inverting input. Referring e
ni
to the
inverting input is easily accomplished by multiplying e
ni
by the ratio of non-inverting to inverting gains.
Noise Figure
Noise Figure (NF) is a measure of the noise degradation caused by an amplifier.
(5)
The noise figure formula is shown in Equation 5. The addition of a terminating resistor R
T
, reduces the external
thermal noise but increases the resulting NF.
The NF is increased because the R
T
reduces the input signal amplitude thus reducing the input SNR.
(6)
The noise figure is related to the equivalent source resistance (R
seq
) and the parallel combination of R
f
and R
g
.
To minimize noise figure, the following steps are recommended:
1. Minimize R
f
||R
g
2. Choose the Optimum R
s
(R
OPT
)
R
OPT
is the point at which the NF curve reaches a minimum and is approximated by:
R
OPT
≈ (e
n
/i
n
)
Copyright © 2001–2013, Texas Instruments Incorporated Submit Documentation Feedback 17
Product Folder Links: LMH6654 LMH6655