lmh6629sdx National Semiconductor Corporation, lmh6629sdx Datasheet - Page 15

• Manufacturer Part Number: lmh6629sdx
• Description: Lmh6629 Ultra-low Noise, High Speed Operational Amplifier
• Manufacturer: National Semiconductor Corporation

TOTAL INPUT NOISE vs. SOURCE RESISTANCE
To determine maximum signal-to-noise ratios from the
LMH6629, an understanding of the interaction between the
amplifier’s intrinsic noise sources and the noise arising from
its external resistors is necessary.
Equation 1
voltage noise density (e
Equation 2
R
Figure 7
V
and external components affecting gain (A
connected to an ideal noiseless amplifier.
S
g
FIGURE 7. Non-Inverting Amplifier Equivalent Noise
= R
source which reaches the non-inverting input of
seq
Equation 1: General Noise Equation
schematically shows e
Equation 2: Noise Equation with
for bias current cancellation:
provides the general form for total equivalent input
is a simplification of
R
Source Schematic
f
|| R
ni
).
g
= R
ni
Equation 1
seq
alongside V
Figure 6
FIGURE 6. Non-Inverting Amplifier Noise Model
that assumes R
v
= 1 + R
IN
describes the
(the portion of
30068023
f
Figure
/ R
g
), all
(1)
(2)
f
4)
||
15
noise model for the non-inverting amplifier configuration
showing all noise sources. In addition to the intrinsic input
voltage noise (e
there is also thermal voltage noise (e
with each of the external resistors.
Figure 8
sumption.
resistance (R
source of
given (R
cancellation. The total equivalent output voltage noise (e
e
As seen in
noise (e
below 15Ω. Between 15Ω and 2.5 kΩ, e
thermal noise (e
sistance R
where the LMH6629 has the best (lowest) Noise Figure (NF)
for the case where R
Above 2.5 kΩ, e
(i
the contribution from voltage noise and current noise of
LMH6629 is equal. For example, configured with a gain of
+10V/V giving a −3dB of 825 MHz and driven from R
R
produces a total equivalent output noise voltage (e
*
ni
n
√
f
||R
*A
=
(1.57 * 825 MHz)) of 385 μV
V
√
G
.
(2) i
= 20Ω (e
FIGURE 8. Noise Model with R
n
seq
illustrates the equivalent noise model using this as-
) of the amplifier for equivalent source resistances
Equation
Figure 9
n
seq
Figure
R
) which assumes R
seq
; incidentally, this is the range of R
seq
ni
ni
). When R
t
n
) with all of the contributing voltage noise
=
= 1.07 nV
is dominated by the amplifier’s current noise
) and current noise (i
9, e
√
is a plot of e
2. This plot gives the expected e
(4kT(2R
seq
30068020
ni
= R
is dominated by the intrinsic voltage
seq
√
f
Hz from
seq
|| R
= 190Ω (i.e., R
)) of the equivalent source re-
rms
f
||R
g
ni
.
.
against equivalent source
g
t
Figure
= R
=
n
√
ni
f
(4KTR)) associated
seq
is dominated by the
= i
||R
9), the LMH6629
n
g
seq
for bias current
+
= R
= i
= e
www.national.com
seq
n
−
seq
ni
n
) source,
/
* 10 V/V
√
ni
values
(2) i
30068021
seq
for a
no
) is
n
),
=