Datasheet

R
f
(Noiseless)
Noiseless Op
Amp
e
ni
R
g
(Noiseless)
V
S
R
T
(Noiseless)
R
S
(Noiseless)
Rseq = R
S
|| R
T
Set R
Seq
= R
f
|| R
g
for bias current offset
cancellation
LMH6629
SNOSB18G –APRIL 2010–REVISED MARCH 2013 www.ti.com
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. Figure 57
describes the noise model for the non-inverting amplifier configuration showing all noise sources. In addition to
the intrinsic input voltage noise (e
n
) and current noise (i
n
= i
n
+
= i
n
−
) source, there is also thermal voltage noise (e
t
= √(4KTR)) associated with each of the external resistors.
Figure 57. Non-Inverting Amplifier Noise Model
Equation 3 provides the general form for total equivalent input voltage noise density (e
ni
).
Equation 3: General Noise Equation (3)
Equation 4 is a simplification of Equation 3 that assumes R
f
|| R
g
= R
seq
for bias current cancellation:
Equation 4: Noise Equation with R
f
|| R
g
= R
seq
(4)
Figure 58 schematically shows e
ni
alongside V
IN
(the portion of V
S
source which reaches the non-inverting input
of Figure 55) and external components affecting gain (A
v
= 1 + R
f
/ R
g
), all connected to an ideal noiseless
amplifier.
Figure 58. Non-Inverting Amplifier Equivalent Noise Source Schematic
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