Datasheet

LTC6406
19
6406fc
Figure 9. Noise Model of the LTC6406
Figure 10. LTC6406 Output Spot Noise vs Spot Noise
Contributed by Feedback Network Alone
APPLICATIONS INFORMATION
Noise Considerations
The LTC6406’s input referred voltage noise is 1.6nV/√Hz.
Its input referred current noise is 2.5pA/√Hz. In addition
to the noise generated by the amplifi er, the surrounding
feedback resistors also contribute noise. A noise model
is shown in Figure 9. The output noise generated by both
the amplifi er and the feedback components is governed
by the equation:
e
no
=
e
ni
•1+
R
F
R
I
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
2
+ 2• I
n
•R
F
()
2
+
2• e
nRI
•
R
F
R
I
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
2
+ 2•e
nRF
2
A plot of this equation, and a plot of the noise generated
by the feedback components for the LTC6406 is shown
in Figure 10.
–
+
e
no
2
R
F
V
OCM
e
nRI
2
R
F
R
I
R
I
e
nRF
2
e
nRI
2
e
ncm
2
e
ni
2
e
nRF
2
i
n
+2
i
n
–2
6406 F09
R
I
= R
F
(Ω)
10
0.1
nV/√Hz
1
10
100
100 1k 10k
6406 F10
FEEDBACK NETWORK
NOISE ALONE
TOTAL (AMPLIFIER AND
FEEDBACK NETWORK)
OUTPUT NOISE
The LTC6406’s input referred voltage noise contributes the
equivalent noise of a 155Ω resistor. When the feedback
network is comprised of resistors whose values are less
than this, the LTC6406’s output noise is voltage noise
dominant (see Figure 10):
e
no
≈e
ni
•1+
R
F
R
I
⎛
⎝
⎜
⎞
⎠
⎟
Feedback networks consisting of resistors with values
greater than about 200Ω will result in output noise which
is resistor noise and amplifi er current noise dominant.
e
no
≈ 2 •I
n
•R
F
()
2
+ 1+
R
F
R
I
⎛
⎝
⎜
⎞
⎠
⎟
•4•k•T•R
F
Lower resistor values (<100Ω) always result in lower noise
at the penalty of increased distortion due to increased load-
ing of the feedback network on the output. Higher resistor
values (but still less than <500Ω) will result in higher
output noise, but typically improved distortion due to less
loading on the output. The optimal feedback resistance for
the LTC6406 runs in between 100Ω to 500Ω.
The differential fi ltered outputs +OUTF and –OUTF will
have a little higher noise than the unfi ltered outputs (due
to the two 50Ω resistors which contribute 0.9nV/√Hz
each), but can provide superior signal-to-noise due to the
output noise fi ltering.