Datasheet

R
F
(Noiseless)
D1
+5VDC
C
D
= 10 pF
Noiseless Op Amp
I
D
i
ni
100 1k 10k
R
f
(:)
Current Noise Density (pA/ Hz)
16
14
12
10
8
6
4
2
0
i
t
i
ni
e
n
/R
F
i
n
LMH6629
www.ti.com SNOSB18G –APRIL 2010–REVISED MARCH 2013
Figure 65. Current Noise Density vs. Feedback Resistance
Equation 10: Noise Equation for Transimpedance Amplifier (10)
Figure 66. Transimpedance Amplifier Equivalent Input Source Model
From Figure 65, it is clear that with LMH6629’s extremely low-noise characteristics, for R
F
< 2.5kΩ, the noise
performance is entirely dominated by R
F
thermal noise. Only above this R
F
threshold, LMH6629’s input noise
current (i
n
) starts being a factor and at no R
F
setting does the LMH6629 input noise voltage play a significant
role. This noise analysis has ignored the possible noise gain increase, due to photo-diode capacitance, at higher
frequencies.
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