lmh6629sdx National Semiconductor Corporation, lmh6629sdx Datasheet - Page 17

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

LOW-NOISE TRANSIMPEDANCE AMPLIFIER
Figure 12
Transimpedance amplifier commonly used with photo-
diodes. The transimpedance gain is set by R
Figure 13
(I-V Gain). As with most Transimpedance amplifiers, it is re-
quired to compensate for the additional phase lag (Noise Gain
zero at f
capacitance) + C
into R
enough phase lead (Noise Gain pole at f
loop.
FIGURE 13. Transimpedance Amplifier Noise Gain &
FIGURE 12. 200MHz Transimpedance Amplifier
F
; this is accomplished by placing C
Z
) created by the total input capacitance ( C
implements a high speed, single supply, low-noise
shows the Noise Gain (NG) and transfer function
CM
Transfer Function
(LMH6629 input capacitance) ) looking
Configuration
F
across R
P
) to stabilize the
F
.
F
to create
D
(diode
30068002
30068011
17
The optimum value of C
the I-V -3dB bandwidth shown in
200MHz in this case (assuming GBWP= 4GHz with COMP
pin = HI). This C
be tuned for the particular application as it is often less than
1pF and thus is easily affected by board parasitics, etc. For
maximum speed, the LMH6629 COMP pin should be HI.
Equation 7
equation for the basic Transimpedance configuration and is
plotted against feedback resistance (R
tributing noise sources in
expected total equivalent input current noise density (i
given feedback resistance (R
schematic of
density (i
noiseless feedback resistor (R
voltage noise density (e
Equation 7: Noise Equation for Transimpedance
FIGURE 14. Current Noise Density vs. Feedback
Equation 6: Resulting -3dB Bandwidth
ni
) is shown at the input of a noiseless amplifier and
Equation 5: Optimum C
provides the total input current noise density (i
Figure 15
F
value is a “starting point” and C
Amplifier
no
where total equivalent current noise
Resistance
F
) is i
is given by
Figure
ni
F
*R
F
). The total equivalent output
). This is depicted in the
F
14. The plot indicates the
.
F
Equation
Value
Equation 5
F
) showing all con-
6, or around
30068028
www.national.com
resulting in
F
needs to
ni
) for a
(5)
(6)
(7)
ni
)