NOISE GAIN (NG)
OP AMP OPEN
LOOP GAIN
I-V GAIN (:)
GAIN (dB)
0 dB
FREQUENCY
1 + sR
F
(C
IN
+ C
F
)
1 + sR
F
C
F
1 +
C
IN
C
F
GBWP
f
z
#
1
2SR
F
C
IN
f
P
=
1
2SR
F
C
F
LMH6624
,
LMH6626
SNOSA42G –NOVEMBER 2002–REVISED DECEMBER 2014
www.ti.com
Typical Application (continued)
8.2.1 Design Requirements
Figure 58 shows the Noise Gain (NG) and transfer function (I-V Gain). As with most Transimpedance amplifiers,
it is required to compensate for the additional phase lag (Noise Gain zero at f
Z
) created by the total input
capacitance: C
D
(diode capacitance) + C
CM
(LMH6624 CM input capacitance) + C
DIFF
(LMH6624 DIFF input
capacitance) looking into R
F
. This is accomplished by placing C
F
across R
F
to create enough phase lead (Noise
Gain pole at f
P
) to stabilize the loop.
Figure 58. Transimpedance Amplifier Noise Gain and Transfer Function
8.2.2 Detailed Design Procedure
The optimum value of C
F
is given by Equation 8 resulting in the I-V -3dB bandwidth shown in Equation 9, or
around 124 MHz in this case, assuming GBWP = 1.5 GHz, C
CM
(LMH6624 CM input capacitance) = 0.9 pF, and
C
DIFF
(LMH6624 DIFF input capacitance) = 2 pF. This C
F
value is a “starting point” and C
F
needs to be tuned for
the particular application as it is often less than 1 pF and thus is easily affected by board parasitics.
Optimum C
F
Value:
(8)
Resulting -3dB Bandwidth:
(9)
Equation 10 provides the total input current noise density (i
ni
) equation for the basic Transimpedance
configuration and is plotted against feedback resistance (R
F
) showing all contributing noise sources in Figure 59.
The plot indicates the expected total equivalent input current noise density (i
ni
) for a given feedback resistance
(R
F
). This is depicted in the schematic of Figure 60 where total equivalent current noise density (i
ni
) is shown at
the input of a noiseless amplifier and noiseless feedback resistor (R
F
). The total equivalent output voltage noise
density (e
no
) is i
ni
*R
F
. Noise Equation for Transimpedance Amplifier:
(10)
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