Datasheet

f #
dB3-
GBWP
INF
CRS2
F
R)GBWP(2S
IN
C
F
C =
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
LMH6629
SNOSB18G –APRIL 2010–REVISED MARCH 2013 www.ti.com
Figure 64. Transimpedance Amplifier Noise Gain & Transfer Function
The optimum value of C
F
is given by Equation 8 resulting in the I-V -3dB bandwidth shown in Equation 9, or
around 200 MHz in this case (assuming GBWP= 4GHz with COMP pin = HI for WSON-8 package). 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 1pF and
thus is easily affected by board parasitics, etc. For maximum speed, the LMH6629 COMP pin should be HI (for
WSON-8 package). This CF value is a “starting point” and CF needs to 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 (or use the SOT-23 package).
Equation 8: Optimum C
F
Value (8)
Equation 9: 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 65.
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 66 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
.
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