Datasheet

2SR
F
C
T
f
-3 dB
GBWP
=
C
F
=
C
T
2SR
F
(GBWP)
NOISE GAIN (NG)
OP AMP OPEN
LOOP GAIN
I-V GAIN (:)
GAIN (dB)
0 dB
FREQUENCY
1 + sR
F
(C
T
+ C
F
)
1 + sR
F
C
F
1 +
C
IN
C
F
GBWP
f
z
#
1
2SR
F
C
T
f
P
=
1
2SR
F
C
F
LMH6618, LMH6619
SNOSAV7E –AUGUST 2007–REVISED OCTOBER 2012 www.ti.com
Figure 72. Bode Plot of Noise Gain Intersecting with Op Amp Open-Loop Gain
Figure 72 shows the bode plot of the noise gain intersecting the op amp open loop gain. With larger values of
gain, C
T
and R
F
create a zero in the transfer function. At higher frequencies the circuit can become unstable due
to excess phase shift around the loop.
A pole at f
P
in the noise gain function is created by placing a feedback capacitor (C
F
) across R
F
. The noise gain
slope is flattened by choosing an appropriate value of C
F
for optimum performance.
Theoretical expressions for calculating the optimum value of C
F
and the expected −3 dB bandwidth are:
(3)
(4)
Equation 4 indicates that the −3 dB bandwidth of the TIA is inversely proportional to the feedback resistor.
Therefore, if the bandwidth is important then the best approach would be to have a moderate transimpedance
gain stage followed by a broadband voltage gain stage.
Table 3 shows the measurement results of the LMH6618 with different photodiodes having various capacitances
(C
PD
) and a feedback resistance (R
F
) of 1 kΩ.
Table 3. TIA (Figure 1) Compensation and Performance Results
C
PD
C
T
C
F CAL
C
F USED
f
−3 dB CAL
f
−3 dB MEAS
Peaking
(pF) (pF) (pF) (pF) (MHz) (MHz) (dB)
22 24 7.7 5.6 23.7 20 0.9
47 49 10.9 10 16.6 15.2 0.8
100 102 15.8 15 11.5 10.8 0.9
222 224 23.4 18 7.81 8 2.9
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