Datasheet
-27
-24
-21
-18
-15
-12
-9
-6
-3
0
3
NORMALIZED I-V GAIN (dB)
1M
10M
100M
FREQUENCY (Hz)
C
PD
= 330 pF,
C
F
= 22 pF
C
PD
= 222 pF,
C
F
= 18 pF
C
PD
= 100 pF,
C
F
= 12 pF
C
PD
= 22 pF,
C
F
= 5.6 pF
C
PD
= 47 pF,
C
F
= 8 pF
2SR
F
C
T
f
-3 dB
GBWP
=
C
F
=
C
T
2SR
F
(GBWP)
LMH6611, LMH6612
SNOSB00K –NOVEMBER 2007–REVISED OCTOBER 2013
www.ti.com
Theoretical expressions for calculating the optimum value of C
F
and the expected −3 dB bandwidth are:
(7)
(8)
Equation 8 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 4 shows the measurement results of the LMH6611 with different photodiodes having various capacitances
(C
PD
) and a feedback resistance (R
F
) of 1 kΩ.
Table 4. TIA (Figure 66) Compensation and Performance Results
(1)
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 5.42 5.6 29.3 27.1 0.5
47 49 7.75 8 20.5 21 0.5
100 102 11.15 12 14.2 15.2 0.5
222 224 20.39 18 9.6 10.7 0.5
330 332 20.2 22 7.9 9 0.8
(1) GBWP = 130 MHz, C
T
= C
PD
+ C
IN
, C
IN
= 2 pF, V
S
= ±2.5V
Figure 79 shows the frequency response for the various photodiodes in Table 4.
Figure 79. Frequency Response for Various Photodiode and Feedback Capacitors
When analyzing the noise at the output of the TIA, it is important to note that the various noise sources (that is,
op amp noise voltage, feedback resistor thermal noise, input noise current, photodiode noise current) do not all
operate over the same frequency band. Therefore, when the noise at the output is calculated, this should be
taken into account. The op amp noise voltage will be gained up in the region between the noise gain’s zero and
pole (f
Z
and f
P
in Figure 78). The higher the values of R
F
and C
T
, the sooner the noise gain peaking starts and
therefore its contribution to the total output noise will be larger. It is advantageous to minimize C
IN
by proper
choice of op amp or by applying a reverse bias across the diode but this will be at the expense of excess dark
current and noise.
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