LMH6618 Datasheet, PDF (23/26 Pages) National Semiconductor (TI) – 130 MHz, 1.25 mA Rail-to-Rail Input and Output Operational Amplifier with Shutdown

TABLE 2. TIA (Figure 1) Compensation and Performance Results
C_PD
(pF)
C_T
(pF)
C_{F CAL}
(pF)
C_{F USED}
(pF)
f _{−3 dB CAL}
(MHz)
f _{−3 dB MEAS}
(MHz)
Peaking
(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
Note:
GBWP = 65 MHz
C_T = C_PD + C_IN
C_IN = 2 pF
V_S = ±2.5V
Figure 11 shows the frequency response for the various pho-
todiodes in Table 2.
When analyzing the noise at the output of the TIA, it is im-
portant to note that the various noise sources (i.e. 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 out-
put 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 10). 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 obvious to note that it is advantageous to
minimize C_IN by proper choice of op amp or by applying a
reverse bias across the diode at the expense of excess dark
current and noise.
20195868
FIGURE 11. Frequency Response for Various Photodiode
and Feedback Capacitors
23
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