LMH6618_0711 Datasheet, PDF (25/28 Pages) National Semiconductor (TI) – PowerWise® 130 MHz, 1.25 mA RRIO Operational Amplifiers

TABLE 3. 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 12 shows the frequency response for the various pho-
todiodes in Table 3.
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 11). 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 12. Frequency Response for Various Photodiode
and Feedback Capacitors
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
DIFFERENTIAL CABLE DRIVER FOR NTSC VIDEO
The LMH6618 and LMH6619 can be used to drive an NTSC
video signal on a twisted-pair cable. Figure 13 shows the
schematic of a differential cable driver for NTSC video. This
circuit can be used to transmit the signal from a camera over
a twisted pair to a monitor or display located a distance. C₁
and C₂ are used to AC couple the video signal into the
LMH6619. The two amplifiers of the LMH6619 are set to a
gain of 2 to compensate for the 75Ω back termination resistors
on the outputs. The LMH6618 is set to a gain of 1. Because
of the DC bias the output of the LMH6618 is AC coupled. Most
monitors and displays will accept AC coupled inputs.
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