Datasheet

100k

1M 10M 100M 1G

FREQUENCY (Hz)

-18

-15

-12

-9

-6

-3

NORMALIZED I-V GAIN (dB)

= 22 pF,

= 5.6 pF

= 47 pF,

= 10 pF

= 100 pF,

= 15 pF

= 222 pF,

= 18 pF

LMH6618, LMH6619

www.ti.com

SNOSAV7E –AUGUST 2007–REVISED OCTOBER 2012

Figure 73 shows the frequency response for the various photodiodes in Table 3.

Figure 73. 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 (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 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

and f

in Figure 72). The higher the values of R

and C

, 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

by proper choice of op amp or by applying a reverse bias across the diode at the expense of

excess dark current and noise.

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 74

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. C

and C

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.

Product Folder Links: LMH6618 LMH6619