100k
1M 10M 100M 1G
FREQUENCY (Hz)
-18
-15
-12
-9
-6
-3
0
3
6
NORMALIZED I-V GAIN (dB)
C
PD
= 22 pF,
C
F
= 5.6 pF
C
PD
= 47 pF,
C
F
= 10 pF
C
PD
= 100 pF,
C
F
= 15 pF
C
PD
= 222 pF,
C
F
= 18 pF
LMH6619Q
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SNOSC78A –JUNE 2012–REVISED NOVEMBER 2012
Table 3 shows the measurement results of the LMH6619Q 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
Figure 11 shows the frequency response for the various photodiodes in Table 3.
Figure 11. 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
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.
DIFFERENTIAL CABLE DRIVER FOR NTSC VIDEO
The LMH6619Q can be used to drive an NTSC video signal on a twisted-pair cable. Figure 12 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
1
and C
2
are used to AC couple the video
signal into the LMH6619Q. The two amplifiers of the LMH6619Q are set to a gain of 2 to compensate for the 75Ω
back termination resistors on the outputs. The LMH6619Q is set to a gain of 1. Because of the DC bias the
output of the LMH6619Q is AC coupled. Most monitors and displays will accept AC coupled inputs.
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