Datasheet

+
-
5V
R
F
620:
R
G
620:
R
O
75:
R
L
75:
V
OUT
LMH6601
U1
C
O
220 PF
CABLE
V
IN
C
G2
C
G
47 PF
+
5V
C
IN
0.47 PF
R
1
510 k:
R
2
510 k:
+
R
IN
75:
LMH6601, LMH6601-Q1
www.ti.com SNOSAK9E –JUNE 2006–REVISED MARCH 2013
Table 1. Finding Figure 54 External Resistor Values by Iteration
Estimate Calculated Equation 1 LHS Comment
R
G
(Ω) (from Equation 2) Calculated (Compare Equation 1 LHS Calculated to RHS)
R3 (Ω)
1k 1.69k 0.988 Increase Equation 1 LHS by reducing R
G
820 1.56k 1.15 Increase Equation 1 LHS by reducing R
G
620 1.37k 1.45 Increase Equation 1 LHS by reducing R
G
390 239 4.18 Reduce Equation 1 LHS by increasing R
G
560 1.30k 1.59 Close to target value of 1.5V/V for Equation 1
The final set of values for R
G
and R
3
in Table 1 are values which will result in the proper gain and correct video
levels (0V to 1V) at the output (V
LOAD
).
AC COUPLED VIDEO
Many monitors and displays accept AC coupled inputs. This simplifies the amplification and buffering task in
some respects. As can be seen in Figure 55, R
1
and R
2
simply set the input to the center of the input linear
range while C
IN
AC couples the video onto the op amp’s input. The op amp is set for a closed loop gain of 2 with
R
F
and R
G
. C
G
is there to make sure the device output is also biased at mid-supply. Because of the DC bias at
the output, the load needs to be AC coupled as well through C
O
. Some applications implement a small valued
ceramic capacitor (not shown) in parallel with C
O
which is electrolytic. The reason for this is that the ceramic
capacitor will tend to shunt the inductive behavior of the Electrolytic capacitor at higher frequencies for an
improved overall low impedance output.
C
G2
is intended to boost the high frequency gain in order to improve the video frequency response. This value is
to be set and trimmed on the board to meet the application’s specific system requirements.
Figure 55. AC Coupled Video Amplifier/Driver
SAG COMPENSATION
The capacitors shown in Figure 55 (except C
G2
), and especially C
O
, are the large electrolytic type which are
considerably costly and take up valuable real estate on the board. It is possible to reduce the value of the output
coupling capacitor, C
O
, which is the largest of all, by using what is called SAG compensation. SAG refers to what
the output video experiences due to the low frequency video content it contains which cannot adequately go
through the output AC coupling scheme due to the low frequency limit of this circuit. The −3 dB low frequency
limit of the output circuit is given by:
f_low_frequency (−3 dB)= 1/ (2*pi* 75*2(Ω) * C
O
) = ∼ 4.82 Hz For C
O
= 220 μF (4)
A possible implementation of the SAG compensation is shown in Figure 56.
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