LMH6574_05 Datasheet, PDF (11/14 Pages) National Semiconductor (TI) – 4:1 High Speed Video Multiplexer

Application Notes (Continued)
The LMH6574 has a current feedback output buffer with gain
determined by external feedback (R_F) and gain set (R_G)
resistors. With current feedback amplifiers, the closed loop
frequency response is a function of R_F. For a gain of 2 V/V,
the recommended value of R_F is 575Ω. For other gains see
the chart “Suggested R_F vs Gain”. Generally, lowering R_F
from the recommended value will peak the frequency re-
sponse and extend the bandwidth while increasing the value
of R_F will cause the frequency response to roll off faster.
Reducing the value of R_F too far below the recommended
value will cause overshoot, ringing and, eventually, oscilla-
tion.
Since all applications are slightly different it is worth some
experimentation to find the optimal R_F for a given circuit. For
more information see Application Note OA-13 which de-
scribes the relationship between R_F and closed-loop fre-
quency response for current feedback operational amplifiers.
The impedance looking into pin 12 is approximately 20Ω.
This allows for good bandwidth at gains up to 10 V/V. When
used with gains over 10 V/V, the LMH6574 will exhibit a “gain
bandwidth product” similar to a typical voltage feedback
amplifier. For gains of over 10 V/V consider selecting a high
performance video amplifier like the LMH6720 to provide
additional gain.
SD vs. EN
The LMH6574 has both shutdown and disable capability.
The shutdown feature affects the entire chip, whereas the
disable function only affects the output buffer. When in shut-
down mode, minimal power is consumed. The shutdown
function is very fast, but causes a very brief spike of about
400 mV to appear on the output. When in shutdown mode
the LMH6574 consumes only 1.8 mA of supply current. For
maximum input to output isolation use the shutdown func-
tion.
The EN pin only disables the output buffer which results in a
substantially reduced output glitch of only 50 mV. While
disabled the chip consumes 4.7 mA, considerably more than
when shutdown. This is because the input buffers are still
active. For minimal output glitch use the EN pin. Also, care
should be taken to ensure that, while in the disabled state,
the voltage differential between the active input buffer (the
one selected by pins A0 and A1) and the output pin stays
less than 2V. As the voltage differential increases, input to
output isolation decreases. Normally this is not an issue. See
the section on MULTIPLEXER EXPANSION for further de-
tails.
To reduce the output glitch when using the SD pin, switch the
EN pin at least 10 ns before switching the SD pin. This can
be accomplished by using an RC delay circuit between the
two pins if only one control signal is available.
Logic inputs "SD" and "EN" will revert to the "High", while
"A₀" and "A₁" will revert to the "Low" state when left floating.
MULITPLEXER EXPANSION
Figure 3 shows an 8:1 MUX using two LMH6574’s.
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FIGURE 3. 8:1 MUX USING TWO LMH6574’s
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