LMH6574MA NSC [National Semiconductor], LMH6574MA Datasheet - Page 11

• Manufacturer Part Number: LMH6574MA
• Description: 4:1 High Speed Video Multiplexer
• Manufacturer: NSC [National Semiconductor]

Application Notes
frequency response is a function of R
the recommended value of R
the chart “Suggested R
from the recommended value will peak the frequency re-
sponse and extend the bandwidth while increasing the value
of R
Reducing the value of R
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
more information see Application Note OA-13 which de-
scribes the relationship between R
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.
EVALUATION BOARDS
National Semiconductor provides the following evaluation
boards as a guide for high frequency layout and as an aid in
device testing and characterization. Many of the data sheet
plots were measured with this board.
F
will cause the frequency response to roll off faster.
F
F
vs Gain”. Generally, lowering R
too far below the recommended
F
is 575Ω. For other gains see
(Continued)
F
F
F
for a given circuit. For
. For a gain of 2 V/V,
and closed-loop fre-
F
11
An evaluation board can be shipped when a sample request
is placed with National Semiconductor. Samples can be
ordered on the National web page. (www.national.com)
MULTIPLEXER EXPANSION
With the SHUTDOWN pin putting the output stage into a
high impedance state, several LMH6574’s can be tied to-
gether to form a larger input MUX. However, there is a
loading effect on the active output caused by the unselected
devices. The circuit in Figure 3 shows how to compensate
for this effect. For the 16:1 MUX function shown in Figure 3
below the gain error would be about −0.8 dB, or about 9%. In
the circuit in Figure 3, resistor ratios have been adjusted to
compensate for this gain error. By adjusting the gain of each
multiplexer circuit the error can be reduced to the tolerance
of the resistors used (1% in this example).
Disabling of the LMH6574 using the EN pin is not recom-
mended for use when doing multiplexer expansion. While
disabled, If the voltage between the selected input and the
chip output exceeds approximately 2V the device will begin
to enter a soft breakdown state. This will show up as reduced
input to output isolation. The signal on the non-inverting
input of the output driver amplifier will leak through to the
inverting input, and then to the output through the feedback
resistor. The worst case is a gain of 1 configuration where
the non inverting input follows the active input buffer and
(through the feedback resistor) the inverting input follows the
voltage driving the output stage. The solution for this is to
use shutdown mode for multiplexer expansion.
BUILDING an 8:1 MULITPLEXER
Figure 4 shows an 8:1 MUX using two LMH6574’s.
Device
LMH6574
FIGURE 3. Multiplexer Gain Compensation
Package
SOIC
Evaluation Board
LMH730276
www.national.com
20119717