LMH6574 Datasheet, PDF (18/27 Pages) National Semiconductor (TI) – 4:1 High Speed Video Multiplexer

LMH6574
SNCS103D – NOVEMBER 2004 – REVISED DECEMBER 2014
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Typical Application (continued)
With the SHUTDOWN pin putting the output stage into a high impedance state, several LMH6574’s can be tied
together 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 33 shows how to compensate for this effect. For the 16:1 MUX function
shown in Figure 33, the gain error would be about −0.8 dB, or about 9%. In the circuit in Figure 33, 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).
Figure 33. Multiplexer Gain Compensation
NOTE
Disabling of the LMH6574 using the EN pin is not recommended 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.
7.2.5 Other Applications
The LMH6574 could support a multi antenna receiver with up to four separate antennas. Monitoring the signal
strength of all 4 antennas and connecting the strongest signal to the final IF stage would provide effective spacial
diversity.
For direction finding, the LMH6574 could be used to provide high speed sampling of four separate antennas to a
single DSP which would use the information to calculate the direction of the received signal.
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