Datasheet

NET GAIN = 0.5 * STAGE GAIN * GAIN ERROR =
0.993 V/V
ON
+
OFF
+
OFF
+
OFF
+
576:
487:
487:
576:
487:
576:
487:
576:
R
L
75:
R
1
75:
R
2
75:
R
3
75:
R
4
75:
STAGE GAIN = 1 + R
F
/R
G
= 2.18 V/V
GAIN
ERROR
[(1138/3) || 75] · 2
[(1138/3) || 75] + 75
=
=
0.91 V/V
LMH6574
SNCS103C –NOVEMBER 2004–REVISED SEPTEMBER 2005 www.ti.com
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 32 shows how to compensate for this effect. For the 16:1 MUX function
shown in Figure 32 below the gain error would be about −0.8 dB, or about 9%. In the circuit in Figure 32, 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 32. Multiplexer Gain Compensation
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.
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.
DRIVING CAPACITIVE LOADS
Capacitive output loading applications will benefit from the use of a series output resistor R
OUT
. Figure 33 shows
the use of a series output resistor, R
OUT
, to stabilize the amplifier output under capacitive loading. Capacitive
loads of 5 to 120 pF are the most critical, causing ringing, frequency response peaking and possible oscillation.
The chart Figure 34 gives a recommended value for selecting a series output resistor for mitigating capacitive
loads. The values suggested in the charts are selected for 0.5 dB or less of peaking in the frequency response.
This gives a good compromise between settling time and bandwidth. For applications where maximum frequency
response is needed and some peaking is tolerable, the value of R
OUT
can be reduced slightly from the
recommended values.
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