OPA4872-EP Datasheet, PDF (11/24 Pages) Texas Instruments – 4:1 HIGH-SPEED MULTIPLEXER

OPA4872-EP
www.ti.com........................................................................................................................................................................................... SBOS444 – DECEMBER 2008
APPLICATION INFORMATION
WIDEBAND MULTIPLEXER OPERATION
The OPA4872 gives a new level of performance in
wideband multiplexers. Figure 27 shows the
dc-coupled, gain of +2 V/V, dual power-supply circuit
used as the basis of the ±5-V Electrical
Characteristics and Typical Characteristic curves. For
test purposes, the input impedance is set to 75 Ω with
a resistor to ground and the output impedance is set
to 75 Ω with a series output resistor. Voltage swings
reported in the specifications are taken directly at the
input and output pins while load powers (in dBm) are
defined at a matched 75-Ω load. For the circuit of
Figure 27, the total effective load will be 150 Ω ||
1046 Ω = 131 Ω. Logic pins A0 and A1 control which
of the four inputs is selected while EN and SD allow
for power reduction. One optional component is
included in Figure 27. In addition to the usual
power-supply decoupling capacitors to ground, a
0.01-µF capacitor is included between the two
power-supply pins. In practical printed circuit board
(PCB) layouts, this optional added capacitor typically
improves the 2nd-harmonic distortion performance by
3 dB to 6 dB for bipolar supply operation.
Even though the internal architecture of the OPA4872
includes current steering, it is advantageous to look
at it as four switches looking into the noninverting
input of a current feedback amplifier. Depending on
the logic applied to channel control pins A0 and A1,
one switch is on at all times. Figure 27 represents the
OPA4872 in this configuration. The truth table for
channel selection is shown in Table 1.
Table 1. TRUTH TABLE
A0 A1 EN SD
0
0
0
0
1
0
0
0
0
1
0
0
1
1
0
0
X
X
1
0
X
X
X
1
V_OUT
IN0
IN1
IN2
IN3
High-Z, I_Q = 3.4 mA
High-Z, I_Q = 1.1 mA
The OPA4872 is in disable mode, with a quiescent
current of 3.4mA typical, when the EN pin is set to
0V. After being placed in disable mode, the OPA4872
is fully enabled in 6ns. For further power savings, the
SD pin can be used. Setting the SD pin to 5V places
the device in shutdown mode with a standing
quiescent current of 1.1 mA. Note that in this
shutdown mode, the OPA4872 requires 15ns to be
fully powered again. The truth table for disable and
shutdown modes can be found in Table 1.
+5V
0.1mF ₊ 6.8mF
OPA4872
SD
IN0
V_IN0
EN
75W
IN1
V_IN1
75W
IN2
V_IN2
75W
IN3
V_IN3
75W
Optional
0.01mF
0.1mF ₊ 6.8mF
75W
V_OUT
523W
To 75W Load
523W
A0
A1
-5V
Figure 27. DC-Coupled, G = +2V/V Bipolar Specification and Test Circuit (Channel 0 Selected)
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