THS4509-Q1 Datasheet, PDF (26/37 Pages) Texas Instruments – WIDEBAND LOW-NOISE LOW-DISTORTION FULLY DIFFERENTIAL AMPLIFIER

THS4509-Q1
SLOS547 – NOVEMBER 2008........................................................................................................................................................................................... www.ti.com
APPLICATION INFORMATION
APPLICATIONS
The following circuits show application information for
the THS4509. For simplicity, power supply decoupling
capacitors are not shown in these diagrams. See
section THS4509 EVM for recommendations. For
more detail on the use and operation of fully
differential op amps see the application report
Fully-Differential Amplifiers (SLOA054) .
Differential Input to Differential Output Amplifier
The THS4509 is a fully differential op amp, and can
be used to amplify differential input signals to
differential output signals. A basic block diagram of
the circuit is shown in Figure 81 (CM input not
shown). The gain of the circuit is set by R_F divided by
R_G.
R_F
Differential
Input
V_S+
Differential
Output
R_G
V_IN+
R_G
V_IN–
+–
THS4509
–+
V_OUT–
V_OUT+
V_S–
R_F
Figure 81. Differential Input to Differential Output
Amplifier
Depending on the source and load, input and output
termination can be accomplished by adding R_IT and
R_O.
Single-Ended Input to Differential Output
Amplifier
The THS4509 can be used to amplify and convert
single-ended input signals to differential output
signals. A basic block diagram of the circuit is shown
in Figure 82 (CM input not shown). The gain of the
circuit is again set by R_F divided by R_G.
Single-Ended
R_G
Input
R_G
R_F
V_S
+–
THS 4509
–+
Differential
Output
V_OUT–
V_OUT+
V_S
R_F
Figure 82. Single-Ended Input to Differential
Output Amplifier
Input Common-Mode Voltage Range
The input common-model voltage of a fully differential
op amp is the voltage at the '+' and '–' input pins of
the op amp.
It is important to not violate the input common-mode
voltage range (V_ICR) of the op amp. Assuming the op
amp is in linear operation the voltage across the input
pins is only a few millivolts at most. So finding the
voltage at one input pin will determine the input
common-mode voltage of the op amp.
Treating the negative input as a summing node, the
voltage is given by Equation 1:
V_IC
=
ççèæ ^VOUT+
´
R_G
R_G + R_F
÷÷øö
+
ççèæ ^VIN-
´
R_F
R_G + R_F
÷÷øö
(1)
To determine the V_ICR of the op amp, the voltage at
the negative input is evaluated at the extremes of
V_OUT+.
As the gain of the op amp increases, the input
common-mode voltage becomes closer and closer to
the input common-mode voltage of the source.
Setting the Output Common-Mode Voltage
The output common-mode voltage is set by the
voltage at the CM pin(s). The internal common-mode
control circuit maintains the output common-mode
voltage within 3-mV offset (typ) from the set voltage,
when set within 0.5 V of mid-supply, with less than
4-mV differential offset voltage. If left unconnected,
the common-mode set point is set to mid-supply by
internal circuitry, which may be overdriven from an
external source. Figure 83 is representative of the
CM input. The internal CM circuit has about 700 MHz
of –3-dB bandwidth, which is required for best
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