THS4509 Datasheet, PDF (21/30 Pages) Texas Instruments – WIDEBAND, LOW NOISE, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIER

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APPLICATION INFORMATION
THS4509
SLOS454 – JANUARY 2005
APPLICATIONS
The following circuits show application information for
the THS4509. For simplicity, power supply decoupling
capacitors are not shown in these diagrams. Please
see the Subsection 1 section for recommendations.
For more detail on the use and operation of fully
differential op amps refer to 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 differ-
ential output signals. A basic block diagram of the
circuit is shown in Figure 80 (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 80. Differential Input to Differential Ouput
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 Ampli-
fier
The THS4509 can be used to amplify and convert
single-ended input signals to differential output sig-
nals. A basic block diagram of the circuit is shown in
Figure 81 (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 81. 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
=
ç^ç_è^{æ VOUT+}
´
R_G
R_G + R_F
÷^÷_ø^ö
+
ç^ç_è^{æ VIN-}
´
R_F
R_G + R_F
÷^÷_ø^ö
(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
4mV differential offset voltage. If left unconnected,
the common-mode set point is set to mid-supply by
internal circuitry, which may be over-driven from an
external source. Figure 82 is representative of the
CM input. The internal CM circuit has about 700 MHz
of –3-dB bandwidth, which is required for best per-
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