THS4508 Datasheet, PDF (16/29 Pages) Texas Instruments – WIDEBAND, LOW NOISE, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIER

THS4508
SLAS459A – SEPTEMBER 2005 – REVISED FEBRUARY 2006
APPLICATION INFORMATION
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APPLICATIONS
The following circuits show application information for
the THS4508. For simplicity, power supply decoupling
capacitors are not shown in these diagrams. For
more detail on the use and operation of fully
differential operational amplifiers, refer to application
report Fully-Differential Amplifiers (SLOA054) .
Differential Input to Differential Output Amplifier
The THS4508 is a fully differential operational
amplifier, and can be used to amplify differential input
signals to differential output signals. A basic block
diagram of the circuit is shown in Figure 49 (CM input
not shown). The gain of the circuit is set by R_F
divided by R_G.
R_F
Differential
Input
5V
Differential
Output
R_G
V_IN+
R_G
V_IN–
+_–
THS4508
–⁺
V_OUT–
V_OUT+
R_F
Figure 49. 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 THS4508 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 50 (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
5V
+–
THS4508
–⁺
Differential
Output
V_OUT–
V_OUT+
R_F
Figure 50. Single-Ended Input to Differential
Output Amplifier
Input Common-Mode Voltage Range
The input common-model voltage of a fully differential
operational amplifier is the voltage at the (+) and (–)
input pins of the operational amplifier.
It is important to not violate the input common-mode
voltage range (V_ICR) of the operational amplifier.
Assuming the operational amplifier 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 determines the input common-mode voltage
of the operational amplifier.
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 operational amplifier, the
voltage at the negative input is evaluated at the
extremes of V_OUT+.
As the gain of the operational amplifier 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 5-mV offset (typical) from the set
voltage, when set within 0.5 V of mid-supply. 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 51 is
representative of the CM input. The internal CM
circuit has about 700 MHz of –3-dB bandwidth, which
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