THS4511 Datasheet, PDF (16/25 Pages) Texas Instruments – WIDEBAND, LOW NOISE, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIER

THS4511
SLOS471 – SEPTEMBER 2005
APPLICATION INFORMATION
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APPLICATIONS
The following circuits show application information for
the THS4511. For simplicity, power supply decoupling
capacitors are not shown in these diagrams. For
more detail on the use and operation of fully differen-
tial operational amplifiers refer to application report
Fully-Differential Amplifiers (SLOA054) .
Differential Input to Differential Output Amplifier
The THS4511 is a fully differential operational ampli-
fier, and can be used to amplify differential input
signals to differential output signals. A basic block
diagram of the circuit is shown in Figure 43 (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–
+_–
THS4511
–⁺
V_OUT–
V_OUT+
R_F
Figure 43. 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
Amplifier
The THS4511 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 44 (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
+–
THS4511
–⁺
Differential
Output
V_OUT–
V_OUT+
R_F
Figure 44. 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 oper-
ation 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.
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