Datasheet

APPLICATION INFORMATION
APPLICATIONS
Differential Input to Differential Output Amplifier
5V
Single-Ended
Input
R
F
R
F
R
G
R
G
THS4508
Differential
Output
V
OUT+
V
OUT–
+
–
–
+
Input Common-Mode Voltage Range
R
F
V
OUT+
V
OUT–
5V
V
IN–
R
F
R
G
R
G
V
IN+
THS4508
Differential
Input
Differential
Output
+
–
–
+
÷
÷
ø
ö
ç
ç
è
æ
+
´+
÷
÷
ø
ö
ç
ç
è
æ
+
´=
-+
FG
F
IN
FG
G
OUT
IC
RR
R
V
RR
R
V
V
(1)
Single-Ended Input to Differential Output
Setting the Output Common-Mode Voltage
THS4508
www.ti.com .................................................................................................................................... SLAS459E – SEPTEMBER 2005 – REVISED SEPTEMBER 2008
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 ) , available for download at the TI web
site.
The THS4508 is a fully differential operational
Figure 50. Single-Ended Input to Differential
amplifier, and can be used to amplify differential input
Output Amplifier
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
.
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 :
Figure 49. Differential Input to Differential Output
To determine the V
ICR
of the operational amplifier, the
Amplifier
voltage at the negative input is evaluated at the
extremes of V
OUT+
.
Depending on the source and load, input and output
termination can be accomplished by adding R
IT
and
As the gain of the operational amplifier increases, the
R
O
.
input common-mode voltage becomes closer and
closer to the input common-mode voltage of the
source.
Amplifier
The THS4508 can be used to amplify and convert
single-ended input signals to differential output
The output common-mode voltage is set by the
signals. A basic block diagram of the circuit is shown
voltage at the CM pin(s). The internal common-mode
in Figure 50 (CM input not shown). The gain of the
control circuit maintains the output common-mode
circuit is again set by R
F
divided by R
G
.
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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