THS4511
www.ti.com
SLOS471–SEPTEMBER 2005
Setting the Output Common-Mode Voltage
operational amplifier. Note RS and RIT are added to
the alternate input from the signal input to balance
the amplifier. One resistor that is equal to the
combined value RI = RG + RS||RIT can be placed at
the alternate input.
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 45 is
representative of the CM input. The internal CM
circuit has about 700 MHz of –3-dB bandwidth, which
is required for best performance, but it is intended to
be a dc-bias input pin. Bypass capacitors are rec-
ommended on this pin to reduce noise at the output.
The external current required to overdrive the internal
resistor divider is given by Equation 2:
RS
RF
RG
V
= 3.75 V to 5 V
S+
R
R
IT
PD
VSignal
RO
VOUT-
THS4511
RG
RO
VOUT+
CM
RS
R
VS–
R
IT
PD
VCM
RF
Figure 46. THS4511 DC Coupled Single-Source
Supply Range From 3.75 V to 5 V With RPD Used
To Set VIC
2V - (V - V )
CM S+ S-
I
=
EXT
50 kW
(2)
where VCM is the voltage applied to the CM pin, and
VS+ ranges from 3.75 V to 5 V, and VS- is 0 V
(ground).
Note that in Figure 46, the source is referenced to
ground as is the input termination resistor RIT. The
proper value of resistance to add can be calculated
from Equation 3:
V
S+
1
R
=
P D
50 kW
I
EXT
é
ê
ê
ê
ù
ú
ú
ú
1
1 .6
S +
1
to internal
CM
-
CM circuit
V
R
R
F
I
- 1 .6
ê
ë
ú
û
2
50 kW
(3)
V
where RI = RG + RS||RIT.
S–
VS+ is the power-supply voltage, RF is the feedback
resistance, RG is the gain-setting resistance, RS is the
signal source resistance, and RIT is the termination
resistance.
Figure 45. CM Input Circuit
Device Operation with Single Power Supplies
Less than 5 V
Table 3 is a modification of Table 1 to add the proper
values with RPD assuming VS+ = 3.75 V, a dc-coupled
50-Ω source impedance, and setting the output com-
mon-mode voltage to mid-supply.
The THS4511 is optimized to work in systems using
5-V single supplies, and the characterization data
presented in this data sheet was taken with 5-V
single-supply inputs. For ac-coupled systems or
dc-coupled systems operating with supplies less than
5 V and greater than 3.75 V, the amplifier input
common-mode range is maximized by adding
pull-down resistors at the device inputs. The
pull-down resistors provide additional loading at the
input, and lower the common-mode voltage that is fed
back into the device input through resistor RF.
Figure 46 shows the circuit configuration for this
mode of operation where RPD is added to the
dc-coupled circuit to avoid violating the VICR of the
Table 3. RPD Values for Various Gains,
VS+ = 3.75 V, DC-coupled Signal Source
Gain
0 dB
6 dB
RF
RG
RIT
RPD
348 Ω
348 Ω
340 Ω
169 Ω
56.2 Ω
64.9 Ω
422 Ω
86.6 Ω
If the signal originates from an ac-coupled 50-Ω
source (see Figure 47), the equivalent dc-source
resistance is an open circuit and RI = RG + RIT.
Table 4 is a modification of Table 1 to add the proper
values with RPD assuming VS+
= 3.75 V, an
ac-coupled 50-Ω source impedance, and setting the
output common-mode voltage to mid-supply.
17
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