
www.ti.com
Setting the Output Common-Mode Voltage
V
=3.75Vto5V
S+
CM
VSignal
VS–
RF
RG
R
IT
R
IT
RO
VOUT+
VOUT-
THS4511
RS
VCM
R
PD
R
PD
RS
(
)
W
-
=
-
+
k
50
V
2
I
S
CM
EXT
(2)
I
R
S
V
F
R
P D
R
1
6
.
1
2
6
.
1
-
ú
ù
ê
é
-
+
=
V
S+
CM
V
S–
50k
ToInternal
CMCircuit
I
EXT
50k
Device Operation with Single Power Supplies
SLOS538A – SEPTEMBER 2007 – REVISED OCTOBER 2007
operational amplifier. Note RS and RIT are added to
the alternate input from the signal input to balance
The output common-mode voltage is set by the
the amplifier. One resistor that is equal to the
voltage at the CM pin(s). The internal common-mode
combined value RI = RG + RS||RIT can be placed at
control circuit maintains the output common-mode
the alternate input.
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 37 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
recommended on this pin to reduce noise at the
output. The external current required to overdrive the
Figure 38. THS4511 DC Coupled Single-Source
Supply Range From 3.75 V to 5 V With RPD Used
To Set VIC
where VCM is the voltage applied to the CM pin, and
Note that in
Figure 38, the source is referenced to
VS+ ranges from 3.75 V to 5 V, and VS- is 0 V
ground as is the input termination resistor RIT. The
(ground).
proper value of resistance to add can be calculated
(3)
where RI = RG + RS||RIT.
VS+ is the power-supply voltage, RF is the feedback
Figure 37. CM Input Circuit
resistance, RG is the gain-setting resistance, RS is the
signal source resistance, and RIT is the termination
resistance.
Less than 5 V
The THS4511 is optimized to work in systems using
values with RPD assuming VS+ = 3.75 V, a dc-coupled
5 V single supplies, and the characterization data
50
source impedance, and setting the output
presented in this data sheet was taken with 5 V
common-mode voltage to mid-supply.
single-supply inputs. For ac-coupled systems or
dc-coupled systems operating with supplies less than
Table 3. RPD Values for Various Gains,
5 V and greater than 3.75 V, the amplifier input
VS+ = 3.75 V, DC-coupled Signal Source
common-mode
range
is
maximized
by
adding
Gain
RF
RG
RIT
RPD
pull-down
resistors
at
the
device
inputs.
The
6 dB
348
169
64.9
80.6
pull-down resistors provide additional loading at the
10 dB
348
102
78.7
90.9
input, and lower the common-mode voltage that is fed
back into the device input through resistor RF.
14 dB
348
61.9
115
90.9
Figure 38 shows the circuit configuration for this
20 dB
348
40.2
221
77.6
mode of operation where RPD is added to the
dc-coupled circuit to avoid violating the VICR of the
If the signal originates from an ac-coupled 50
source (see
Figure 39), the equivalent dc-source
16
Copyright 2007, Texas Instruments Incorporated