Datasheet

V =3.75Vto5V
S+
CM
V
Signal
V
S–
R
F
R
F
R
G
R
G
R
IT
R
IT
R
O
R
O
V
OUT+
V
OUT-
THS4511
R
S
V
CM
R
PD
R
PD
R
S
( )
W
--
=
-+
k50
VVV2
I
SSCM
EXT
I
R
S
V
F
R
P D
R
1
6.1
2
6.11
1
-
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ú
û
ù
ê
ê
ê
ê
ë
é
-
+
=
V
S+
CM
V
S–
50kW
tointernal
CMcircuit
I
EXT
50kW
THS4511
SLOS471E –SEPTEMBER 2005–REVISED NOVEMBER 2009 www.ti.com
Setting the Output Common-Mode Voltage
operational amplifier. Note R
S
and R
IT
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 R
I
= R
G
+ R
S
|| R
IT
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 midsupply. If left
unconnected, the common-mode set point is set to
midsupply by internal circuitry, which may be
overdriven 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
recommended on this pin to reduce noise at the
output. The external current required to overdrive the
internal resistor divider is given by Equation 2:
Figure 46. THS4511 DC-Coupled Single-Source
Supply Range From 3.75 V to 5 V With R
PD
Used
(2)
To Set V
IC
where V
CM
is the voltage applied to the CM pin, and
V
S+
ranges from 3.75 V to 5 V, and V
S–
is 0 V
Note that in Figure 46, the source is referenced to
(ground).
ground, as is the input termination resistor R
IT
. The
proper value of resistance to add can be calculated
from Equation 3:
(3)
where R
I
= R
G
+ R
S
|| R
IT
.
V
S+
is the power-supply voltage, R
F
is the feedback
Figure 45. CM Input Circuit
resistance, R
G
is the gain-setting resistance, R
S
is the
signal source resistance, and R
IT
is the termination
resistance.
Device Operation with Single Power Supplies
Less than 5 V
Table 3 is a modification of Table 1 to add the proper
values with R
PD
assuming V
S+
= 3.75 V, a dc-coupled
The THS4511 is optimized to work in systems using
50-Ω source impedance, and setting the output
5-V single supplies, and the characterization data
common-mode voltage to midsupply.
presented in this data sheet were taken with 5-V
single-supply inputs. For ac-coupled systems or
Table 3. R
PD
Values for Various Gains,
dc-coupled systems operating with supplies less than
V
S+
= 3.75 V, DC-coupled Signal Source
5 V and greater than 3.75 V, the amplifier input
common-mode range is maximized by adding
GAIN R
F
R
G
R
IT
R
PD
pull-down resistors at the device inputs. The
0 dB 348 Ω 340 Ω 56.2 Ω 422 Ω
pull-down resistors provide additional loading at the
6 dB 348 Ω 169 Ω 64.9 Ω 86.6 Ω
input, and lower the common-mode voltage that is fed
back into the device input through resistor R
F
.
Figure 46 shows the circuit configuration for this
mode of operation where R
PD
is added to the
dc-coupled circuit to avoid violating the V
ICR
of the
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