THS4511_14 Datasheet, PDF (20/36 Pages) Texas Instruments – WIDEBAND, LOW-NOISE, LOW-DISTORTION FULLY-DIFFERENTIAL AMPLIFIER

THS4511
SLOS471E – SEPTEMBER 2005 – REVISED NOVEMBER 2009
Setting the Output Common-Mode Voltage
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 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:
( ) _IEXT
=
2V_CM
- V_S+ - V_S-
50 kW
(2)
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
(ground).
V_S+
to internal
CM circuit
50 kW
50 kW
I_EXT
CM
V _S–
Figure 45. CM Input Circuit
Device Operation with Single Power Supplies
Less than 5 V
The THS4511 is optimized to work in systems using
5-V single supplies, and the characterization data
presented in this data sheet were 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 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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operational amplifier. Note R_S and R_IT are added to
the alternate input from the signal input to balance
the amplifier. One resistor that is equal to the
combined value R_I = R_G + R_S || R_IT can be placed at
the alternate input.
R_S
V_Signal
R_G
R_F
R_IT
R_PD ^VS+ ^{= 3.75 V to 5 V}
R_G
THS4511
CM
R_S
R_IT
R_PD VS–
V_CM
R_F
R_O
V_OUT-
R_O
V_OUT+
Figure 46. THS4511 DC-Coupled Single-Source
Supply Range From 3.75 V to 5 V With R_PD Used
To Set V_IC
Note that in Figure 46, the source is referenced to
ground, as is the input termination resistor R_IT. The
proper value of resistance to add can be calculated
from Equation 3:
RPD =
é
1
ù
1
RF
ê
ê
ê
êë
1 .6
VS _{+ -}
2
1 .6
ú
ú
ú
úû
-
1
RI
(3)
where R_I = R_G + R_S || R_IT.
V_S+ is the power-supply voltage, R_F is the feedback
resistance, R_G is the gain-setting resistance, R_S is the
signal source resistance, and R_IT is the termination
resistance.
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
50-Ω source impedance, and setting the output
common-mode voltage to midsupply.
Table 3. R_PD Values for Various Gains,
V_S+ = 3.75 V, DC-coupled Signal Source
GAIN
R_F
R_G
R_IT
R_PD
0 dB
348 Ω
340 Ω
56.2 Ω
422 Ω
6 dB
348 Ω
169 Ω
64.9 Ω
86.6 Ω
20
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