THS4508 Datasheet, PDF (17/29 Pages) Texas Instruments – WIDEBAND, LOW NOISE, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIER

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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 51. CM Input Circuit
Device Operation with Single Power Supplies
Less than 5 V
The THS4508 is optimized to work in systems using a
5-V single supply, 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 R_F.
Figure 52 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
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.
THS4508
SLAS459A – SEPTEMBER 2005 – REVISED FEBRUARY 2006
R_S
R_G
R_F
V_Signal
R_IT
R_PD ^VS+ ^{= 3.75 V to 5 V}
R_G
THS4508
CM
R_S
R_IT
R_PD VS–
V_CM
R_F
R_O
V_OUT-
R_O
V_OUT+
Figure 52. THS4508 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 52, 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:
R_PD =
é
1
ù
1
R_F
ê
ê
ê
ê_ë
V
1 .6
S+ _-
2
1 .6
ú
ú
ú
ú_û
-
1
R_I
(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 mid-supply.
Table 3. R_PD Values for Various Gains,
V_S+ = 3.75 V, DC-coupled Signal Source
Gain
6 dB
R_F
348 Ω
R_G
169 Ω
R_IT
64.9 Ω
R_PD
86.6 Ω
10 dB
348 Ω
102 Ω
78.7 Ω
110 Ω
14 dB
348 Ω
61.9 Ω
115 Ω
158 Ω
20 dB
348 Ω
40.2 Ω
221 Ω
226 Ω
If the signal originates from an ac-coupled 50-Ω
source (see Figure 53), the equivalent dc-source
resistance is an open circuit and R_I = R_G + R_IT.
Table 4 is a modification of Table 1 to add the proper
values with R_PD assuming V_S+ = 3.75 V, an
ac-coupled 50-Ω source impedance, and setting the
output common-mode voltage to mid-supply.
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