THS4509-Q1 Datasheet, PDF (27/37 Pages) Texas Instruments – WIDEBAND LOW-NOISE LOW-DISTORTION FULLY DIFFERENTIAL AMPLIFIER

THS4509-Q1
www.ti.com........................................................................................................................................................................................... SLOS547 – NOVEMBER 2008
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+ -
50 kW
V_S-
(2)
where V_CM is the voltage applied to the CM pin.
V_S+
to internal
CM circuit
50 kW
50 kW
I_EXT
CM
V _S–
Figure 83. CM Input Circuit
Single-Supply Operation (3 V to 5 V)
To facilitate testing with common lab equipment, the
THS4509 EVM allows split-supply operation, and the
characterization data presented in this data sheet
was taken with split-supply power inputs. The device
can easily be used with a single-supply power input
without degrading the performance. Figure 84,
Figure 85, and Figure 86 show dc and ac-coupled
single-supply circuits with single-ended inputs. These
configurations all allow the input and output
common-mode voltage to be set to mid-supply
allowing for optimum performance. The information
presented here can also be applied to differential
input sources.
In Figure 84, the source is referenced to the same
voltage as the CM pin (V_CM). V_CM is set by the
internal circuit to mid-supply. R_T along with the input
impedance of the amplifier circuit provides input
termination, which is also referenced to V_CM.
Note R_S and R_T are added to the alternate input from
the signal input to balance the amplifier. Alternately,
one resistor can be used equal to the combined value
R_G+ R_S||R_T on this input. This is also true of the
circuits shown in Figure 85 and Figure 86.
R_S
R_G
R_F
V_Signal
V_Bias= V_CM
R_T
V_CM
R_G
R_S
R_T
V_CM V_CM
V_S+
THS4509
CM
V_S–
V_CM
R_F
R_O
V_OUT-
R_O
V_OUT+
Figure 84. THS4509 DC-Coupled Single Supply
With Input Biased to V_CM
In Figure 85 the source is referenced to ground and
so is the input termination resistor. R_PU is added to
the circuit to avoid violating the V_ICR of the op amp.
The proper value of resistor to add can be calculated
from Equation 3:
( ) _RPU
=
V_CM
ççèæ
1
R_F
V_IC - V_S+
÷÷øö
-
V_IC
ççèæ
1
R_IN
+
1
R_F
÷÷øö
(3)
V_IC is the desire input common-mode voltage, V_CM =
CM, and R_IN = R_G + R_S||R_T. To set to mid-supply,
make the value of R_PU = R_G+ R_S||R_T.
Table 3 is a modification of Table 1 to add the proper
values with R_PU assuming a 50 Ω source impedance
and setting the input and output common-mode
voltage to mid-supply.
There are two drawbacks to this configuration. One is
it requires additional current from the power supply.
Using the values shown for a gain of 10 dB requires
37 mA more current with 5-V supply, and 22 mA
more current with 3-V supply.
The other drawback is this configuration also
increases the noise gain of the circuit. In the 10-dB
gain case, noise gain increases by a factor of 1.5.
Table 3. RPU Values for Various Gains
Gain
6 dB
10 dB
14 dB
20 dB
R_F
348 Ω
348 Ω
348 Ω
348 Ω
R_G
169 Ω
102 Ω
61.9 Ω
40.2 Ω
R_IT
64.9 Ω
78.7 Ω
115 Ω
221 Ω
R_PU
200 Ω
133 Ω
97.6 Ω
80.6 Ω
Copyright © 2008, Texas Instruments Incorporated
Product Folder Link(s): THS4509-Q1
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