THS4509 Datasheet, PDF (27/42 Pages) Texas Instruments – WIDEBAND, LOW NOISE, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIER

THS4509
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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 3-mV offset (typ) from the set voltage,
when set within 0.5 V of midsupply, with less than
4-mV differential offset voltage. If left unconnected,
the common-mode set point is set to midsupply by
internal circuitry, which may be overdriven from an
external source. Figure 83 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.
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
were 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 midsupply
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 midsupply. R_T along with the input
impedance of the amplifier circuit provides input
termination, which is also referenced to V_CM.
SLOS454H – JANUARY 2005 – REVISED NOVEMBER 2009
Note that 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
V_Signal
V_Bias= V_CM
R_T
V_CM
R_G
R_S
R_T
V_CM V_CM
R_F
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)
RS
V Signal
RS
V S+
R PU
RG
RF
RT
V S+
V S+
R PU
RG
RT
THS 4509
CM
V S-
RO
V OUT-
RO
V OUT+
RF
Figure 85. THS4509 DC-Coupled Single-Supply
with R_PU Used to Set V_IC
V_IC is the desired input common-mode voltage, V_CM =
CM, and R_IN = R_G+ R_S || R_T. To set to midsupply,
make the value of R_PU = R_G+ R_S || R_T.
Copyright © 2005–2009, Texas Instruments Incorporated
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