Datasheet

V
S+
CM
V
Signal
V
S–
R
F
R
F
R
G
R
G
R
T
R
O
R
O
V
OUT+
V
OUT-
THS4509
V
Bias
= V
CM
R
S
R
T
V
CM
V
CM
V
CM
V
CM
R
S
( )
W
--
=
-+
k50
VVV2
I
SSCM
EXT
( )
÷
÷
ø
ö
ç
ç
è
æ
+-
÷
÷
ø
ö
ç
ç
è
æ
-
=
+
FIN
IC
F
CM
SIC
PU
R
1
R
1
V
R
1
V
VV
R
V
S+
CM
V
S–
50kW
tointernal
CMcircuit
I
EXT
50kW
V
S+
CM
V
Signal
V
S-
R
F
R
F
R
G
R
G
R
T
R
O
R
O
V
OUT+
V
OUT-
THS 4509
R
S
R
T
R
S
V
S+
V
S+
R
PU
R
PU
THS4509
www.ti.com SLOS454H –JANUARY 2005–REVISED NOVEMBER 2009
Setting the Output Common-Mode Voltage
Note that R
S
and R
T
are added to the alternate input
from the signal input to balance the amplifier.
The output common-mode voltage is set by the
Alternately, one resistor can be used equal to the
voltage at the CM pin(s). The internal common-mode
combined value R
G
+ R
S
|| R
T
on this input. This is
control circuit maintains the output common-mode
also true of the circuits shown in Figure 85 and
voltage within 3-mV offset (typ) from the set voltage,
Figure 86.
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:
Figure 84. THS4509 DC-Coupled Single-Supply
with Input Biased to V
CM
(2)
where V
CM
is the voltage applied to the CM pin.
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:
(3)
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. THS4509 DC-Coupled Single-Supply
Figure 85, and Figure 86 show dc and ac-coupled
with R
PU
Used to Set V
IC
single-supply circuits with single-ended inputs. These
configurations all allow the input and output
V
IC
is the desired input common-mode voltage, V
CM
=
common-mode voltage to be set to midsupply
CM, and R
IN
= R
G
+ R
S
|| R
T
. To set to midsupply,
allowing for optimum performance. The information
make the value of R
PU
= R
G
+ R
S
|| R
T
.
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
.
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