THS4513_13 Datasheet, PDF (26/41 Pages) Texas Instruments – WIDEBAND, LOW-NOISE, LOW-DISTORTION,FULLY-DIFFERENTIAL AMPLIFIER

THS4513
SLOS472E – AUGUST 2005 – REVISED JANUARY 2013
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 over-driven from an
external source. Figure 78 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 78. CM Input Circuit
Single-Supply Operation (3 V to 5 V)
To facilitate testing with common lab equipment, the
THS4513 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 79,
Figure 80, and Figure 81 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.
www.ti.com
In Figure 79, the signal source is referenced to a
voltage derived from the CM pin via a unity-gain
wideband buffer such as the BUF602. V_CM is set to
midsupply by THS4513 internal circuitry. R_T along
with the input impedance of the amplifier provides
input termination, which is also referenced to V_CM.
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
technique is also true of the circuits shown in
Figure 80 and Figure 81.
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+
THS4513
CM
V_S-
R_F
R_O
V_OUT-
R_O
V_OUT+
G=1 ^VCM
Wideband
Buffer
Figure 79. THS4513 DC-Coupled Single-Supply
with Input Biased to V_CM
In Figure 80, 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 4513
CM
V S-
RO
V OUT-
RO
V OUT+
RF
Figure 80. THS4513 DC-Coupled Single-Supply
with R_PU Used to Set V_IC
26
Submit Documentation Feedback
Product Folder Links: THS4513
Copyright © 2005–2013, Texas Instruments Incorporated