THS4513 Datasheet, PDF (26/34 Pages) Texas Instruments – WIDEBAND, LOW NOISE, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIER

THS4513
SLOS472 – AUGUST 2005
formance, 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 (3V to 5V)
To facilitate testing with common lab equipment, the
THS4513 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 79, Fig-
ure 80, and Figure 81 show DC and AC-coupled
single-supply circuits with single-ended inputs. These
configurations all allow the input and output com-
mon-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 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
mid-supply 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. Alter-
nately, one resistor can be used equal to the com-
bined value R_G+ R_S||R_T on this input. This is also true
of the circuits shown in Figure 80 and Figure 81.
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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
ç^ç_è^æ
1
R_F
V_IC - V_S+
÷^÷_ø^ö
-
V_IC
ç^ç_è^æ
1
R_IN
+
1
R_F
÷^÷_ø^ö
(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 0 dB requires 14
mA more current with 5 V supply, and 8.2 mA more
current with 3 V supply.
The other drawback is this configuration also in-
creases 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
0 dB
6 dB
R_F
348 Ω
348 Ω
R_G
340 Ω
168 Ω
R_IT
56.2 Ω
64.9 Ω
R_PU
365 Ω
200 Ω
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