THS4211 Datasheet, PDF (20/41 Pages) Texas Instruments – LOW-DISTORTION HIGH-SPEED VOLTAGE FEEDBACK AMPLIFIER

THS4211
THS4215
SLOS400D – SEPTEMBER 2002 – REVISED NOVEMBER 2004
5 V _+VS
100 pF
0.1 µF
+
6.8 µF
C_T
0.1 µF
R_T
200 Ω
50 Ω Source
R_g
V_I
392 Ω
R_M
57.6 Ω
+
THS4211
_
R_f
392 Ω
100 pF
V_O
499 Ω
0.1 µF 6.8 µF
+
-5 V ^-VS
Figure 76. Wideband, Inverting Gain
Configuration
In the inverting configuration, some key design con-
siderations must be noted. One is that the gain
resistor (R_g) becomes part of the signal-channel input
impedance. If input impedance matching is desired
(beneficial when the signal is coupled through a
cable, twisted pair, long PC board trace, or other
transmission line conductor), R_g may be set equal to
the required termination value and R_f adjusted to give
the desired gain. However, care must be taken when
dealing with low inverting gains, as the resultant
feedback resistor value can present a significant load
to the amplifier output. For an inverting gain of 2,
setting R_g to 49.9 Ω for input matching eliminates the
need for R_M but requires a 100-Ω feedback resistor.
This has the advantage that the noise gain becomes
equal to 2 for a 50-Ω source impedance—the same
as the noninverting circuit in Figure 75. However, the
amplifier output now sees the 100-Ω feedback re-
sistor in parallel with the external load. To eliminate
this excessive loading, it is preferable to increase
both R_g and R_f, values, as shown in Figure 76, and
then achieve the input matching impedance with a
third resistor (R_M) to ground. The total input im-
pedance becomes the parallel combination of R_g and
R_M.
The next major consideration is that the signal source
impedance becomes part of the noise gain equation
and hence influences the bandwidth. For example,
the R_M value combines in parallel with the external
50-Ω source impedance (at high frequencies), yield-
ing an effective source impedance of 50 Ω || 57.6 Ω =
26.8 Ω. This impedance is then added in series with
R_g for calculating the noise gain. The result is 1.9 for
Figure 76, as opposed to the 1.8 if R_M is eliminated.
The bandwidth is lower for the inverting gain-of-2
circuit in Figure 76 (NG=+1.9), than for the
noninverting gain of 2 circuit in Figure 75.
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The last major consideration in inverting amplifier
design is setting the bias-current cancellation resistor
on the noninverting input. If the resistance is set
equal to the total dc resistance looking out of the
inverting terminal, the output dc error, due to the input
bias currents, is reduced to (input offset current) × R_f
in Figure 76, the dc source impedance looking out of
the inverting terminal is 392 Ω || (392 Ω + 26.8 Ω) =
200 Ω. To reduce the additional high-frequency noise
introduced by the resistor at the noninverting input,
and power-supply feedback, R_T is bypassed with a
capacitor to ground.
SINGLE SUPPLY OPERATION
The THS4211 is designed to operate from a single
5-V to 15-V power supply. When operating from a
single power supply, care must be taken to ensure
the input signal and amplifier are biased appropriately
to maximize output voltage swing. The circuits shown
in Figure 77 demonstrate methods to configure an
amplifier for single-supply operation.
+V_S
50 Ω Source
V_I
R_T 49.9 Ω
+
THS4211
_
+V_S
R_f
2
R_g
392 Ω
392 Ω
V_O
499 Ω
+V_S
2
R_f
50 Ω Source
R_g
V_I
392 Ω
57.6 Ω R_T
+V_S
+V_S
2
2
392 Ω
V_S
_
THS4211
+
V_O
499 Ω
Figure 77. DC-Coupled Single Supply Operation
Saving Power With Power-Down
Functionality and Setting Threshold Levels
With the Reference Pin
The THS4215 features a power-down pin (PD) which
lowers the quiescent current from 19-mA down to
650-µA, ideal for reducing system power.
The power-down pin of the amplifiers defaults to the
positive supply voltage in the absence of an applied
voltage, putting the amplifier in the power-on mode of
operation. To conserve power, the amplifier is turned
off by driving the power-down pin towards the nega-