THS4281 Datasheet, PDF (25/43 Pages) Texas Instruments – VERY LOW-POWER, HIGH-SPEED, RAIL-TO-RAIL INPUT AND OUTPUT VOLTAGE-FEEDBACK OPERATIONAL AMPLIFIER

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Feature Description (continued)
THS4281
SLOS432B – APRIL 2004 – REVISED OCTOBER 2015
+V_S
C_T
0.1 µF
+
R_T
1.24 kW _
50-W Source
R_g
V_I
2.49 kW
R_M
51.1 W
R_f
2.49 kW
+
0.1 µF 6.8 µF
V_O
To Load
0.1 µF 6.8 µF
+
−V_S
Figure 69. Wideband, Inverting Gain Configuration
In the inverting configuration, some key design considerations must be noted. One is that the gain resistor (R_g)
becomes part of the signal channel input impedance. If the input impedance matching is desired (which is
beneficial whenever the signal is coupled through a cable, twisted pair, long PCB trace, or other transmission line
conductors), 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 resulting feedback resistor value can
present a significant load to the amplifier output. For example, 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. The 100-Ω feedback resistor,
in parallel with the external load, causes excessive loading on the amplifier output. To eliminate this excessive
loading, it is preferable to increase both R_g and R_f values, as shown in Figure 69, and then achieve the input
matching impedance with a third resistor (R_M) to ground. The total input impedance is the parallel combination of
R_g and R_M.
Another consideration in inverting amplifier design is setting the bias current cancellation resistor (R_T) on the
noninverting input. If the resistance is set equal to the total dc resistance presented to the device at the inverting
terminal, the output dc error (due to the input bias currents) is reduced to the input offset current multiplied by R_T.
In Figure 69, the dc source impedance presented at the inverting terminal is 2.49 kΩ || (2.49 kΩ + 25.3 Ω) ≈ 1.24
kΩ. To reduce the additional high-frequency noise introduced by the resistor at the noninverting input, R_T is
bypassed with a 0.1-μF capacitor to ground (C_T).
7.3 Device Functional Modes
This device has no specific function modes.
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