TLC082-Q1_16 Datasheet, PDF (19/38 Pages) Texas Instruments – Wide-Bandwidth High-Output-Drive Single-Supply Operational Amplifiers

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TLC082-Q1, TLC084-Q1
SLOS510D – SEPTEMBER 2006 – REVISED AUGUST 2016
Typical Applications (continued)
9.2.1.2 Detailed Design Procedure
9.2.1.2.1 Driving a Capacitive Load
When the amplifier is configured in this manner, capacitive loading directly on the output decreases the device
phase margin, leading to high-frequency ringing or oscillations. Therefore, for capacitive loads of greater than
10 pF, TI recommends placing a resistor in series (R_NULL) with the output of the amplifier, as shown in Figure 40.
A minimum value of 20 Ω should work well for most applications.
R_F
R_G
Input
_
+
R_NULL
Output
C_LOAD
Figure 40. Driving a Capacitive Load
9.2.1.2.2 Offset Voltage
The output offset voltage, (V_OO) is the sum of the input offset voltage (V_IO) and both input bias currents (I_IB) times
the corresponding gains. The schematic and formula in Figure 41 can be used to calculate the output offset
voltage.
R_F
I_IB−
R_G
+
−
V_I
+
V_O
R_S
I_IB+
V_OO
=
V_IO
æ
ççè1+
æ
ç
R_F
è ^RG
öö
÷
ø
÷÷ø
±
I_IB+
R_S
æ
ççè1
+
æ
ç
è
R_F
R_G
öö
÷
ø
÷÷ø
± I_IB-
R_F
Figure 41. Output Offset Voltage Model
9.2.1.2.3 High-Speed CMOS Input Amplifiers
The TLC08x-Q1 is a family of high-speed low-noise CMOS input operational amplifiers that has an input
capacitance on the order of 20 pF. Any resistor used in the feedback path adds a pole in the transfer function
equivalent to the input capacitance multiplied by the combination of source resistance and feedback resistance.
For example, a gain of –10, a source resistance of 1 kΩ, and a feedback resistance of 10 kΩ add an additional
pole at approximately 8 MHz. This is more apparent with CMOS amplifiers than bipolar amplifiers due to their
greater input capacitance.
This is of little consequence on slower CMOS amplifiers, as this pole normally occurs at frequencies above their
unity-gain bandwidth. However, the TLC08x-Q1 with its 10-MHz bandwidth means that this pole normally occurs
at frequencies where there is on the order of 5-dB gain left and the phase shift adds considerably.
The effect of this pole is the strongest with large feedback resistances at small closed-loop gains. As the
feedback resistance is increased, the gain peaking increases at a lower frequency and the 180° phase-shift
crossover point also moves down in frequency, decreasing the phase margin.
For the TLC08x-Q1, the maximum feedback resistor recommended is 5 kΩ; larger resistances can be used but a
capacitor in parallel with the feedback resistor is recommended to counter the effects of the input capacitance
pole.
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