OPAX170 datasheet(21/64 Pages) TI2

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OPA170

R =

10kW

OUT

=10kW

+18V

-18V

+18V

-18V

OUT

OPA170

G=+1

OPA170, OPA2170, OPA4170

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SBOS557E – AUGUST 2011 – REVISED APRIL 2018

Product Folder Links: OPA170 OPA2170 OPA4170

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Feature Description (continued)

Another common question involves what happens to the amplifier if an input signal is applied to the input when

the power supplies (V+ or V–) are at 0 V. Again, this question depends on the supply characteristic when at 0 V,

or at a level below the input signal amplitude. If the supplies appear as high impedance, then the input source

supplies the operational amplifier current through the current-steering diodes. This state is not a normal bias

condition; most likely, the amplifier does not operate normally. If the supplies are low impedance, then the current

through the steering diodes can become quite high. The current level depends on the ability of the input source

to deliver current, and any resistance in the input path.

If there is any uncertainty about the ability of the supply to absorb this current, add external Zener diodes to the

supply pins; see Figure 38. Select the Zener voltage so that the diode does not turn on during normal operation.

However, the Zener voltage must be low enough so that the Zener diode conducts if the supply pin begins to rise

above the safe-operating, supply-voltage level.

The OPAx170 input pins are protected from excessive differential voltage with back-to-back diodes; see

Figure 38. In most circuit applications, the input protection circuitry has no effect. However, in low-gain or G = 1

circuits, fast-ramping input signals can forward-bias these diodes because the output of the amplifier cannot

respond rapidly enough to the input ramp. If the input signal is fast enough to create this forward-bias condition,

limit the input signal current to 10 mA or less. If the input signal current is not inherently limited, an input series

resistor can be used to limit the input signal current. This input series resistor degrades the low-noise

performance of the OPAx170. Figure 38 illustrates an example configuration that implements a current-limiting

feedback resistor.

8.3.4 Capacitive Load and Stability

The dynamic characteristics of the OPAx170 have been optimized for common operating conditions. The

combination of low closed-loop gain and high capacitive loads decreases the phase margin of the amplifier and

can lead to gain peaking or oscillations. As a result, heavier capacitive loads must be isolated from the output.

The simplest way to achieve this isolation is to add a small resistor (for example, ROUT equal to 50 Ω) in series

with the output. Refer to Figure 39 and Figure 40 illustrate graphs of small-signal overshoot versus capacitive

load for several values of ROUT. Also, refer to applications bulletin AB-028, Feedback Plots Define Op Amp AC

Performance, for details of analysis techniques and application circuits.

100-mV Output Step

G = 1

Figure 39. Small-Signal Overshoot vs Capacitive Load

100-mV Output Step

G = –1

Figure 40. Small-Signal Overshoot vs Capacitive Load

OPAX170 Datasheet(PDF) 21 Page - Texas Instruments

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