OPA835 datasheet(8/21 Pages) TI1

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Frequency (Hz)

-9

-6

-3

10k

100k

10M

100M

Gain

RF = 1 k:

RF = 2 k:

RF = 3.6 k:

RF = 5 k:

RF = 10 k:

RF = 100 k:

OPA2834

SBOS973 – JUNE 2019

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Feature Description (continued)

8.3.2 Output Voltage Range

The OPA2834 is a rail-to-rail output (RRO) op amp. Rail-to-rail output typically means that the output voltage

swings within a couple hundred millivolts of the supply rails. There are different ways to specify this: one is with

the output still in linear operation and another is with the output saturated. Saturated output voltages are closer to

the power-supply rails than linear outputs, but the signal is not a linear representation of the input. Linear output

is a better representation of how well a device performs when used as a linear amplifier. Saturation and linear

operation limits are affected by the output current, where higher currents lead to more loss in the output

transistors.

The specification tables list the saturated output voltage specifications with a 5-kΩ load. Given a light load, the

output voltage limits have nearly constant headroom to the power rails and track the power-supply voltages. For

example, with a 5-kΩ load and a single 5-V supply, the saturation output voltage ranges from 0.1 V to 4.9 V and

ranges from 0.1 V to 2.6 V for a 2.7-V supply. The delta from each power-supply rail is the same in either case:

0.1 V and 0.1 V.

With a device such as the OPA2834 where the input range is lower than the output range, typically the input

limits the available signal swing only in a noninverting gain of 1. Signal swing in noninverting configurations in

gains greater than +1 and inverting configurations in any gain is typically limited by the output voltage limits of

the op amp.

8.3.3 Low-Power Applications and the Effects of Resistor Values on Bandwidth

The OPA2834 is designed for the nominal value of RF to be 3.6 kΩ in gains other than +1. This setting gives

excellent distortion performance, maximum bandwidth, best flatness, and best pulse response. This feedback

resistor also loads the amplifier. For example, in a gain of 2 with RF = RG = 3.6 kΩ, RG to ground, and VOUT = 4

V, 555 µA of current flows through the feedback path to ground. In a gain of +1, RG is open and no current flows

to ground. In low-power applications, reduce the current in the feedback path by increasing the gain-setting

resistors values. Using larger value gain resistors has two primary side effects (other than lower power) because

of their interaction with parasitic circuit capacitance:

•

Lowers the bandwidth

•

Lowers the phase margin

–

Causes peaking in the frequency response

–

Causes overshoot and ringing in the pulse response

Figure 3 shows the small-signal frequency response for a noninverting gain of 2 with RF and RG equal to 2 kΩ,

3.6 kΩ, 5 kΩ, 10 kΩ, and 100 kΩ. The test was done with RL = 5 kΩ. Because of the loading effects of RL, lower

RL values can reduce the peaking, but higher values do not have a significant effect.

Figure 3. Frequency Response With Various Gain-Setting Resistor Values

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