OPA830 datasheet(20/56 Pages) TI

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7.3 Feature Description

7.3.1 Input Common-Mode Voltage Range

When the primary design goal is a linear amplifier with high CMRR, the input pins must stay within the input

operating range (VICR). These input pins are referenced off of each supply as an input headroom requirement.

Specified operation at 25°C is maintained to the negative supply voltage, and to within 1.3 V of the positive

supply voltage. The common-mode input range specifications in the table data use CMRR to set the limit. The

limits are selected to make sure CMRR does not degrade more than 3 dB less than the minimum CMRR value if

the input voltage is within the specified range.

During linear operation, the voltage difference between the input pins is small (0 V) and the input common-mode

voltage is analyzed at either input pin, as both pins are at the same potential. The voltage at VIN+ is simple

to evaluate. In noninverting configuration (see Figure 7-1), the input signal (VIN) must not violate the VICR. In

inverting configuration (see Figure 7-2), the reference voltage (VREF), must be within the VICR.

The input voltage limits have fixed headroom to the power rails and track the power supply voltages. For a single

5-V supply, the linear 25°C minimum input voltage ranges from 0 V to 3.7 V, and 0 V to 1.4 V for a single 2.7

‑V

supply. The delta headroom from each power supply rail is the same in each case (0 V and 1.3 V).

7.3.2 Output Voltage Range

The OPA838 device is a rail-to-rail output op amp. Rail-to-rail output typically means that the output voltage

swings to within 100 mV 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. Saturation and linear

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

transistors; see Figure 6-56.

The specification tables show saturated output voltage specifications with a 2-kΩ load. Figure 6-11 and Figure

6-43 illustrate saturated voltage-swing limits versus output load resistance, and Figure 6-12 and Figure 6-44

illustrate the output saturation voltage versus load current. With a light load, the output voltage limits have

constant headroom to the power rails and track the power supply voltages. For example, with a 1-kΩ load and a

single 5-V supply, the linear output voltage ranges from 0.12 V to 4.88 V and ranges from 0.12 V to 2.58 V for a

2.7-V supply. The delta from each power supply rail is the same in each case: 0.12 V.

With devices like the OPA838 where the input range is lower than the output range, the input limits the

available signal swing at low gains. Because the OPA838 is intended for higher gains, the smaller input swing

range does not limit operation and full rail-to-rail output is available. Inverting voltage gain and transimpedance

configurations are typically limited by the output voltage limits of the op amp if the noninverting input pin is

biased in range.

OPA838

SBOS867D – AUGUST 2017 – REVISED SEPTEMBER 2024

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OPA830 Datasheet(PDF) 20 Page - Texas Instruments