OPA37EZ datasheet(9/33 Pages) TI

9 / 33 page

OPA347, 2347, 4347

SBOS167D

www.ti.com

RAIL-TO-RAIL OUTPUT

A class AB output stage with common-source transistors is

used to achieve rail-to-rail output. This output stage is ca-

pable of driving 5k

Ω loads connected to any potential be-

tween V+ and ground. For light resistive loads (> 100k

Ω), the

output voltage can typically swing to within 5mV from supply

rail. With moderate resistive loads (10k

Ω to 50kΩ), the output

can swing to within a few tens of millivolts from the supply

rails while maintaining high open-loop gain (see the typical

characteristic Output Voltage Swing vs Output Current).

CAPACITIVE LOAD AND STABILITY

The OPA347 in a unity-gain configuration can directly drive

up to 250pF pure capacitive load. Increasing the gain en-

hances the amplifier’s ability to drive greater capacitive loads

(see the characteristic curve Small-Signal Overshoot vs

Capacitive Load). In unity-gain configurations, capacitive

load drive can be improved by inserting a small (10

Ω to 20Ω)

resistor, RS, in series with the output, as shown in Figure 6.

This significantly reduces ringing while maintaining Direct

Current (DC) performance for purely capacitive loads. How-

ever, if there is a resistive load in parallel with the capacitive

load, a voltage divider is created, introducing a DC error at

the output and slightly reducing the output swing. The error

introduced is proportional to the ratio RS/RL, and is generally

negligible.

load, reducing the resistor values from 100k

Ω to 5kΩ de-

creases overshoot from 40% to 8% (see the characteristic

curve Small-Signal Overshoot vs Load Capacitance). How-

ever, when large-valued resistors can not be avoided, a

small (4pF to 6pF) capacitor, CFB, can be inserted in the

feedback, as shown in Figure 7. This significantly reduces

overshoot by compensating the effect of capacitance, CIN,

which includes the amplifier input capacitance and PC board

parasitic capacitance.

FIGURE 6. Series Resistor in Unity-Gain Buffer Configura-

tion Improves Capacitive Load Drive.

Ω to

Ω

OPA347

V+

OUT

FIGURE 7. Adding a Feedback Capacitor In the Unity-Gain

Inverter Configuration Improves Capacitative

Load.

OPA347

OUT

DRIVING ADCs

The OPA347 series op amps are optimized for driving

medium-speed sampling Analog-to-Digital Converters (ADCs).

The OPA347 op amps buffer the ADC’s input capacitance

and resulting charge injection while providing signal gain.

See Figure 8 for the OPA347 in a basic noninverting configu-

ration driving the ADS7822. The ADS7822 is a 12-bit,

microPower sampling converter in the MSOP-8 package.

When used with the low-power, miniature packages of the

OPA347, the combination is ideal for space-limited, low-

power applications. In this configuration, an RC network at

the ADC input can be used to provide for anti-aliasing filter

and charge injection current.

See Figure 9 for the OPA2347 driving an ADS7822 in a

speech bandpass filtered data acquisition system. This small,

low-cost solution provides the necessary amplification and

signal conditioning to interface directly with an electret micro-

phone. This circuit will operate with VS = 2.7V to 5V with less

than 250

µA typical quiescent current.

In unity-gain inverter configuration, phase margin can be

reduced by the reaction between the capacitance at the op

amp input, and the gain setting resistors, thus degrading

capacitive load drive. Best performance is achieved by using

small valued resistors. For example, when driving a 500pF

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