V

OUT

R

Op Amp V

− = Gnd

OPA333A

V

IN

V+ = +5V

−5V

Additional

Negative

Supply

OPA333-Q1

www.ti.com

SBOS522 – JUNE 2010

Achieving Output Swing to the Op Amp Negative Rail

Some applications require output voltage swings from 0 V to a positive full-scale voltage (such as 2.5 V) with

excellent accuracy. With most single-supply op amps, problems arise when the output signal approaches 0 V,

near the lower output swing limit of a single-supply op amp. A good single-supply op amp may swing close to

single-supply ground, but will not reach ground. The output of the OPA333 can be made to swing to ground, or

slightly below, on a single-supply power source. To do so requires the use of another resistor and an additional,

more negative, power supply than the op amp negative supply. A pulldown resistor may be connected between

the output and the additional negative supply to pull the output down below the value that the output would

otherwise achieve (see Figure 2).

The OPA333 has an output stage that allows the output voltage to be pulled to its negative supply rail, or slightly

below, using the technique previously described. This technique only works with some types of output stages.

The OPA333 has been characterized to perform with this technique; however, the recommended resistor value is

approximately 20 k

Ω. Note that this configuration will increase the current consumption by several hundreds of

microamps.

Accuracy

is

excellent

down

to

0

V

and

as

low

as

–2 mV. Limiting and nonlinearity occurs below –2 mV, but excellent accuracy returns as the output is again

driven above –2 mV. Lowering the resistance of the pulldown resistor allows the op amp to swing even further

below the negative rail. Resistances as low as 10 k

Ω can be used to achieve excellent accuracy down to

–10 mV.

General Layout Guidelines

Attention to good layout practices is always recommended. Keep traces short and, when possible, use a printed

circuit board (PCB) ground plane with surface-mount components placed as close to the device pins as possible.

Place a 0.1-mF capacitor closely across the supply pins. These guidelines should be applied throughout the

analog circuit to improve performance and provide benefits, such as reducing the electromagnetic interference

(EMI) susceptibility.

Operational amplifiers vary in their susceptibility to radio frequency interference (RFI). RFI can generally be

identified as a variation in offset voltage or dc signal levels with changes in the interfering RF signal. The

OPA333 has been specifically designed to minimize susceptibility to RFI and demonstrates remarkably low

sensitivity compared to previous-generation devices. Strong RF fields may still cause varying offset levels.

Copyright © 2010, Texas Instruments Incorporated

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Product Folder Link(s): OPA333-Q1