OPA365-EP_15 Datasheet, PDF (12/22 Pages) Texas Instruments – 50-MHz Low-Distortion High-CMRR Rail-to-Rail I/O, Single-Supply Operational Amplifier

OPA365-EP
SLOS735 – AUGUST 2011
www.ti.com
Achieving an Output Level of Zero Volts (0V)
Certain single-supply applications require the op amp output to swing from 0 V to a positive full-scale voltage and
have high accuracy. An example is an op amp employed to drive a single-supply ADC having an input range
from 0 V to 5 V. Rail-to-rail output amplifiers with very light output loading may achieve an output level within
millivolts of 0 V (or +V_S at the high end), but not 0 V. Furthermore, the deviation from 0V only becomes greater
as the load current required increases. This increased deviation is a result of limitations of the CMOS output
stage.
When a pulldown resistor is connected from the amplifier output to a negative voltage source, the OPA365 can
achieve an output level of 0 V, and even a few millivolts below 0 V. Below this limit, nonlinearity and limiting
conditions become evident. Figure 8 illustrates a circuit using this technique.
V+=+5V
OPA365
V_IN
500µA
Op Amp
Negative
Supply
Grounded
V_OUT
R_P = 10 kΩ
−V = −5V
(Add itio na l
Negative Supply)
Figure 8. Swing-to-Ground
A pulldown current of approximately 500 µA is required when OPA365 is connected as a unity-gain buffer. A
practical termination voltage (V_NEG) is −5 V, but other convenient negative voltages also may be used. The
pulldown resistor R_L is calculated from R_L = [(V_O −V_NEG)/(500 µA)]. Using a minimum output voltage (V_O) of 0 V,
R_L = [0 V−(−5 V)]/(500 µA)] = 10 kΩ. Keep in mind that lower termination voltages result in smaller pulldown
resistors that load the output during positive output voltage excursions.
Note that this technique does not work with all op amps and should only be applied to op amps such as the
OPA365 that have been specifically designed to operate in this manner. Also, operating the OPA365 output at
0 V changes the output stage operating conditions, resulting in somewhat lower open-loop gain and bandwidth.
Keep these precautions in mind when driving a capacitive load because these conditions can affect circuit
transient response and stability.
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