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OPA2335AIDGKR Datasheet(PDF) 8 Page - Texas Instruments |
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OPA2335AIDGKR Datasheet(HTML) 8 Page - Texas Instruments |
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8 / 31 page  8 www.ti.com OPA334, OPA2334, OPA335, OPA2335 SBOS245D FIGURE 1. Input Current Protection. 5k Ω OPA335 10mA max +5V V IN V OUT I OVERLOAD Current-limiting resistor required if input voltage exceeds supply rails by ≥ 0.5V. Normally, input bias current is approximately 70pA; however, input voltages exceeding the power supplies can cause excessive current to flow in or out of the input pins. Momen- tary voltages greater than the power supply can be tolerated if the input current is limited to 10mA. This is easily accom- plished with an input resistor, as shown in Figure 1. INTERNAL OFFSET CORRECTION The OPA334 and OPA335 series op amps use an auto-zero topology with a time-continuous 2MHz op amp in the signal path. This amplifier is zero-corrected every 100 µs using a proprietary technique. Upon power-up, the amplifier requires one full auto-zero cycle of approximately 100 µs to achieve specified VOS accuracy. Prior to this time, the amplifier functions properly but with unspecified offset voltage. This design has remarkably little aliasing and noise. Zero correction occurs at a 10kHz rate, but there is virtually no fundamental noise energy present at that frequency. For all practical purposes, any glitches have energy at 20MHz or higher and are easily filtered, if required. Most applications are not sensitive to such high-frequency noise, and no filtering is required. Unity-gain operation demands that the auto-zero circuitry correct for common-mode rejection errors of the main ampli- fier. Because these errors can be larger than 0.01% of a full- scale input step change, one calibration cycle (100 µs) can be required to achieve full accuracy. This behavior is shown in the typical characteristic section, see Settling Time vs Closed- Loop Gain. ACHIEVING OUTPUT SWING TO THE OP AMP’S NEGATIVE RAIL Some applications require output voltage swing from 0V to a positive full-scale voltage (such as +2.5V) with excellent accuracy. With most single-supply op amps, problems arise when the output signal approaches 0V, near the lower output FIGURE 2. Op Amp with Pull-Down Resistor to Achieve VOUT = Ground. 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 OPA334 or OPA335 can be made to swing to ground, or slightly below, on a single- supply power source. To do so requires use of another resistor and an additional, more negative, power supply than the op amp’s negative supply. A pull-down 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, as shown in Figure 2. The OPA334 and OPA335 have an output stage that allows the output voltage to be pulled to its negative supply rail, or slightly below using the above technique. This technique only works with some types of output stages. The OPA334 and OPA335 have been characterized to perform well with this technique. Accuracy is excellent down to 0V and as low as –2mV. Limiting and non-linearity occurs below –2mV, but excellent accuracy returns as the output is again driven above –2mV. Lowering the resistance of the pull-down resis- tor will allow the op amp to swing even further below the negative rail. Resistances as low as 10k Ω can be used to achieve excellent accuracy down to –10mV. LAYOUT GUIDELINES Attention to good layout practices is always recommended. Keep traces short. When possible, use a PCB ground plane with surface-mount components placed as close to the de- vice pins as possible. Place a 0.1 µF capacitor closely across the supply pins. These guidelines should be applied through- out the analog circuit to improve performance and provide benefits such as reducing the EMI (electromagnetic-interfer- ence) susceptibility. V OUT R P = 40kΩ Op Amp’s V– = Gnd OPA335 V IN V+ = +5V –5V Additional Negative Supply |
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