OPA1S2384 Datasheet, PDF (12/23 Pages) Texas Instruments – 250-MHz, CMOS Transimpedance Amplifier (TIA) with Integrated Switch and Buffer

OPA1S2384
OPA1S2385
SBOS645A – DECEMBER 2012 – REVISED JUNE 2013
APPLICATION INFORMATION
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OPERATING VOLTAGE
The OPA1S238x operates over a power-supply range of +2.7 V to +5.5 V. Supply voltages higher than +6 V
(absolute maximum) can permanently damage the device. Parameters that vary over supply voltage or over
temperature are shown in the Typical Characteristics section of this data sheet.
INPUT VOLTAGE
The OPA1S238x input common-mode voltage range extends 0.1 V beyond the supply rails. Under normal
operating conditions, the input bias current is approximately 3 pA. Input voltages exceeding the supply voltage
can cause excessive current to flow into or out of the input pins. If there is a possibility that this operating
condition may occur, the inputs must be protected. Momentary voltages that exceed the supply voltage can be
tolerated if the input current is limited to 10 mA. This limitation is easily accomplished with an input resistor
between the signal and the input pin of the device.
OUTPUT VOLTAGE
Rail-to-rail output is achieved by using a class AB output stage with common-source transistors. For high-
impedance loads (> 200 Ω), the output voltage swing is typically 100 mV from the supply rails. With 10-Ω loads,
a useful output swing can be achieved while maintaining high open-loop gain; see Figure 13.
OUTPUT DRIVE
The OPA1S238x output stage can supply a continuous output current of ±100 mA and still provide approximately
2.7 V of output swing on a 5-V supply; see Figure 13.
The OPA1S238x provides peak currents of up to 200 mA, which corresponds to the typical short-circuit current.
Therefore, an on-chip thermal shutdown circuit is provided to protect the OPA1S238x from dangerously-high
junction temperatures. At +160°C, the protection circuit shuts down the amplifier. Normal operation resumes
when the junction temperature cools to below +140°C.
CAPACITIVE LOAD AND STABILITY
The OPA1S238x can drive a wide range of capacitive loads. However, all op amps can become unstable under
certain conditions. Op amp configuration, gain, and load value are just a few of the factors to consider when
determining stability. An op amp in a unity-gain configuration is most susceptible to the effects of capacitive
loading. The capacitive load reacts with the op amp output resistance, along with any additional load resistance,
to create a pole in the small-signal response that degrades the phase margin; see Figure 12 for details.
The OPA1S238x topology enhances its ability to drive capacitive loads. In unity gain, these op amps perform well
with large capacitive loads. See Figure 10 and Figure 11 for details.
One method of improving capacitive load drive in the unity-gain configuration is to insert a 10-Ω to 20-Ω resistor
in series with the output. This resistor significantly reduces ringing with large capacitive loads. For details about
stability with certain output capacitors, see Figure 11. However, if there is a resistive load in parallel with the
capacitive load, R_S creates a voltage divider. This voltage divider introduces a dc error at the output and slightly
reduces output swing. This error may be insignificant. For instance, with R_L = 10 kΩ and R_S = 20 Ω, there is only
about a 0.2% error at the output.
WIDEBAND TRANSIMPEDANCE AMPLIFIER
Wide bandwidth, low input bias current and low current noise make the OPA1S238x an ideal wideband,
photodiode, transimpedance amplifier for low-voltage, single-supply applications. Low-voltage noise is important
because photodiode capacitance causes the effective noise gain of the circuit to increase at high frequencies.
POWER DISSIPATION
Power dissipation depends on power-supply voltage, signal, and load conditions. With dc signals, power
dissipation is equal to the product of output current times the voltage across the conducting output transistor.
Power dissipation can be minimized by using the lowest possible power-supply voltage necessary to assure the
required output voltage swing.
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