OPA1S2384 Datasheet, PDF (13/23 Pages) Texas Instruments – 250-MHz, CMOS Transimpedance Amplifier (TIA) with Integrated Switch and Buffer

OPA1S2384
OPA1S2385
www.ti.com
SBOS645A – DECEMBER 2012 – REVISED JUNE 2013
For resistive loads, the maximum power dissipation occurs at a dc output voltage of one-half the power-supply
voltage. Dissipation with ac signals is lower. Application bulletin AB-039 (SBOA022), Power Amplifier Stress and
Power Handling Limitations, explains how to calculate or measure power dissipation with unusual signals and
loads, and is available for download at www.ti.com.
Repeated activation of the thermal protection circuit indicates excessive power dissipation or an inadequate
heatsink. For reliable operation, junction temperature should be limited to +150°C, maximum. To estimate the
margin of safety in a complete design, increase the ambient temperature until the thermal protection is triggered
at +160°C. However, for reliable operation, design your system to operate at a maximum of 35°C below the
thermal protection trigger temperature (that is, +125°C or less).
TYPICAL APPLICATIONS
The following sections show typical applications of the OPA1S238x and explain their basic functionality.
Signal Strength Detection
The OPA1S238x can be used to detect the signal strength of a fast changing optical signal. Figure 27 shows a
simplified circuit for this application.
Optical sensors like photodiodes often generate a current that is proportional to the amount of light detected by
these sensors. The current generated by this sensor is represented by the current source I_IN, as shown in
Figure 27. One of the OPA1S238x op amps is configured in a transimpedance configuration. If it is assumed that
this op amp behaves like an ideal op amp, then all the current generated by I_IN flows through R1 and generates a
voltage drop of I_IN × R1. The voltage at the output of this op amp can then be calculated by V_TIA = V_BIAS + I_IN ×
R1. This calulation assumes ideal components.
In real-life applications, the current generated by I_IN can change very quickly. The current at a specific point in
time can be measured by using the internal switch of the OPA1S238x. When the switch is closed, the C2
capacitor is charged to the output voltage level of the first amplifier (V_TIA). By opening the switch, the output is
disconnected from C2, and the voltage at the noninverting terminal of the second op amp remains at the same
voltage level as when the switch was opened. The second op amp is configured in a buffer configuration and
prevents the C2 capacitor from being discharged by a load at the V_OUT terminal.
V– V+ SC
+
– ^VBIAS
+
+
R1
C2
I_IN
V_TIA
Figure 27. Signal Strength Detection
V_OUT
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