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OPA378 Datasheet(PDF) 11 Page - Texas Instruments |
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OPA378 Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 29 page  CORE -In +In Clamp 1.5kW 1.5kW 1k Frequency(Hz) 100k 10k 1M 10M 100M 1G 0 10 20 30 40 - - - - f =25MHzwithParasitics OverTemperature 29dBat800MHz - C OPA378 OPA2378 www.ti.com SBOS417D – JANUARY 2008 – REVISED OCTOBER 2009 INPUT DIFFERENTIAL VOLTAGE OPA378 operational amplifier family incorporates an internal input low-pass filter that reduces the amplifier The typical input bias current of the OPA378 during response to EMI. Both common-mode and normal operation is approximately 150pA. In differential-mode filtering are provided by the input over-driven conditions, the bias current can increase filter. The filter is designed for a cutoff frequency of significantly (see Figure 24). The most common approximately 25MHz (–3dB), with a roll-off of 20dB cause of an over-driven condition occurs when the op per decade. Figure 29 shows the EMI filter. amp is outside of the linear range of operation. When the output of the op amp is driven to one of the supply rails the feedback loop requirements cannot be satisfied and a differential input voltage develops across the input pins. This differential input voltage results in activation of parasitic diodes inside the front end input chopping switches that combine with 1.5k Ω EMI filter resistors to create the equivalent circuit shown in Figure 28. Figure 28. Equivalent Input Circuit Figure 29. EMI Filter INTERNAL OFFSET CORRECTION GENERAL LAYOUT GUIDELINES The OPA378 and OPA2378 family of op amps use an auto-calibration technique with a time-continuous Attention to good layout practices is always 350kHz op amp in the signal path. This amplifier is recommended. Keep traces short and, when zero-corrected every 3 μs using a proprietary possible, use a printed circuit board (PCB) ground technique. Upon power-up, the amplifier requires plane with surface-mount components placed as approximately 100 μs to achieve specified VOS close to the device pins as possible. Place a 0.1 μF accuracy. This architecture has no aliasing or flicker capacitor closely across the supply pins. These noise. guidelines should be applied throughout the analog circuit to improve performance. NOISE For lowest offset voltage and precision performance, The OPA378 series of op amps have excellent circuit layout and mechanical conditions should be distortion characteristics. Total harmonic distortion + optimized. Avoid temperature gradients that create noise is below 0.003% (G = +1, VO = 3VRMS, and f = thermoelectric (Seebeck) effects in the thermocouple 1kHz, with a 10k Ω load). Design of low-noise op amp junctions formed from connecting dissimilar circuits requires careful consideration of a variety of conductors. These thermally-generated potentials can possible noise contributors: noise from the signal be made to cancel by assuring they are equal on source, noise generated in the op amp, and noise both input terminals. Other layout and design from the feedback network resistors. The total noise considerations include: of the circuit is the root-sum-square combination of all • Use low thermoelectric-coefficient conditions the noise components. (avoid dissimilar metals). • Thermally isolate components from power EMI SUSCEPTIBILITY AND INPUT FILTERING supplies or other heat sources. Operational amplifiers vary in their susceptibility to • Shield op amp and input circuitry from air electromagnetic interference (EMI). If conducted EMI currents, such as cooling fans. enters the operational amplifier, the dc offset Following these guidelines reduces the likelihood of observed at the amplifier output may shift from its junctions being at different temperatures, which can nominal value while the EMI is present. This shift is a cause thermoelectric voltages of 0.1 μV/°C or higher, result of signal rectification associated with the depending on materials used. internal semiconductor junctions. While all operational amplifier pin functions can be affected by EMI, the input pins are likely to be the most susceptible. The Copyright © 2008–2009, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Link(s): OPA378 OPA2378 |
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