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LT1996AIDD Datasheet(PDF) 9 Page - Linear Technology |
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LT1996AIDD Datasheet(HTML) 9 Page - Linear Technology |
9 / 24 page LT1996 9 1996f maximum voltage allowed on the input and REF pins. The 50k resistors connected to the M9 and P9 inputs are isolated from the substrate, and can therefore be taken beyond the supply voltages. The naming of the pins “P9,” “P27,” “P81,” etc., is based on their admittances relative to the feedback and REF admittances. Because it has 9 times the admittance, the voltage applied to the P9 input has 9 times the effect of the voltage applied to the REF input. Bandwidth The bandwidth of the LT1996 will depend on the gain you select (or more accurately the noise gain resulting from the gain you select). In the lowest configurable gain of 1, the –3dB bandwidth is limited to 450kHz, with peaking of about 2dB at 280kHz. In the highest configurable gains, bandwidth is limited to 5kHz. Input Noise The LT1996 input noise is comprised of the Johnson noise of the internal resistors (√4kTR), and the input voltage noise of the op amp. Paralleling all four resistors to the +input gives a 3.8kΩ resistance, for 8nV/√Hz of voltage noise. The equivalent network on the –input gives another 8nV/√Hz, and the op amp 14nV/√Hz. Taking their RMS sum gives a total 18nV/√Hz input referred noise floor. Output noise depends on configuration and noise gain. Input Resistance The LT1996 input resistances vary with configuration, but once configured are apparent on inspection. Note that resistors connected to the op amp’s –input are looking into a virtual ground, so they simply parallel. Any feedback resistance around the op amp does not contribute to input resistance. Resistors connected to the op amp’s +input are looking into a high impedance, so they add as parallel or series depending on how they are connected, and whether or not some of them are grounded. The op amp +input itself presents a very high GΩ impedance. In the classical noninverting op amp configuration, the LT1996 presents the high input impedance of the op amp, as is usual for the noninverting case. Common Mode Input Voltage Range The LT1996 valid common mode input range is limited by three factors: 1. Maximum allowed voltage on the pins 2. The input voltage range of the internal op amp 3. Valid output voltage The maximum voltage allowed on the P27, M27, P81 and M81 inputs includes the positive and negative supply plus a diode drop. These pins should not be driven more than a diode drop outside of the supply rails. This is because they are connected through diodes to internal manufactur- ing post-package trim circuitry, and through a substrate diode to VEE. If more than 10mA is allowed to flow through these pins, there is a risk that the LT1996 will be detrimmed or damaged. The P9 and M9 inputs do not have clamp diodes or substrate diodes or trim circuitry and can be taken well outside the supply rails. The maximum allowed voltage on the P9 and M9 pins is ±60V. The input voltage range of the internal op amp extends to within 1.2V of VCC and 1V of VEE. The voltage at which the op amp inputs common mode is determined by the voltage at the op amp’s +input, and this is determined by the voltages on pins P9, P27, P81 and REF. (See “Calcu- lating Input Voltage Range” section.) This is true provided that the op amp is functioning and feedback is maintaining the inputs at the same voltage, which brings us to the third requirement. For valid circuit function, the op amp output must not be clipped. The output will clip if the input signals are attempt- ing to force it to within 40mV of its supply voltages. This usually happens due to too large a signal level, but it can also occur with zero input differential and must therefore be included as an example of a common mode problem. APPLICATIO S I FOR ATIO |
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