Datasheet LT1996 (Analog Devices) - 9

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