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LT1991ACMS-TRPBF Datasheet(PDF) 11 Page - Linear Technology |
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LT1991ACMS-TRPBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 28 page  LT1991 11 1991fg The maximum voltage allowed on the P3, M3, P9, and M9 inputs includes the positive and negative supply plus a diode drop. These pins should not be driven more than 0.2V outside of the supply rails. This is because they are connected through diodes to internal manufacturing 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 LT1991 will be detrimmed or damaged. The P1 and M1 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 P1 and M1 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 P1, P3, P9 and REF (see “Calculating 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. Consider Figure 1. This shows the LT1991 configured as a gain of 13 difference amplifier on a single supply with the output REF connected to ground. This is a great circuit, but it does not support VDM = 0V at any common mode because the output clips into ground while trying to produce 0VOUT. It can be fixed simply by declaring the valid input differential range not to extend below +4mV, or by elevating the REF pin above 40mV, or by providing a negative supply. Calculating Input Voltage Range Figure 2 shows the LT1991 in the generalized case of a difference amplifier, with the inputs shorted for the common mode calculation. The values of RF and RG are dictated by how the P inputs and REF pin are connected. By superposition we can write: VINT = VEXT • (RF/(RF + RG)) + VREF • (RG/(RF + RG)) Or, solving for VEXT: VEXT = VINT • (1 + RG/RF) – VREF • RG/RF But valid VINT voltages are limited to VCC – 1.2V and VEE + 1V, so: MAX VEXT = (VCC – 1.2) • (1 + RG/RF) – VREF • RG/RF and: MIN VEXT = (VEE + 1) • (1 + RG/RF) – VREF • RG/RF These two voltages represent the high and low extremes of the common mode input range, if the other limits have not already been exceeded (1 and 3, above). In most cases, the inverting inputs M1 through M9 can be taken further than these two extremes because doing this does not move the op amp input common mode. To calculate the limit on this additional range, see Figure 3. Note that, – + 1991 F01 50k 150k 450k 50k 150k 450k 450k 450k REF 5V VCM 2.5V VDM 0V – + 8 7 6 5 4 9 10 1 2 3 LT1991 VOUT = 13 • VDM 4pF 4pF – + VREF RF RF RG RG 1991 F02 VEXT VINT VCC VEE Figure 1. Difference Amplifier Cannot Produce 0V on a Single Supply. Provide a Negative Supply, or Raise Pin 5, or Provide 4mV of VDM Figure 2. Calculating CM Input Voltage Range APPLICATIONS INFORMATION |
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