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LT1991IMS Datasheet(PDF) 10 Page - Linear Integrated Systems |
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LT1991IMS Datasheet(HTML) 10 Page - Linear Integrated Systems |
10 / 24 page LT1991 10 1991fb representation of the circuit on the top. The LT1991 is shown on the bottom configured in a precision gain of 5.5. One of the benefits of the noninverting op amp configura- tion is that the input impedance is extremely high. The LT1991 maintains this benefit. Given the finite number of available feedback resistors in the LT1991, the number of gain configurations is also finite. The complete list of such Hi-Z input noninverting gain configurations is shown in Table 1. Many of these are also represented in Figure 5 in schematic form. Note that the P-side resistor inputs have been connected so as to match the source impedance seen by the internal op amp inputs. Note also that gain and noise gain are identical, for optimal precision. Figure 3. Calculating Additional Voltage Range of Inverting Inputs VEXT (the high cm limit), as VMORE goes positive, the op amp output will go more negative from VREF by the amount VMORE • RF/RG, so: VOUT = VREF – VMORE • RF/RG Or: VMORE = (VREF – VOUT) • RG/RF The most negative that VOUT can go is VEE + 0.04V, so: Max VMORE = (VREF – VEE – 0.04V) • RG/RF (should be positive) The situation where this function is negative, and therefore problematic, when VREF = 0 and VEE = 0, has already been dealt with in Figure 1. The strength of the equation is demonstrated in that it provides the three solutions sug- gested in Figure 1: raise VREF, lower VEE, or provide some negative VMORE. Likewise, from the lower common mode extreme, making the negative input more negative will raise the output voltage, limited by VCC – 0.04V. MIN VMORE = (VREF – VCC + 0.04V) • RG/RF (should be negative) Figure 4. The LT1991 as a Classical Noninverting Op Amp APPLICATIO S I FOR ATIO – + VREF RF RF RG RG 1991 F03 VEXT MAX OR MIN VINT VMORE VCC VEE – + RF RG VIN VIN VOUT VOUT VOUT = GAIN • VIN GAIN = 1 + RF/RG – + 1991 F04 50k 150k 450k 50k 150k 450k 450k 450k 8 6 5 9 10 1 2 3 LT1991 CLASSICAL NONINVERTING OP AMP CONFIGURATION. YOU PROVIDE THE RESISTORS. CLASSICAL NONINVERTING OP AMP CONFIGURATION IMPLEMENTED WITH LT1991. RF = 225k, RG = 50k, GAIN = 5.5. GAIN IS ACHIEVED BY GROUNDING, FLOATING OR FEEDING BACK THE AVAILABLE RESISTORS TO ARRIVE AT DESIRED RF AND RG. WE PROVIDE YOU WITH <0.1% RESISTORS. 4pF 4pF Again, the additional input range calculated here is only available provided the other remaining constraint is not violated, the maximum voltage allowed on the pin. The Classical Noninverting Amplifier: High Input Z Perhaps the most common op amp configuration is the noninverting amplifier. Figure 4 shows the textbook |
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