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LT1114C Datasheet(PDF) 10 Page - Linear Technology |
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LT1114C Datasheet(HTML) 10 Page - Linear Technology |
10 / 12 page 10 LT1112/LT1114 S APPLICATI I FOR ATIO 800mV of the supplies. The effect of input and output overdrive on the other amplifiers in the LT1112 or LT1114 packages is negligible, as each amplifier is biased independently. Advantages of Matched Dual and Quad Op Amps In many applications the performance of a system de- pends on the matching between two operational amplifi- ers rather than the individual characteristics of the two op amps. Two or three op amp instrumentation amplifiers, tracking voltage references and low drift active filters are some of the circuits requiring matching between two op amps. The well-known triple op amp configuration illustrates these concepts. Output offset is a function of the differ- ence between the offsets of the two halves of the LT1112. This error cancellation principle holds for a considerable number of input referred parameters in addition to offset voltage and its drift with temperature. Input bias current will be the average of the two noninverting input currents (IB+). The difference between these two currents (∆IB+) is the offset current of the instrumentation amplifier. Com- mon-mode and power supply rejections will be dependent only on the match between the two amplifiers (assuming perfect resistor matching). The concepts of common-mode and power supply rejec- tion ratio match ( ∆CMRR and ∆PSRR) are best demon- strated with a numerical example: Assume CMRRA = +1µV/V or 120dB, and CMRRB = +0.75µV/V or 122.5dB, then ∆CMRR = 0.25µV/V or 132dB; if CMRRB = – 0.75µV/V which is still 122.5dB, then ∆CMRR = 1.75µV/V or 115dB. Clearly the LT1112/LT1114, by specifying and guarantee- ing all of these matching parameters, can significantly improve the performance of matching-dependent circuits. Typical performance of the instrumentation amplifier: Input offset voltage = 35 µV Offset voltage drift = 0.3 µV/°C Input bias current = 80pA Input offset current = 100pA Input resistance = 800G Ω Input noise = 0.42 µVP-P Three Op Amp Instrumentation Amplifier When the instrumentation amplifier is used with high impedance sources, the LT1114 is recommended be- cause its CMRR vs frequency performance is better than the LT1112’s. For example, with two matched 1M Ωsource resistors, CMRR at 100Hz is 100dB with the LT1114, 76dB with the LT1112. This difference is explained by the fact that capacitance between adjacent pins on an IC package is about 0.25pF (including package, socket and PC board trace capaci- tances). On the dual op amp package, positive input A is next to the V – pin (AC ground), while positive input B has no AC ground pin adjacent to it, resulting in a 0.25pF input capacitance mismatch. At 100Hz, 0.25pF represents a 6.4 × 109 input impedance mismatch, which is only 76dB higher than the 1M Ω source resistors. On the quad package, all four inputs are adjacent to a power supply terminal–therefore, there is no mismatch. – + 1/2 LT1112 OR 1/4 LT1114 A 1/2 LT1112 OR 1/4 LT1114 D IN– IN+ R1 10k 1% R3 2.1k 1% R8 200 Ω R2 10k 1% R4 100 Ω 0.5% R5 100 Ω 0.5% C1 33pF R10 1M R6 10k 0.5% R7 9.88k 0.5% R9 200 Ω OUTPUT LT1097 OR 1/4LT1114 B OR C GAIN = 1000 TRIM R8 FOR GAIN TRIM R9 FOR DC COMMON-MODE REJECTION TRIM R10 FOR AC COMMON-MODE REJECTION LT1112/14 • AI02 |
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