Datasheet LT1002 (Analog Devices) - 10

LT1002
U U W U APPLICATIONS INFORMATION Advantages of Matched Dual Op Amps
In many applica-
Three Op Amp Instrumentation Amplifier
tions the performance of a system depends on the match- ing between two operational amplifiers rather than the – individual characteristics of the two op amps. Two or three INPUT + A op amp instrumentation amplifiers, tracking voltage refer- 1/2 LT1002 R1 R4 R6 – ences and low drift active filters are some of the circuits 10k 100Ω 10k 1% 1% 1% requiring matching between two op amps. R3 2.1k – The well-known triple op amp configuration illustrates 1% R10 LT1037 OUTPUT 100k these concepts. Output offset is a function of the differ- R8 200Ω + C1 ence between the offsets of the two halves of the LT1002. 100pF This error cancellation principle holds for a considerable R2 GAIN = 1000 10k number of input referred parameters in addition to offset – 1% B voltage and its drift with temperature. Input bias current 1/2 LT1002 R5 R7 will be the average of the two non-inverting input currents INPUT + 100 + Ω 9.76k 1% 1% (I + + B ). The difference between these two currents (IOS ) is R9 the offset current of the instrumentation amplifier. The 200Ω 1002 TA07 difference between the inverting input currents (I – OS ) will cause errors flowing through R1, R2, and R3. Common- Trim R8 for gain mode and power supply rejections will be dependent only Trim R9 for DC common mode rejection on the match between the two amplifiers (assuming Trim R10 for AC common mode rejection perfect resistor matching). The concepts of common mode and power supply rejec- Typical performance of the instrumentation amplifier: tion ratio match (∆CMRR and ∆PSRR) are best demon- strated with a numerical example: Input offset voltage = 25µV Input bias current = 0.7nA Assume CMRRA = + 1.0µV/V or 120dB, Input resistance = 200 GΩ and CMRRB = + 0.75µV/V or 122.5dB, Input offset current = 0.6nA then ∆CMRR = 0.25µV/V or 132dB; Input noise = 0.5µV p-p if CMRRB = – 0.75µV/V which is still 122.5dB, Power bandwidth (V then ∆CMRR = 1.75µV/V or 115dB. 0 = ±10V) = 80kHz Clearly, the LT1002, by specifying and guaranteeing all of these matching parameters, can significantly improve the performance of matching dependent circuits. 1002fb 10