LMC6001_16 Datasheet, PDF (18/25 Pages) Texas Instruments – Ultra, Ultra-Low Input Current Amplifier

LMC6001
SNOS694I – MARCH 1995 – REVISED SEPTEMBER 2015
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Typical Application (continued)
1. Disconnect the pH probe and with R3 set to about mid-range and the noninverting input of the LMC6001
grounded, adjust R8 until the output is 700 mV.
2. Apply −414.1 mV to the noninverting input of the LMC6001. Adjust R3 for and output of 1400 mV. This
completes the calibration. As real pH probes may not perform exactly to theory, minor gain and offset
adjustments should be made by trimming while measuring a precision buffer solution.
8.2.1.3 Application Curve
V_S = ±5 V
Figure 24. Input Current vs. V_CM
8.3 System Example
8.3.1 Ultra-Low Input Current Instrumentation Amplifier
Figure 25 shows an instrumentation amplifier that features high-differential and common-mode input resistance
(>10¹⁴Ω), 0.01% gain accuracy at A_V = 1000, excellent CMRR with 1-MΩ imbalance in source resistance. Input
current is less than 20 fA and offset drift is less than 2.5 μV/°C. R₂ provides a simple means of adjusting gain
over a wide range without degrading CMRR. R₇ is an initial trim used to maximize CMRR without using super
precision matched resistors. For good CMRR over temperature, low-drift resistors should be used.
If R₁ = R₅, R₃ = R₆, and R₄ = R₇; then
∴A_V ≈ 100 for circuit shown (R₂ = 9.85k).
Figure 25. Instrumentation Amplifier
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