LMC6001-MIL Datasheet, PDF (12/18 Pages) Texas Instruments – Ultra, Ultra-Low Input Current Amplifier

LMC6001-MIL
SNOSD60 – JUNE 2017
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Typical Application (continued)
7.2.1 Two Op Amp, Temperature Compensated Ph Probe Amplifier
The signal from a pH probe has a typical resistance between 10 MΩ and 1000 MΩ. Because of this high value, it
is very important that the amplifier input currents be as small as possible. The LMC6001 with less than 25-fA
input current is an ideal choice for this application.
The LMC6001 amplifies the probe output providing a scaled voltage of ±100 mV/pH from a pH of 7. The second
op amp, a micropower LMC6041 provides phase inversion and offset so that the output is directly proportional to
pH, over the full range of the probe. The pH reading can now be directly displayed on a low-cost, low-power
digital panel meter. Total current consumption will be about 1 mA for the whole system.
The micropower dual-operational amplifier, LMC6042, would optimize power consumption but not offer these
advantages:
1. The LMC6001A ensures a 25-fA limit on input current at 25°C.
2. The input ESD protection diodes in the LMC6042 are only rated at 500 V while the LMC6001 has much more
robust protection that is rated at 2000 V.
(1)
R1 100 k + 3500 ppm/°C
R2 68.1 k
R3, 8 5 k
R4, 9 100 k
R5 36.5 k
R6 619 k
R7 97.6 k
D1 LM4040D1Z-2.5
C1 2.2 µF
(2) µΩ style 137 or similar
Figure 23. Ph Probe Amplifier
7.2.1.1 Design Requirements
The theoretical output of the standard Ag/AgCl pH probe is 59.16 mV/pH at 25°C with 0 V out at a pH of 7.00.
This output is proportional to absolute temperature. To compensate for this, a temperature-compensating
resistor, R1, is placed in the feedback loop. This cancels the temperature dependence of the probe. This resistor
must be mounted where it will be at the same temperature as the liquid being measured.
7.2.1.2 Detailed Design Procedure
The set-up and calibration is simple with no interactions to cause problems.
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