Datasheet

LMC6001

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SNOS694H –MARCH 1995–REVISED MARCH 2013

(Input pins are lifted out of PC board and soldered directly to components. All other pins connected to PC board).

Figure 28. Air Wiring

Another potential source of leakage that might be overlooked is the device package. When the LMC6001 is

manufactured, the device is always handled with conductive finger cots. This is to assure that salts and skin oils

do not cause leakage paths on the surface of the package. We recommend that these same precautions be

adhered to, during all phases of inspection, test and assembly.

LATCHUP

CMOS devices tend to be susceptible to latchup due to their internal parasitic SCR effects. The (I/O) input and

output pins look similar to the gate of the SCR. There is a minimum current required to trigger the SCR gate

lead. The LMC6001 is designed to withstand 100 mA surge current on the I/O pins. Some resistive method

should be used to isolate any capacitance from supplying excess current to the I/O pins. In addition, like an SCR,

there is a minimum holding current for any latchup mode. Limiting current to the supply pins will also inhibit

latchup susceptibility.

Typical Applications

The extremely high input resistance, and low power consumption, of the LMC6001 make it ideal for applications

that require battery-powered instrumentation amplifiers. Examples of these types of applications are hand-held

pH probes, analytic medical instruments, electrostatic field detectors and gas chromotographs.

TWO OPAMP, 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 theoretical output of the standard Ag/AgCl pH probe is 59.16 mV/pH at 25°C with 0V 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.

The LMC6001 amplifies the probe output providing a scaled voltage of ±100 mV/pH from a pH of 7. The second

opamp, 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 500V while the LMC6001 has much more

robust protection that is rated at 2000V.

The setup and calibration is simple with no interactions to cause problems.

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.

Product Folder Links: LMC6001