LMC6001AIH datasheet(10/14 Pages) NSC

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Two Opamp, Temperature

Compensated pH Probe Amplifier

(Continued)

the feedback loop. This cancels the temperature depen-

dence of the probe. This resistor must be mounted where it

will be at the same temperature as the liquid being mea-

sured.

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 dis-

played on a low cost, low power digital panel meter. Total

current consumption will be about 1 mA for the whole sys-

tem.

The micropower dual operational amplifier, LMC6042, would

optimize power consumption but not offer these advantages:

The LMC6001A guarantees a 25 fA limit on input current

at 25˚C.

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.

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.

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 adjust-

ments should be made by trimming while measuring a

precision buffer solution.

Ultra-Low Input Current Instrumentation Amplifier

Figure 8 shows an instrumentation amplifier that features

high differential and common mode input resistance

(>10

Ω), 0.01% gain accuracy at A

V = 1000, excellent

CMRR with 1 M

Ω imbalance in source resistance. Input cur-

rent is less than 20 fA and offset drift is less than 2.5 µV/˚C.

2 provides a simple means of adjusting gain over a wide

range without degrading CMRR. R

7 is an initial trim used to

maximize CMRR without using super precision matched re-

sistors. For good CMRR over temperature, low drift resistors

should be used.

DS011887-15

R1 100k + 3500 ppm/˚C (Note 12)

R2 68.1k

R3,85k

R4, 9 100k

R5 36.5k

R6 619k

R7 97.6k

D1 LM4040D1Z-2.5

C1 2.2 µF

Note 12: (Micro-ohm style 144 or similar)

FIGURE 7. pH Probe Amplifier

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