LMC6001AIH NSC [National Semiconductor], LMC6001AIH Datasheet - Page 10

• Manufacturer Part Number: LMC6001AIH
• Description: Ultra Ultra-Low Input Current Amplifier
• Manufacturer: NSC [National Semiconductor]

www.national.com
Two Opamp, Temperature
Compensated pH Probe Amplifier
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
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:
Note 12: (Micro-ohm style 144 or similar)
Ultra-Low Input Current Instrumentation Amplifier
Figure 8 shows an instrumentation amplifier that features
high differential and common mode input resistance
(
CMRR with 1 M
rent is less than 20 fA and offset drift is less than 2.5 µV/˚C.
>
(Continued)
10
R1 100k + 3500 ppm/˚C (Note 12)
R2 68.1k
R3, 8 5k
R4, 9 100k
R5 36.5k
R6 619k
R7 97.6k
D1 LM4040D1Z-2.5
C1 2.2 µF
14
), 0.01% gain accuracy at A
±
100 mV/pH from a pH of 7. The second opamp,
imbalance in source resistance. Input cur-
V
= 1000, excellent
FIGURE 7. pH Probe Amplifier
10
1. The LMC6001A guarantees a 25 fA limit on input current
2. The input ESD protection diodes in the LMC6042 are
The setup and calibration is simple with no interactions to
cause problems.
1. Disconnect the pH probe and with R3 set to about
2. Apply −414.1 mV to the noninverting input of the
R
range without degrading CMRR. R
maximize CMRR without using super precision matched re-
sistors. For good CMRR over temperature, low drift resistors
should be used.
2
provides a simple means of adjusting gain over a wide
at 25˚C.
only rated at 500V while the LMC6001 has much more
robust protection that is rated at 2000V.
mid-range and the noninverting input of the LMC6001
grounded, adjust R8 until the output is 700 mV.
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.
DS011887-15
7
is an initial trim used to