TOTAL NOISE CONTRIBUTION

The LMP7701/LMP7702/LMP7704 have very low input bias

current, very low input current noise, and very low input

voltage noise. As a result, these amplifiers are ideal choices

for circuits with high impedance sensor applications.

Figure 8 shows the typical input noise of the LMP7701/

LMP7702/LMP7704 as a function of source resistance

where:

e

e

referred current noise or e

e

e

Where:

The input current noise of the LMP7701/LMP7702/LMP7704

is so low that it will not become the dominant factor in the

total noise unless source resistance exceeds 300 M

Ω, which

is an unrealistically high value.

As is evident in Figure 8, at lower R

dominated by the amplifier’s input voltage noise. Once R

larger than a few kilo-Ohms, then the dominant noise factor

becomes the thermal noise of R

current noise will not be the dominant noise factor for any

practical application.

HIGH IMPEDANCE SENSOR INTERFACE

Many sensors have high source impedances that may range

up to 10 M

Ω. The output signal of sensors often needs to be

amplified or otherwise conditioned by means of an amplifier.

The input bias current of this amplifier can load the sensor’s

output and cause a voltage drop across the source resis-

tance as shown in Figure 9, where V

IN

The last term, I

To prevent errors introduced to the system due to this volt-

age, an op amp with very low input bias current must be

used with high impedance sensors. This is to keep the error

contribution by I

the amplifier, so that it will not become the dominant noise

factor.

pH electrodes are very high impedance sensors. As their

name indicates, they are used to measure the pH of a

solution. They usually do this by generating an output volt-

age which is proportional to the pH of the solution. pH

electrodes are calibrated so that they have zero output for a

neutral solution, pH = 7, and positive and negative voltages

for acidic or alkaline solutions. This means that the output of

a pH electrode is bipolar and has to be level shifted to be

used in a single supply system. The rate of change of this

voltage is usually shown in mV/pH and is different for differ-

ent pH sensors. Temperature is also an important factor in a

pH electrode reading. The output voltage of the senor will

change with temperature.

Figure 10 shows a typical output voltage spectrum of a pH

electrode. Note that the exact values of output voltage will be

different for different sensors. In this example, the pH elec-

trode has an output voltage of 59.15 mV/pH at 25˚C.

The temperature dependence of a typical pH electrode is

shown in Figure 11. As is evident, the output voltage

changes with changes in temperature.

20127358

FIGURE 8. Total Input Noise

20127359

FIGURE 9. Noise Due to I

BIAS

20127360

FIGURE 10. Output Voltage of a pH Electrode

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