SENSOR

V

+

V

-

+

-

+

-

10

1k

100k

10M

0.1

1

10

1000

1M

10k

100

100

LMP7701, LMP7702, LMP7704

www.ti.com

SNOSAI9H – SEPTEMBER 2005 – REVISED MARCH 2013

Figure 52. Total Input Noise

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 resistance as shown in Figure 53,

voltage, an op amp with very low input bias current must be used with high impedance sensors. This is to keep

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 voltage 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 different pH sensors. Temperature is also an important factor in a pH electrode reading. The

output voltage of the senor will change with temperature.

Figure 54 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 electrode has an output voltage of 59.15 mV/pH at

25°C.

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