Datasheet

SENSOR
V
+
V
-
R
S
+
-
I
B
V
IN
+
V
S
+
-
10
1k 100k 10M
R
S
(:)
0.1
1
10
1000
1M10k
100
100
VOLTAGE NOISE DENSITY (nV/
Hz)
e
ni
e
n
e
i
e
t
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 MΩ. 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,
where V
IN
+
= V
S
– I
BIAS
*R
S
The last term, I
BIAS
*R
S
, shows the voltage drop across R
S
. To prevent errors introduced to the system due to this
voltage, 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
BIAS
*R
S
less than the input voltage noise of the amplifier, so that it will not become the
dominant noise factor.
Figure 53. Noise Due to I
BIAS
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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