Datasheet

SENSOR
V
+
V
-
R
S
+
-
I
B
V
IN
+
V
S
+
-
LMV841, LMV842, LMV844
www.ti.com SNOSAT1G –OCTOBER 2006–REVISED FEBRUARY 2013
HIGH IMPEDANCE SENSOR INTERFACE
With CMOS inputs, the LMV841/LMV842/LMV844 are particularly suited to be used as high impedance sensor
interfaces.
Many sensors have high source impedances that may range up to 10MΩ. The input bias current of an amplifier
will load the output of the sensor, and thus cause a voltage drop across the source resistance, as shown in
Figure 43. When an op amp is selected with a relatively high input bias current, this error may be unacceptable.
The low input current of the LMV841/LMV842/LMV844 significantly reduces such errors. The following examples
show the difference between a standard op amp input and the CMOS input of the LMV841/LMV842/LMV844.
The voltage at the input of the op amp can be calculated with
V
IN+
= V
S
- I
B
* R
S
(21)
For a standard op amp the input bias Ib can be 10nA. When the sensor generates a signal of 1V (V
S
) and the
sensors impedance is 10MΩ (R
S
), the signal at the op amp input will be
V
IN
= 1V - 10nA * 10MΩ = 1V - 0.1V = 0.9V (22)
For the CMOS input of the LMV841/LMV842/LMV844, which has an input bias current of only 0.3pA, this would
give
V
IN
= 1V – 0.3pA * 10MΩ = 1V - 3μV = 0.999997V (23)
The conclusion is that a standard op amp, with its high input bias current input, is not a good choice for use in
impedance sensor applications. The LMV841/LMV842/LMV844, in contrast, are much more suitable due to the
low input bias current. The error is negligibly small; therefore, the LMV841/LMV842/LMV844 are a must for use
with high impedance sensors.
Figure 43. High Impedance Sensor Interface
THERMOCOUPLE AMPLIFIER
The following is a typical example for a thermocouple amplifier application using an LMV841, LMV842, or
LMV844. A thermocouple senses a temperature and converts it into a voltage. This signal is then amplified by
the LMV841, LMV842, or LMV844. An ADC can then convert the amplified signal to a digital signal. For further
processing the digital signal can be processed by a microprocessor, and can be used to display or log the
temperature, or the temperature data can be used in a fabrication process.
Characteristics of a Thermocouple
A thermocouple is a junction of two different metals. These metals produce a small voltage that increases with
temperature.
The thermocouple used in this application is a K-type thermocouple. A K-type thermocouple is a junction
between Nickel-Chromium and Nickel-Aluminum. This is one of the most commonly used thermocouples. There
are several reasons for using the K-type thermocouple. These include temperature range, the linearity, the
sensitivity, and the cost.
A K-type thermocouple has a wide temperature range. The range of this thermocouple is from approximately
−200°C to approximately 1200°C, as can be seen in Figure 44. This covers the generally used temperature
ranges.
Over the main part of the range the behavior is linear. This is important for converting the analog signal to a
digital signal.
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