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LMV841, LMV842, LMV844

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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 10M

Ω. 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

(21)

sensors impedance is 10M

(22)

For the CMOS input of the LMV841/LMV842/LMV844, which has an input bias current of only 0.3pA, this would

give

(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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Product Folder Links: LMV841 LMV842 LMV844