Certain gases absorb IR radiation at a specific wavelength. Each channel has a specific wavelength filter. The

active channel has a filter centered on gas absorption while the reference channel has a filter on another

wavelength which is still in the IR range.

When a gas enters the optical cavity, the radiation hitting the active channel decreases, whereas it remains the

same on the reference channel.

The difference between the reference and active channel gives the concentration of gas present in the optical

cavity.

As the thermopile delivers extremely low voltages (hundreds of µV to several mV) the output signal must be

amplified with a high gain and a very low offset in order to minimize DC errors.

Moreover, the drift of Vio depending on temperature must be as low as possible not to impact the measurement

once the calibration has been made.

An NDIR sensor generally works at low frequency and the noise of the amplifiers must be as low as possible

of 0.1 µV/°C, no 1/f noise in low frequency, and a white noise of 37 nV/√Hz.

Figure 62. Principle schematic shows an NDIR gas sensing schematic where the active and reference channels

are pre-amplified before treatment by an ADC thanks to the TSZ18x.

A Vref voltage (in hundreds of mV) can be used to ensure the amplifiers are not saturated when the signal is close

to the low rail. A gain of 1000 is used to allow amplification of the signal coming from the NDIR sensor (3 mV).

VL

Ref.ch

Act. ch

Vref

10 Ω

10 nF

10 nF

ADC2

ADC1

ADC3

10 Ω

100 nF

100 nF

10 kΩ

10 kΩ

Vcc

+

-

TSZ182_a

TSZ182_b

+

-

5.14.2

Precision instrumentation amplifier

The instrumentation amplifier uses three op amps. The circuit, shown in Figure 63. Precision instrumentation

amplifier schematic, exhibits high input impedance, so that the source impedance of the connected sensor has no

impact on the amplification.

TSZ181, TSZ182

Application examples

DS11863 - Rev 5

page 25/43