TSZ182 datasheet(29/39 Pages) STMICROELECTRONICS

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TSZ182

Application information

DocID029836 Rev 1

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5.13

Open loop gain close to the rail

One of the key parameters of current measurement in low-side applications is precision.

Moreover, it is generally interesting to be able to make a measurement when there is no

current through the shunt resistance. But, when the output voltage gets close the rail some

internal transistors saturate resulting in a loss of open loop gain. Therefore, the output

voltage can be as high as several mV while it is expected to be close to 0 V.

The TSZ182 has been designed to keep a high gain even when the op amp output is very

close to the rail, to ensure good accuracy at low current.

Figure 61: "Gain vs. output voltage, VCC = 5 V, RI = 10 kΩ to GND" shows the open loop

gain of the TSZ182 vs. output voltage. A single power supply of 5 V and a common-mode

voltage of 0 V is used, with a 10 kΩ resistor connected to GND.

Figure 61: Gain vs. output voltage, VCC

= 5 V, RI = 10 kΩ to GND

5.14

Application examples

5.14.1

Measuring gas concentration using the NDIR principle (thermopile)

A thermopile is a serial interconnected array of thermocouples. Based on the Seebeck

principle, a thermocouple is able to deliver an output voltage which depends on the

temperature difference between a reference junction and an active junction.

An NDIR sensor (non dispersive infrared) is generally composed of an infrared (IR) source,

an optical cavity, a dual channel detector, and an internal thermistor. Both channels are

made with a thermopile. One channel is considered as a reference and the other is

considered as the active channel.

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.

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