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 TSZ18x 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 TSZ18x 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.

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.

TSZ181, TSZ182

Open loop gain close to the rail

DS11863 - Rev 5

page 24/43