TSU111

Application information

DocID029790 Rev 2

17/30

5.1.1

Schematic optimization aiming for nanopower

To benefit from the full performance of the TSU111, the impedances must be maximized so

that current consumption is not lost where it is not required.

For example, an aluminum electrolytic capacitance can have significantly high leakage.

This leakage may be greater than the current consumption of the op amp. For this reason,

ceramic type capacitors are preferred.

For the same reason, big resistor values should be used in the feedback loop. However,

there are two main limitations to be considered when choosing a resistor.

1.

Noise generated: a 100 kΩ resistor generates 40 nV/√Hz, a bigger resistor value

generates even more noise.

2.

Leakage on the PCB: leakage can be generated by moisture. This can be improved by

using a specific coating process on the PCB.

5.1.2

PCB layout considerations

For correct operation, it is advised to add 10 nF decoupling capacitors as close as possible

to the power supply pins.

Minimizing the leakage from sensitive high impedance nodes on the inputs of the TSU111

can be performed with a guarding technique. The technique consists of surrounding high

impedance tracks by a low impedance track (the ring). The ring is at the same electrical

potential as the high impedance node.

Therefore, even if some parasitic impedance exists between the tracks, no leakage current

can flow through them as they are at the same potential (see Figure 33: "Guarding on the

PCB").

Figure 33: Guarding on the PCB