Inpact of input current spikes on real applications

AN4587

10/14

DocID026923 Rev 1

Figure 17. Common mode capacitor, Cg

At high frequency (current spikes) the common mode capacitor, Cg, shows lower

impedance than that of the sensor. In this case, Cg draws the main part of the current as a

consequence of filtering the input current spikes.

Figure 18 and 19 are snapshot probes measured on the TSZ121. They show that the Iib

spikes have been filtered by adding a common mode capacitor of 10 pF.

Adding a common mode capacitor, helps significantly to filter the Iib spikes generated by the

chopper input stage. Obviously, the higher the capacitor, the better the filtering. But we

cannot increase the capacitance value too much without risking system instability.

Below, we calculate the maximum value of the Cg capacitor to ensure good stability.

The loop gain is given by Equation 5.

Equation 5

Where A is the open loop transfer function of the op amp.

One pole appears as shown in Equation 6.

Figure 18. TSZ121 input bias current spike,

Vcc = 5.5 V

Figure 19. TSZ121 Iib filtered by a common

mode capacitor, Cg = 10 pF, Vcc = 5.5 V

-A

Rg

Rg Rf

+

--------------------

1

1jw

Rg

Rf

⋅

Rg Rf

+

---------------------

Cg

⋅

⎝⎠

⎛⎞

+

----------------------------------------------------------

×

(5)