DocID026923 Rev 1

11/14

AN4587

Inpact of input current spikes on real applications

14

Equation 6

In addition to the pole of Equation 6, we have to consider the low frequency pole of the open

loop transfer function of the operational amplifier as shown in Equation 7.

Equation 7

The bode diagram of this system can be plotted as shown in Figure 20.

Figure 20. Bode diagram of the open loop transfer function

To guaranty the stability of the system, the bode diagram must cross the X-axis with a slope

of -20 dB/decade.

So, considering Figure 20 and to ensure stability, the gain at frequency fp, must be lower

than 1 (see Equation 8).

Equation 8

fp

1

2

π

Rf

Rg

⋅

Rf Rg

+

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

Cg

⋅⋅

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

=

(6)

fop

GBP

Avd

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

=

(7)

GBP

2

π Cg

Rf

Rg

2

⋅

Rf Rg

+

()

2

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

⋅⋅

⋅

1

<

(8)