AN3222

Frequency behavior

Doc ID 17551 Rev 1

13/18

6

Frequency behavior

This chapter provides different AC cases permitting the filtering of the measurements.

To sense in a large bandwidth, the gain of the application must not exceed the capability of

the amplifier. If the gain is too big, you will be limited by the gain-bandwidth product or by the

amplifier’s slew rate. As such, for test purposes, we have selected the same conditions as in

Case 4 (Figure 9).

Rshunt = 100 m

Ω Rg = 100 Ω and Rf = 4.75 kΩ

The two first cases deal with the filtering of the measurement for a current source which

should be constant. The easiest way to achieve it is to add the capacitor named Cf in

Figure 2.

In Figure 11 and Figure 12, you can see that the period of oscillations is T = 300 µs, so the

frequency equals 3.33 kHz.

The following equation demonstrates how to select the value of Cf to filter the oscillations.

Equation 8

To efficiently cut this frequency, you have to cut one decade earlier, so you have to set Cf to

100 nF.

On the left part of Figure 11, the measurement has been performed without the capacitor,

and on the right part a capacitor of 100nF has been added. A current of about 1 A is applied.

You can see that the signal is correctly filtered by the capacitor.

Figure 12 shows another example with a lower Isense.

F

1

2

π Rf Cf

⋅⋅

---------------------------------- so Cf

1

2

π FRf

⋅⋅

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

1

2

π 3330 4750

⋅⋅

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

10 nF

==

=

=

Figure 11.

Filtering: first example

without capacitor

with capacitor: Cf = 100 nf

Vout and Isense vs Time

0

1

2

3

4

5

6

0.0E+00

2.0E-04

4.0E-04

6.0E-04

8.0E-04

Time (s)

0

0.3

0.6

0.9

1.2

1.5

1.8

Vout_th

Isense

Vout and Isense vs Time

0

1

2

3

4

5

6

0.0E +00

2.0E -04

4.0E-04

6.0E-04

8.0E-04

Time (s)

0

0.3

0.6

0.9

1.2

1.5

1.8

Vout

Isense