A21SP16 datasheet(28/37 Pages) STMICROELECTRONICS

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Application information

A21SP16

28/37

DocID026037 Rev 1

5.3

Common mode feedback loop limitations

As explained previously, the common mode feedback loop allows the output DC bias

voltage to be averaged at VCC/2 for any DC common mode bias input voltage.

However, due to Vicm limitation in the input stage (see Table 3: Operating conditions on

page 3), the common mode feedback loop can ensure its role only within a defined range.

This range depends upon the values of VCC and Rin (AVdiff). To have a good estimation of

the Vicm value, we can apply this formula (no tolerance on Rin):

with

and the result of the calculation must be in the range:

Due to the +/-9% tolerance on the 150k

 resistor, it’s also important to check V

icm in these

conditions:

If the result of Vicm calculation is not in the previous range, input coupling capacitors must

be used (with VCC from 2.4 V to 2.5 V, input coupling capacitors are mandatory).

For example:

With VCC =3 V, Rin = 150 k and VIC = 2.5 V, we typically find Vicm = 2 V and this is lower

than 3V - 0.8 V = 2.2 V. With 136.5 k

 we find 1.97 V, and with 163.5 k we have 2.02 V.

So, no input coupling capacitors are required.

5.4

Low frequency response

If a low frequency bandwidth limitation is requested, it is possible to use input coupling

capacitors.

In the low frequency region, Cin (input coupling capacitor) starts to have an effect. Cin forms,

with Rin, a first order high-pass filter with a -3dB cut-off frequency:

So, for a desired cut-off frequency we can calculate Cin,

with Rin in  and FCL in Hz.

icm



150k





150k







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

(V)

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

(V)

0.5V

icm

0.8V

–





136.5k





136.5k







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

icm



163.5k





163.5k







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



 R



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

(Hz)

 R



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

(F)

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