TS4962 datasheet(31/44 Pages) STMICROELECTRONICS

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TS4962

Application information

4.3

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 a Vicm limitation in the input stage (see Table 3: Operating conditions on

page 4

), the common-mode feedback loop can play 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 150 k

Ω resistor, it is also important to check V

icm in these

conditions.

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

must be used. With VCC between 2.4 and 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, which is lower than

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

Ω we find 1.97 V and with 163.5 kΩ we have 2.02 V.

Therefore, no input coupling capacitors are required.

4.4

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 -3 dB cut-off frequency.

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

with Rin in Ω and FCL in Hz.

icm

150k

Ω

150k

Ω

(

)

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

(V)

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

(V)

0.5V

icm

0.8V

–

≤

136.5k

Ω

136.5k

Ω

(

)

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

icm

163.5k

Ω

163.5k

Ω

(

)

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

≤

π R

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

(Hz)

π R

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

(F)

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