Datasheet

TS4962 Application information
Doc ID 10968 Rev 8 31/44
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 V
CC
/2 for any DC common-mode bias input voltage.
However, due to a V
icm
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 V
CC
and R
in
(A
Vdiff
). To have a good estimation of the
V
icm
value, we can apply this formula (no tolerance on R
in
):
with
And the result of the calculation must be in the range:
Due to the +/-9% tolerance on the 150 kΩ resistor, it is also important to check V
icm
in these
conditions.
If the result of the V
icm
calculation is not in the previous range, input coupling capacitors
must be used. With V
CC
between 2.4 and 2.5 V, input coupling capacitors are mandatory.
For example:
With V
CC
=3V, R
in
= 150 k and V
IC
= 2.5 V, we typically find V
icm
= 2 V, which is lower than
3 V-0.8 V = 2.2 V. With 136.5 kΩ 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, C
in
(input coupling capacitor) starts to have an effect. C
in
forms,
with R
in
, a first order high-pass filter with a -3 dB cut-off frequency.
So, for a desired cut-off frequency we can calculate C
in
,
with R
in
in Ω and F
CL
in Hz.
V
icm
V
CC
R
in
× 2V
IC
× 150kΩ×+
2R
in
150kΩ+()×
------------------------------------------------------------------------------
(V)=
V
IC
In
+
In
-
+
2
---------------------
(V)=
0.5V V
icm
V
CC
0.8V–≤≤
V
CC
R
in
× 2V
IC
× 136.5kΩ×+
2R
in
136.5kΩ+()×
-----------------------------------------------------------------------------------
V
icm
V
CC
R
in
× 2V
IC
× 163.5kΩ×+
2R
in
163.5kΩ+()×
-----------------------------------------------------------------------------------
≤≤
F
CL
1
2π R
in
× C
in
×
--------------------------------------
(Hz)=
C
in
1
2π R
in
× F
CL
×
--------------------------------------- -
(F)=