TS4962M
Application information
6.3
Common-mode feedback loop limitations
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 5), 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):
Vicm = V-----C---C-----×--2---R--×---i-n(---R-+---i--n2----+-×----1-V--5--I-0-C---k--×--Ω--1--)--5---0----k---Ω---
(V)
with
VIC = I--n----+----2+-----I--n----- (V)
and the result of the calculation must be in the range:
0.5V ≤ Vicm ≤ VCC – 0.8V
Due to the ±9% tolerance on the 150kΩ resistor, it is also important to check Vicm in these
conditions:
-V----C----C----×--2---R--×---i-n(---R-+---i--n2----+-×----1-V--3--I-6-C---.-5-×---k-1--Ω--3---6-)--.--5----k---Ω--- ≤ Vicm ≤ -V----C---C-----×--2---R--×---i-n(---R-+---i--n2----+-×----1-V--6--I-3-C---.-5×----k-1--Ω--6---3-)--.--5----k---Ω---
If the result of the Vicm calculation is not in the previous range, input coupling capacitors
must be used (with VCC from 2.4V to 2.5V, input coupling capacitors are mandatory).
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 kΩ we find 1.97 V, and with 163.5 kΩ we have 2.02 V.
So, no input coupling capacitors are required.
6.4
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:
FCL = 2----π-----×-----R---1-i--n----×-----C----i--n-
(Hz)
So, for a desired cut-off frequency we can calculate Cin,
Cin = -2---π-----×-----R----1i--n----×-----F----C---L-
(F)
with Rin in Ω and FCL in Hz.
DocID11703 Rev 6
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