TS4998 datasheet(21/33 Pages) STMICROELECTRONICS

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TS4998

Application information

21/33

Due to the tolerance on the internal 50k

Ωfeedback resistors, the differential gain will be in

the range (no tolerance on RIN):

The difference of resistance between input resistors of each channel have direct influence

on the PSRR, CMRR and other amplifier parameters. In order to reach maximum

performance, we recommend matching the input resistors RIN1, RIN2, RIN3, and RIN4 with a

maximum tolerance of 1%.

Note:

For the rest of this section, Avdiff will be called AV to simplify the mathematical expressions.

4.4

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.

Due to the VICM limitation of the input stage (see Table 3 on page 4), the common mode

feedback loop can fulfil its role only within the defined range. This range depends upon the

values of VCC, RIN and Rfeed (AV). To have a good estimation of the VICM value, use the

following formula:

Equation 2

with VCC in volts, RIN in kΩ and

The result of the calculation must be in the range:

Due to the +/-20% tolerance on the 50k

Ω feedback resistors Rfeed (no tolerance on RIN), it is

also important to check that the V

ICM remains in this range at the tolerance limits:

If the result of the VICM calculation is not in this range, an input coupling capacitor must be

used.

Example: VCC =2.7V, AV = 2, and Vic =2.2V.

With internal resistors Rfeed = 50kΩ, calculated external resistors are RIN = Rfeed/AV = 25kΩ,

VCC = 2.7V and Vic = 2.2V, which gives VICM = 1.92V. Taking into account the tolerance on

the feedback resistors, with Rfeed = 40kΩ the common mode input voltage is VICM =1.87V

and with Rfeed = 60kΩ, it is VICM =1.95V.

These values are not in range from GND to VCC - 1V = 1.7V, therefore input coupling

capacitors are required. Alternatively, you can change the Vic value.

40k

-------------- A

diff

60k

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

≤≤

ICM

feed

()

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

50k

()

--------------------------------------------------------------------------- V

()

Diff

input+

Diff

input-

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

(V)

GND V

ICM

–

≤≤

40k

()

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

ICM

≤

60k

()

-------------------------------------------------------------------------- V

()

≤

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