TS4994 datasheet(20/31 Pages) STMICROELECTRONICS

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TS4994

Application Information

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

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

Example: With Vcc=2.5V, Rin=Rfeed=20k and VIC=2V, we found VICM=1.63V. This is higher than 2.5V-

0.9V=1.6V, so input coupling capacitors are required or you will have to change the VIC value.

4.4 Low and high frequency response

In the low frequency region, Cin starts to have an effect. Cin forms, with Rin, a high-pass filter with a -3dB

cut-off frequency. FCL is in Hz.

In the high-frequency region, you can limit the bandwidth by adding a capacitor (Cfeed) in parallel with

Rfeed. It forms a low-pass filter with a -3dB cut-off frequency. FCH is in Hz.

While these bandwidth limitations are in theory attractive, in practice, because of low performance in

terms of capacitor precision (and by consequence in terms of mismatching), they deteriorate the values of

PSRR and CMRR.

We will discuss the influence of mismatching on PSRR and CMRR performance in more detail in the

following paragraphs.

Example: A typical application with input coupling and feedback capacitor with FCL=50Hz and

FCH=8kHz. We assume that the mismatching between Rin1,2 and Cfeed1,2 can be neglected. If we sweep

the frequency from DC to 20kHz we observe the following with respect to the PSRR value:

l From DC to 200Hz, the Cin impedance decreases from infinite to a finite value and the Cfeed

impedance is high enough to be neglected. Due to the tolerance of Cin1,2, we must introduce a

mismatch factor (Rin1 xCin ≠ Rin2 xCin2) that will decrease the PSRR performance.

l From 200Hz to 5kHz, the Cin impedance is low enough to be neglected when compare to Rin, and

the Cfeed impedance is high enough to be neglected as well. In this range, we can reach the PSRR

performance of the TS4994 itself.

l From 5kHz to 20kHz, the Cin impedance is low to be neglected when compared to Rin, and the Cfeed

impedance decreases to a finite value. Due to tolerance of Cfeed1,2, we introduce a mismatching

factor (Rfeed1 xCfeed1 ≠ Rfeed2 xCfeed2) that will decrease the PSRR performance.

Vcc

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feed

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