TS4994FC datasheet(25/35 Pages) STMICROELECTRONICS

25 / 35 page

TS4994FC

Application information

25/35

must introduce a mismatch factor (Rin1 xCin ≠ Rin2 xCin2) that will decrease the PSRR

performance.

●

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

compared with 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.

●

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.

4.5

Calculating the influence of mismatching on PSRR

performance

For calculating PSRR performance, we consider that Cin and Cfeed have no influence. We

use the same kind of resistor (same tolerance) and

ΔR is the tolerance value in %.

The following PSRR equation is valid for frequencies ranging from DC to about 1kHz.

The PSRR equation is (

ΔR in %):

This equation doesn't include the additional performance provided by bypass capacitor

filtering. If a bypass capacitor is added, it acts, together with the internal high output

impedance bias, as a low-pass filter, and the result is a quite important PSRR improvement

with a relatively small bypass capacitor.

The complete PSRR equation (

ΔR in %, Cb in microFarad and F in Hz) is:

Example: With

ΔR = 0.1% and Cb = 0, the minimum PSRR is -60dB. With a 100nF bypass

capacitor, at 100Hz the new PSRR would be -93dB.

This example is a worst case scenario, where each resistor has extreme tolerance. It

illustrates the fact that with only a small bypass capacitor, the TS4994 provides high PSRR

performance.

Note also that this is a theoretical formula. Because the TS4994 has self-generated noise,

you should consider that the highest practical PSRR reachable is about -110dB. It is

therefore unreasonable to target a -120dB PSRR.

)

(

)

10000

(

100

Log

PSRR

2 ⎥

⎦

⎤

⎢

⎣

⎡

−

≤

PSRR

log

≤

100

(1000

)

22.2

–

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()

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