Application Information

TS4994

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4.5 Calculating the influence of mismatching

On PSRR performance:

We use the same kind of resistor (same tolerance) and

∆R is the tolerance value in %.

The following equation is valid for frequencies ranging from DC to about 1kHz. Above this frequency,

parasitic effects start to be significant and a literal equation is not possible to write.

The PSRR equation is (

∆R 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 (

Example: With

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

This example is a worst case scenario, where each resistor has extreme tolerance and illustrates the fact

that with only a small bypass capacitor, the TS4994 produce high PSRR performance.

In addition, it’s important to note that this is a theoretical formula. As 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.

condition (

∆R=0.1%).

Figure 70. PSRR vs. frequency worst case

condition

Figure 71. PSRR vs. frequency worst case

condition

)

dB

(

)

R

10000

(

100

R

Log

20

PSRR

⎦

⎤

⎢

⎣

⎡

∆

−

×

∆

×

≤

PSRR

20

∆R 100

×

10000

∆R

2

–

()

1F

2

+

C

b

2

22.2

×

×

×

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

log

×

(dB)

≤

100

1000

10000

-140

-130

-120

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

Cb=0.1

µF

Cb=1

µF

Cb=0

20k

20

Cb=0.47

µF

Vcc = 5V, Vripple = 200mVpp

Av = 1, Cin = 4.7

µF

∆R/R = 0.1%, RL ≥ 8Ω

Tamb = 25

°C, Inputs = Grounded

Frequency (Hz)

100

1000

10000

-140

-130

-120

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

Cb=0.1

µF

Cb=1

µF

Cb=0

20k

20

Cb=0.47

µF

Vcc = 3.3V, Vripple = 200mVpp

Av = 1, Cin = 4.7

µF

∆R/R = 0.1%, RL ≥ 8Ω

Tamb = 25

°C, Inputs = Grounded

Frequency (Hz)