TS4890IST datasheet(29/32 Pages) STMICROELECTRONICS

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TS4890

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s Note on how to use the PSRR curves

(page 8)

We have finished a design and we have chosen for

the components :

• Rin=Rfeed=22k

Ω

• Cin=100nF

• Cb=1µF

Now, on fig. 16, we can see the PSRR (input

grounded) vs frequency curves. At 217Hz, we

have a PSRR value of -36dB.

In reality we want a value about -70dB. So, we

need a gain of 34dB !

Now, on fig. 15 we can see the effect of Cb on the

PSRR (input grounded) vs. frequency. With

Cb=100µF, we can reach the -70dB value.

The process to obtain the final curve (Cb=100µF,

Cin=100nF, Rin=Rfeed=22k

Ω) is a simple transfer

point by point on each frequency of the curve on

fig. 16 to the curve on fig. 15.

The measurement result is shown on the next

figure.

Fig. 107 : PSRR changes with Cb

s Note on PSRR measurement

What is the PSRR ?

The PSRR is the Power Supply Rejection Ratio.

It's a kind of SVR in a determined frequency range.

The PSRR of a device, is the ratio between a

power supply disturbance and the result on the

output. We can say that the PSRR is the ability of

a device to minimize the impact of power supply

disturbances to the output.

How do we measure the PSRR ?

Fig. 108 : PSRR measurement schematic

s Principle of operation

• We fixed the DC voltage supply (Vcc)

• We fixed the AC sinusoidal ripple voltage

(Vripple)

• No bypass capacitor Cs is used

The PSRR value for each frequency is :

Remark : The measure of the Rms voltage is not a

Rms selective measure but a full range (2 Hz to

125 kHz) Rms measure. It means that we

measure the effective Rms signal + the noise.

100

1000

10000

100000

-70

-60

-50

-40

-30

Cin=100nF

Cb=100

µF

Cin=100nF

Cb=1

µF

Vcc = 5 & 2.2V

Rfeed = 22k, Rin = 22k

Rg = 100

Ω, RL = 8Ω

Tamb = 25

°C

Frequency (Hz)

Vripple

Vcc

Rin

Cin

100 Ohms

Rfeed

Vin-

Vin+

Bypass

Standby

Bias

Vout1

Vout2

Av=-1

TS4890

Vs-

Vs+













−

)

(

Rms

)

(

Rms

Log

)

(

PSRR

ripple

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