TS4890 ST Microelectronics, Inc., TS4890 Datasheet - Page 29

• Manufacturer Part Number: TS4890
• Description: 1W Mono Audio Amplifier With Standby Active High
• Manufacturer: ST Microelectronics, Inc.

We have finished a design and we have chosen for
the components :
• Rin=Rfeed=22k
• 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 ) 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
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.
Note on how to use the PSRR curves
(page 8)
Note on PSRR measurement
-30
-40
-50
-60
-70
10
Cin=100nF
Cb=1 F
100
Cin=100nF
Cb=100 F
Frequency (Hz)
Vcc = 5 & 2.2V
Rfeed = 22k, Rin = 22k
Rg = 100 , RL = 8
Tamb = 25 C
1000
10000
100000
How do we measure the PSRR ?
Fig. 108 : PSRR measurement schematic
• 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.
Principle of operation
PSRR
(
dB
)
20
Log
10
Rms
Rms
(
Vs
V (
ripple
Vs
TS4890
)
)
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