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s Note on how to use the PSRR curves
(page 7)
We have finished a design and we have chosen
the components values :
Rin=Rfeed=22k
Cin=100nF
Cb=1F
Now, on fig. 13, 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. 12 we can see the effect of Cb on the
PSRR (input grounded) vs. frequency. With
Cb=100F, we can reach the -70dB value.
The process to obtain the final curve (Cb=100F,
Cin=100nF, Rin=Rfeed=22k
) is a simple transfer
point by point on each frequency of the curve on
fig. 13 to the curve on fig. 12. The measurement
result is shown on the next figure.
Fig. 85 : 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.
How do we measure the PSRR ?
Fig. 86 : PSRR measurement schematic
s Principle of operation
We fixed the DC voltage supply (Vcc), the AC
sinusoidal ripple voltage (Vripple) and no supply
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.
sHigh/low cut-off frequencies
For their calculation, please check this "Frequency
Response Gain vs Cin, & Cfeed" graph:
10
100
1000
10000
100000
-70
-60
-50
-40
-30
Cin=100nF
Cb=100
F
Cin=100nF
Cb=1
F
Vcc = 5, 3.3 & 2.6V
Rfeed = 22k, Rin = 22k
Rg = 100
, RL = 8
Tamb = 25
°C
P
S
RR
(
d
B)
Frequency (Hz)
Vripple
Vcc
Rin
Cin
Rg
100 Ohms
Cb
Rfeed
4
3
2
1
5
8
Vin-
Vin+
-
+
-
+
Bypass
Standby
Bias
6
Vout1
Vout2
Av=-1
TS4871
Vs-
Vs+
RL
Vcc
GND
7
PSRR d B
() = 20 x Log
10
Rm s Vrippl e
()
Rm s Vs + - Vs-
()
---------------------------------------------
10
100
1000
10000
-25
-20
-15
-10
-5
0
5
10
Rin = Rfeed = 22k
Tamb = 25
°C
Cfeed = 2.2nF
Cfeed = 680pF
Cfeed = 330pF
Cin = 470nF
Cin = 82nF
Cin = 22nF
G
a
in
(dB
)
Frequency (Hz)