TS4994 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
TS4994
STMicroelectronics
'TS4994' PDF : 31 Pages View PDF
Application Information
TS4994
4.5 Calculating the influence of mismatching
On PSRR performance:
For this calculation, we consider that Cin and Cfeed have no influence.
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 %):
PSRR
≤
20
×
Log⎢⎣⎡(10∆0R00×−10∆0R2
)
⎤
⎥
⎦
(dB)
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 (∆R in %, Cb in microFarad and F in Hz) is:
PSRR ≤ 20 × log ---------------------------------------∆----R------×-----1---0---0----------------------------------------
(dB)
(10000 – ∆R2) ×
1
+
F2
×
C
2
b
×
22.2
Example: With ∆R=0.1% and Cb=0, the minimum PSRR would be -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 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.
The three following graphs show PSRR versus frequency and versus bypass capacitor Cb in worst-case
condition (∆R=0.1%).
Figure 70. PSRR vs. frequency worst case
condition
0
-10 Vcc = 5V, Vripple = 200mVpp
-20 Av = 1, Cin = 4.7µF
-30 ∆R/R = 0.1%, RL ≥ 8Ω
-40 Tamb = 25°C, Inputs = Grounded
-50
-60
-70
-80
Cb=0.1µF
-90
-100
-110
-120
-130
Cb=1µF Cb=0.47µF
-140
20
100
1000
Frequency (Hz)
Cb=0
10000 20k
Figure 71. PSRR vs. frequency worst case
condition
0
-10 Vcc = 3.3V, Vripple = 200mVpp
-20 Av = 1, Cin = 4.7µF
-30 ∆R/R = 0.1%, RL ≥ 8Ω
-40 Tamb = 25°C, Inputs = Grounded
-50
-60
-70
-80
Cb=0.1µF
-90
-100
-110
-120
-130
Cb=1µF Cb=0.47µF
-140
20
100
1000
Frequency (Hz)
Cb=0
10000 20k
21/31
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