TS616
ACTIVE FILTERING
Figure 67: Low-Pass Active Filtering. Sallen-Key
R1
IN
C1
R2
+
C2
TS616
_
Rfb
RG
910Ω
OUT
25Ω
The resistors Rfb and RG give the gain of the filter
as a classical non-inverting amplification configu-
ration :
AV = g = 1 + R--R---f-g-b--
Assume the following expression is the response
of the system:
Tjω = -V--V--o--i--un---j-t-ωj--ω--- = -1----+-----2----ζ----ω-j----ω--g--c-----+------(---ω--j----ω---c-----)--2---2----
the cut-off frequency is not gain-dependent and it
becomes:
ωc
=
------------------1-------------------
R1R2C1C2
The damping factor becomes:
ζ = 12-- ωc(C1R1 + C1R2 + C2R1 – C1R1g)
The higher the gain, the more sensitive the damp-
ing factor is. When the gain is higher than 1 it is
preferable to use very stable resistors and capac-
itances values.
In the case of R1=R2:
ζ = 2----C-----2-----–----C-----1----R--R-------f--g--b----
2 C1C2
INCREASING THE LINE LEVEL BY USING AN
ACTIVE IMPEDANCE MATCHING
With passive matching, the output signal ampli-
tude of the driver must be twice the amplitude on
the load. To go beyond this limitation, active
matching impedance can be used. With this tech-
nique, it is possible to keep good impedance
matching with an amplitude on the load higher
than half of the output driver amplitude. This con-
cept is shown in Figure 68 for a differential line.
Figure 68: TS616 as a differential line driver with
an active impedance matching
100n
+
Vcc+
Vcc+
1k
Vi
_
GND
R2
R3
1/2 R1
Vcc/2
1/2 R1
R5
Vi
10µ
100n
1k GND
R4
+ Vcc+
100n
_
GND
Rs1
Vo°
Vo°
Rs2
1µ
10n
Vo
RL
1:n
Hybrid
& 100Ω
Transformer
Vo
24/27