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RC6601
PRODUCT SPECIFICATION
2
Functional Description
Digitizing video signals requires high-order anti-aliasing fil-
ters that can handle large signal swings with low distortion.
CCIR601 standards recommend equi-ripple gain and group
delay characteristics for filtering NTSC and PAL signals.
RC6601 is a single-chip solution that matches the require-
ments with less than
±
0.25dB gain ripple,
variation in the passband, and more than 40dB attenuation in
the stop band. The block diagram in Figure 1a shows the
direct synthesis of the filter transfer function as a fifth order
elliptic with third order phase equalization. The cut-off fre-
quency, nominally set at 5.5MHz, is continuously program-
mable over a decade. Using current mode techniques, the IC
can drive 2Vpp signals into 75
quiescent current.
±
20ns group delay
W
load drawing only 35mA
The architecture of the complete filter as illustrated by
Figure 1a is a 5th-order elliptic transfer function in tandem
with a 3rd-order all-pass phase equalizer. The Cauer-elliptic
response function has an equi-ripple passband with a sharp
roll-off into stop-band in the magnitude transfer function but
causes excessive group delay peaking. The equalizer main-
tains this magnitude response while compensating for the
group delay peaking. These two filters are represented in
Figure 1a by a series of 2nd-order expressions that can be
realized as biquads using transimpedance-based integrators.
Elliptic poles and zeros give a flat magnitude response in
passband and a 40dB roll-off from 5.5MHz to 8MHz. The
equalizer transfer function corrects group delay to
90% of the cut-off frequency. These pole-zero values deter-
mine biquad coefficients as shown in Figure 1b. A supply-
independent band-gap cell generates and distributes bias cur-
rents for all the transimpedance integrators as in Figure 1b.
The cut-off frequency is programmed by globally scaling the
currents, using a single external setting.
±
15ns to
The entire filter, including the programmable bias genera-
tors, is integrated on a single chip using complementary
bipolar technology. The npn and pnp transistors have a cut-
off frequency of 4GHz and of 1.5GHz respectively. Gate-
oxide-based capacitors and thin film resistors with 0.5%
match set filter time constants. At 5.5MHz cut-off, the filter
averages 2.5mA/pole. Nearly 15mA of the supply current is
used for the output driver. The cut-off frequency is actually
programmable beyond the 1–10 MHz, with an external volt-
age control. Measured differential gain of 0.25% and a dif-
ferential phase of 0.2
°
make it well suited for video
applications.
Figure 1a. Composite Video Filter
5th Order Elliptic Function
3rd Order Bessel Equalizer
Magnitude Selectivity
Gain
dB
Equalizer
1MHz
5.5MHz
WZ1
WZ2
Elliptic Transfer
65-6601-02
Group Delay
ns
Equalizer
Elliptic
Phase Equalization
K
1
(s
2
+
w
Z12
)
s
2
+
(
w
1
/Q
1
)
s
+
w
12
K
2
(s
2
+
w
Z22
)
s
2
+
(
w
2
/Q
2
)
s
+
w
22
K
w
R
s
+
w
R
s
–
w
B
s
+
w
B
Y
IN
Y
OUT
s
2
– (
w
A
/Q
A
)
s
+
w
A2
s
2
+ (
w
A
/Q
A
)
s
+
w
A2