SC-15, NO. 6, DECEMBER
WILLIAM C. BLACK, JR.,
DAVID J. ALLSTOT, MEMBER,IEEE, AND RAY A. REED,
and receive filters on a single die.
This chip displays
noise of typically
O dBrnCO, a
ratio of 40-50
dB at 1 kHz, and a fully operational
mW, making it very cost effective
this chip, including
filter, and amplifier considerations
is described, and
results are presented.
become quite common in telephone switching and trans-
mission systems, and are ahnost universally employed in new
In these techniques, the analog subscriber lines are
digitrd switching network
by way of a
subscriber-line unit, which performs all necessary interface and
interface circuit, three precision fiiters (line-frequency
reject, anti-aliasing, and output smoothing fiiters), and a Codec
circuit, which performs channel encoding and decoding at an
8 kHz rate.
As circuitry to perform these functions must exist for each
subscriber line, a significant portion of the total switching sys-
tem cost rests in each component of the line unit.
effort in the area of monolithic A/D and D/A conversion tech-
niques have succeeded in reducing the Codec to a fairly cost-
effective integrated form
 - , although attempts to inte-
grate the fiiter and line-interface
functions have, in general,
proven less successful.
niques have offered the potential of very low-cost fiiters .
Many of these techniques have been employed in monolithic
channel bank falters, which have often met the required fre-
quency response characteristics
tions to date, however, have suffered from high-noise output,
which have severely degraded the cost-effectiveness of these
falters in switching systems.
In the device presented here, a number of new circuit tech-
niques are used to provide improved performance
Manuscript received April 28, 1980; revised August 15, 1980.
W. C. Black, Jr. is with
versity of California, Berkeley, CA 94720.
D. J. Ellstot
was with the Electronics
sity of California, Berkeley, CA 94720.
He is now with MOSTEK, Inc.,
R. A. Reed is with NationaJ Semiconductor