MT8960/61/62/63/64/65/66/67
Data Sheet
injects this voltage level into the non-inverting input of the comparator. An integrating capacitor (of value between
0.1 and 1
µF)
must be externally connected from this point (ANUL) to the Analog Ground (GNDA).
The absolute gain of the transmit filter (nominally 0 dB at 1 kHz) can be adjusted from 0 dB to 7 dB in 1 dB steps by
means of three binary controlled gain pads.
The resulting bandpass characteristics with the limits shown in Figure 10 meet the CCITT and AT&T recommended
specifications. Typical attenuations are 30 dB for 0-60 Hz and 35 dB for 4.6 kHz and above.
The filter output signal is an 8 kHz staircase waveform which is fed into the codec capacitor array, or alternatively,
into an external capacitive load of 250 pF when the chip is in the test mode. The digital encoder generates an eight-
bit digital word representation of the 8 kHz sampled analog signal. The first bit of serial data stream is bit 7 (MSB)
and represents the sign of the analog signal. Bits 4-6 represent the chord which contains the analog sample value.
Bits 0-3 represent the step value of the analog sample within the selected chord. The MT8960-63 provide a sign
plus magnitude PCM output code format. The MT8964/66 PCM output code conforms to the AT &T D3
specification, i.e., true sign bit and inverted magnitude bits. The MT8965/67 PCM output code conforms to the
CCITT specifications with alternate digit inversion (even bits inverted). See Figs. 3 and 4 for the digital output code
corresponding to the analog voltage, V
IN
, at V
X
input.
The eight-bit digital word is output at DSTo at a nominal rate of 2.048 MHz, via the output buffer as the first 8-bits of
the 125
µs
sampling frame.
Receive Path
An eight-bit PCM encoded digital word is received on DSTi input once during the 125
µs
period and is loaded into
the input register. A charge proportional to the received PCM word appears on the capacitor array and an 8 kHz
sample and hold circuit integrates this charge and holds it for the rest of the sampling period.
The receive (D/A) filter provides interpolation filtering on the 8 kHz sample and hold signal from the codec. The filter
consists of a 3.4 kHz lowpass fifth-order elliptic section clocked at 128 kHz and performs bandlimiting and
smoothing of the 8 kHz "staircase" waveform. In addition, sinx/x gain correction is applied to the signal to
compensate for the attenuation of higher frequencies caused by the capacitive sample and hold circuit. The
absolute gain of the receive filter can be adjusted from 0 dB to -7 dB in 1 dB steps by means of three binary
controlled gain pads. The resulting lowpass characteristics, with the limits shown in Figure 11, meet the CCITT and
AT & T recommended specifications.
Typical attenuation at 4.6 kHz and above is 30 dB. The filter is followed by a buffer amplifier which will drive 5V
peak/peak into a 10k ohm load, suitable for driving electronic 2-4 wire circuits.
V
Ref
An external voltage must be supplied to the V
Ref
pin which provides the reference voltage for the digital encoding
and decoding of the analog signal. For V
Ref
= 2.5 V, the digital encode decision value for overload (maximum
analog signal detect level) is equal to an analog input V
IN
= 2.415 V (µ-Law version) or 2.5 V (A-Law version) and is
equivalent to a signal level of 3.17 dBm0 or 3.14 dBm0 respectively, at the codec.
The analog output voltage from the decoder at V
R
is defined as:
µ-Law:
V
Ref
X
[(
-0.5
128
) ( )(
+
2
C
128
16.5 + S
33
)]
±
V
OFFSET
A-Law:
V
Ref
X
[( )(
2
C+1
128
0.5 + S
32
)]
±
V
OFFSET
C=0
5
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