MT92303 Zarlink Semiconductor, MT92303 Datasheet - Page 6

• Manufacturer Part Number: MT92303
• Manufacturer: Zarlink Semiconductor

MT92303
6
Loud Speaker Drive
The MT92303 provides four Audio Interfaces, all of
which provide the same gain adjustments and output
drive voltage levels. However, Audio Interface #0 has
an output buffer which is capable of delivering more
current than the others, and is intended for use with
a loud speaker. This output can drive 150mW r.m.s.
into a 16 Ohm load - this represents almost
rail-to-rail operation from the ‘bridge’ configured
outputs.
Other loud speaker impedances may be used (as
long as they are greater than or equal to 16 Ohms)
and will allow a power output (t.h.d. < 1%) of:
•
TX Channel
An optional pre-amp and a variable gain stage
provide a very large range of gain adjustment for the
microphone signal. The boosted signal is passed
through an anti-alias filter before it is digitised by the
ADC and routed to the DSP circuitry. The DSP
band-pass filtering provides a sharp cut-off above
3.4kHz to prevent aliasing by the 8kHz sampling, and
also provides a high-pass function at 150Hz to
remove low-frequency voice energy.
As shown in Figure 4, the pre-amp and variable gain
block combine to allow the TX gain to be adjusted
from 0dB to +46.5dB in 1.5dB steps.
The 8kHz samples are available in either 16-bit linear
form, or 8-bit u-Law or A-Law companded form.
The differential amplifiers may be used with electret
microphones as shown in Figure 8, or with dynamic
microphones which have no bias requirements.
Dynamic microphones may be connected directly to
-1dB
-2dB
-3dB
0dB
Power = 150mW x 16 / Rload
Figure 6 - TX Band-Pass Filter Response
1kHz
2kHz
3kHz
the chip inputs, without the use of coupling
capacitors.
DC offset voltages which potentially exist on the
amplifier inputs are automatically cancelled by a
sequence which runs each time the circuit is
powered-up (see section ‘Automatic DC Offset
Cancellation’).
The ‘Mic’ inputs of the TX channel have a differential
input impedance which is sufficiently high to allow
the use of small value coupling capacitors. The input
impedance is formed by the internal gain-setting
resistors which are used around the op-amps, and
consequently the value of the input impedance is
dependent on the selected gain as shown in Table 1.
‡ These figures are for design aid only: not guaranteed and not
subject to production testing.
Automatic DC Offset Cancellation (TX)
The amplifiers in the TX channel can provide gains
as high as 46.5dB (linear gain of x211). Potential DC
input offset voltages on the amplifier inputs need to
be controlled so that they do not affect the dynamic
range of the channel. These DC offset voltages are
removed by a sequence which automatically runs
each time the TX ‘analog’ circuitry is enabled/
powered-up (see control register ‘Audio Interface:
Enable’).
The ‘offset cancellation’ routine works by using
internal DAC’s to introduce small correction voltages
into the amplifiers so that their outputs have a
nominal DC level of 0V (measured differentially). The
digital portion of this auto calibration circuitry is
clocked by the ‘Frame Alignment’ input and requires
up to 128 clock pulses. It is important that the on-chip
voltage reference is powered-up and allowed to settle
before the ‘offset cancellation’ is initiated (by enabling
the TX ‘analog’ circuitry).
There are two TX ‘analog’ channels, each of which
has its own ‘offset cancellation’ circuitry. These can
operate completely independently of each other if
required.
0dB to 10.5dB
12dB to 22.5dB
24dB to 46.5dB
Table 1. Microphone Input Impedance
Gain
Preliminary Information
Input Impedance
Min
49
29
52
Typ
107
54
80
‡
(kOhms)
Max
213
113
89