ACE9040 datasheet(27/33 Pages) MITEL

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ACE9040

OP-AMP Reference Current

Reference currents for all the internal op-amps are set by

an external resistor connected from pin RREF to ground (V

SS).

Nominal values are 100 k

Ω for V

DD = 4·85 V and 68 kΩ for

DD = 3·75 V. A stable discrete resistor should be used to

ensure consistent operation over a wide temperature range.

Power Supply Comparator - Reset Output

A power supply comparator is provided to give a reset at

power-on and enable the system controller to initiate a clean

shut-down sequence if the battery voltage falls too low. When

DD is below a band-gap derived threshold the open-drain

outut pin LVN drives to a logic low. This occurs for V

exceeding 1 V but less than a typical threshold of 3·35 V. An

external resistor at LVN provides a pull-up to V

DD with a

capacitor to ground (V

SS) to give a power-on reset delay.

Typical values for RC are 220 k

Ω and 150 nF. This RC

combination also removes short transients or noise pulses

from the signal at LVN during power up. If this comparator is

not required the bandgap and comparator can be powered

down by setting bit PDLVC in the “Initializing mode 0” control

message to a “1”.

Serial Data Clock

All switched capacitor filter switching clocks are derived

from the serial data clock SCLK which must be fixed at

1·008 MHz to ensure correct frequency responses.

AMPLIFIER

An uncommitted op-amp is provided with its non-inverting

input internally connected to VMID, inverting input at pin AMPI

and output at pin AMPO.

pins TXC and RXC respectively, to give d.c. voltages corre-

sponding to the signal levels.

A switch HFS with its output at pin HF can be internally

toggled between TXC and RXC to allow measurement of the

two levels at these pins by an external level sensing circuit

such as an analog to digital converter input of ACE9030. The

HFS switch is controlled by bit HFS in the “Handsfree mode”

control message.

The system controller after comparison of the voltage

levels at TXC and RXC pins can attenuate the weaker signal

path by up to 49 dB, in 7 dB steps using blocks HFGAIN for

transmit and HFATTEN for receive. Bits THF[2:0] and

RHF[2:0] in the “Handsfree mode” control message are used

to set the gains of HFGAIN and HFATTEN respectively. The

rate of change of gain should be limited in the system control-

ler to allow normal conversation.

Attack and decay time constants are set by the resistance

and capacitance on the TXC and RXC pins. With the internal

resistor to ground of approximately 500 k

Ω and an external

capacitor to ground of 68 nF the normal attack time of 1 ms

and decay time of 35 ms is achieved. By adding a parallel

resistor the ratio of attack to decay time can be altered.

To save power in a hand portable when handsfree opera-

tion is not needed, the transmit and receive signal rectifiers

can be switched off by setting bit HFP in the “Handsfree mode”

control message to “0”.

BIASES AND REFERENCES

BIAS, VMID and MICBIAS

Within ACE9040 most signals are single ended and swing

either side of a mid-supply reference voltage. These internal

references are all labelled VMIDxx in this data sheet.

A low impedance voltage source at mid-supply for use as

an external signal ground is available on pin BIAS. This is a

buffered copy of the voltage at pin DEC which is from an

internal high impedance potential divider between V

DD and

SS. The DEC pin should be decoupled to ground with a

capacitor of greater than 3.3

µF. Two additional buffers pro-

vide copies of DEC’s voltage at pins VMIDTX and VMIDRX,

these are used as internal signal grounds for the transmit and

receive paths respectively. VMIDTX and VMIDRX pins should

be decoupled to GND with 82 nF capacitors.

By using

separate mid-supply signal grounds crosstalk due to the

compander time constant circuits and the speech and tone

signals are kept to a minimum.

Pin MICBIAS gives the bias needed for an electret micro-

phone nominally 0·8 times V

DD, e.g. when VDD is 3·75 V

MICBIAS = 3 V

Fig. 16 Bias Circuits

3.3

µF

100k

Ω/

68k

Ω*

*VDD = 3.75V

DEC

BIAS

RREF

MICBIAS

BGAP

10nF

VMIDRX

VMIDTX

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