MT8870 datasheet(5/12 Pages) MITEL

Part #	MT8870

Description	ISO2-CMOS Integrated DTMF Receiver

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Manufacturer	MITEL [Mitel Networks Corporation]

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ISO2-CMOS

MT8870D/MT8870D-1

4-15

Power-down and Inhibit Mode

A logic high applied to pin 6 (PWDN) will power down

the device to minimize the power consumption in a

standby mode. It stops the oscillator and the

functions of the filters.

Inhibit mode is enabled by a logic high input to the

pin 5 (INH). It inhibits the detection of tones

representing characters A, B, C, and D. The output

code will remain the same as the previous detected

code (see Table 1).

Differential Input Configuration

The input arrangement of the MT8870D/MT8870D-1

provides a differential-input operational amplifier as

well as a bias source (VRef) which is used to bias the

inputs at mid-rail. Provision is made for connection of

a feedback resistor to the op-amp output (GS) for

adjustment of gain. In a single-ended configuration,

the input pins are connected as shown in Figure 10

with the op-amp connected for unity gain and VRef

biasing the input at 1/2VDD. Figure 6 shows the

differential

configuration,

which

permits

the

adjustment of gain with the feedback resistor R5.

Crystal Oscillator

The internal clock circuit is completed with the

addition of an external 3.579545 MHz crystal and is

normally connected as shown in Figure 10 (Single-

Ended Input Configuration). However, it is possible

to configure several MT8870D/MT8870D-1 devices

employing only a single oscillator crystal. The

oscillator output of the first device in the chain is

coupled through a 30 pF capacitor to the oscillator

input (OSC1) of the next device. Subsequent devices

are connected in a similar fashion. Refer to Figure 7

for

details.

The

problems

associated

with

unbalanced loading are not a concern with the

arrangement

shown,

i.e.,

precision

balancing

capacitors are not required.

Figure 6 - Differential Input Configuration

Figure 7 - Oscillator Connection

Table 2. Recommended Resonator Specifications

Note: Qm=quality factor of RLC model, i.e., 1/2

ΠƒR1C1.

Parameter

Unit

Resonator

R1

Ohms

10.752

L1

mH

.432

C1

pF

4.984

C0

pF

37.915

Qm

-

896.37

∆f

%

±0.2%

C1

R1

C2

R4

R3

IN+

IN-

+

-

R5

GS

R2

VRef

MT8870D/

MT8870D-1

Differential Input Amplifier

C1=C2=10 nF

R1=R4=R5=100 kΩ

R2=60kΩ, R3=37.5 kΩ

All resistors are

±1% tolerance.

All capacitors are

±5% tolerance.

R3=

R2R5

R2+R5

VOLTAGE GAIN (Av diff)=

R5

R1

INPUT IMPEDANCE

(ZINDIFF) = 2

R1

2+

1

ωc

2

OSC1

OSC2

OSC1

C

X-tal

C

To OSC1 of next

MT8870D/MT8870D-1

C=30 pF

X-tal=3.579545 MHz

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