75T2090 Datasheet, PDF (2/9 Pages) TDK Electronics – DTMF Transceivers

75T2089/2090/2091
DTMF Transceivers
DESCRIPTION (continued)
The 75T2091 also incorporates an early detect
function which is useful in multi-channel radio
scanning applications. The only external
components necessary for the 75T2089/2090/2091
are a 3.58 MHz “colorburst” crystal with a parallel 1
MΩ resistor. This provides the time base for digital
functions and switched-capacitor filters in the device.
No external filtering is required.
CIRCUIT OPERATION
RECEIVER
The DTMF Receiver in the 75T2089/2090/2091 detects
the presence of a valid tone pair (indicating a single
dialed digit) on a telephone line or other transmission
medium. The analog input is pre-processed by 60 Hz
reject and band-splitting filters, then hard-limited to
provide Automatic Gain Control. Eight bandpass filters
detect the individual tones. The digital post-processor
times the tone durations and provides the correctly
coded digital outputs. The outputs will drive standard
CMOS circuitry, and are three-state enabled to facilitate
bus-oriented architectures.
DIN
This pin accepts the analog input. It is internally
biased so that the input signal may be AC coupled.
The input may be DC coupled as long as it does not
exceed the positive supply. Proper input coupling is
illustrated in Figure 1.
Vp
Vp
VIN<VP
VIN>VP
8
DIN
10 pF
>100 k Ω
0.01 µF
8
DIN
10 pF
>100 k Ω
GND
GND
FIGURE 1: Input Coupling
The IC is designed to accept sinusoidal input
waveforms but will operate satisfactorily with any
input that has the correct fundamental frequency
with harmonics greater than -20 dB below the
fundamental.
CRYSTAL OSCILLATOR
The IC contains an onboard inverter with sufficient
gain to provide oscillation when connected to a low-
cost television “colorburst” crystal. The crystal is
placed between XIN and XOUT in parallel with a
1Mz resistor, while XEN is tied high. Since the
switched-capacitor-filter time base is derived from
the crystal oscillator, the frequency accuracy of all
portions of the IC depends on the time base
tolerance. The DTMF Receiver frequency response
and timing is specified for a time base accuracy of at
least =0.005%. ATB is a clock output with the
frequency of 1/8 of crystal. Other devices may use
the same frequency reference by tying their ATB
pins to the ATB of a crystal connected device. XIN
and XEN of the auxiliary devices must then be tied
high and low respectively, XOUT is left floating.
XOUT is designed to drive a resonant circuit only
and is not intended to drive additional devices. Ten
devices may run off a single crystal-connected
transceiver as shown in Figure 2.
XIN
ATB
VP
XOUT
75T20XX
XEN
ATB
XIN Connected to VP
75T20XX
XEN
Up to 10 Devices
FIGURE 2: Crystal Connections
2