TLC32044C Datasheet, PDF (28/39 Pages) Texas Instruments – VOICE-BAND ANALOG INTERFACE CIRCUITS

TLC32044C, TLC32044E, TLC32044I, TLC32044M, TLC32045C, TLC32045I
VOICE-BAND ANALOG INTERFACE CIRCUITS
SLAS017F – MARCH 1988 – REVISED MAY 1995
Table 3. Gain Control Table (Analog Input Signal Required for Full-Scale A/D Conversion)
INPUT CONFIGURATIONS
CONTROL REGISTER
BITS
d6
d7
ANALOG INPUT‡
A/D
CONVERSION
RESULT
1
1
Differential configuration
0
0
Analog input = IN + – IN –
= AUX IN + – AUX IN –
1
0
0
1
±6 V
±3 V
± 1.5 V
Full - scale
Full - scale
Full - scale
1
1
Single-ended configuration
0
0
Analog input = IN + – ANLG GND
= AUX IN + – ANLG GND
1
0
±3 V
±3 V
Half - scale
Full - s ale
0
1
± 1.5 V
Full - scale
‡ _{In this example, Vref is assumed to be 3 V. In order to minimize distortion, it is recommended that the analog}
input not exceed 0.1 dB below full scale.
Rfb
Rfb
R
IN +
IN –
R
+
–
–
+
To Multiplexer
R
AUX IN +
AUX IN –
R
+
–
–
+
To Multiplexer
Rfb
Rfb = R for d6 = 1, d7 = 1
d6 = 0, d7 = 0
Rfb = 2R for d6 = 1, d7 = 0
Rfb = 4R for d6 = 0, d7 = 1
Figure 7. IN + and IN – Gain Control Circuitry
Rfb
Rfb = R for d6 = 1, d7 = 1
d6 = 0, d7 = 0
Rfb = 2R for d6 = 1, d7 = 0
Rfb = 4R for d6 = 0, d7 = 1
Figure 8. AUX IN + and AUX IN –
Gain Control Circuitry
(sin x)/x correction
The AIC does not have (sin x)/x correction circuitry after the digital-to-analog converter. (Sin x)/x correction can
be accomplished easily and efficiently in digital signal processor (DSP) software. Excellent correction accuracy
can be achieved to a band edge of 3000 Hz by using a first-order digital correction filter. The results, which are
shown in Table 4, are typical of the numerical correction accuracy that can be achieved for sample rates of
interest. The filter requires only seven instruction cycles per sample on the TMS(SMJ)320 DSPs. With a 200-ns
instruction cycle, nine instructions per sample represents an overhead factor of 1.4% and 1.7% for sampling
rates of 8000 Hz and 9600 Hz, respectively. This correction adds a slight amount of group delay at the upper
edge of the 300 – 3000-Hz band.
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