73M1903C-EVM Maxim Integrated Products, 73M1903C-EVM Datasheet - Page 24

• Manufacturer Part Number: 73M1903C-EVM
• Description: BOARD DEMO 73M1903C WORLDWIDE
• Manufacturer: Maxim Integrated Products

Specifications of 73M1903C-EVM

• Main Purpose: Interface, Analog Front End (AFE)
• Utilized Ic / Part: *
• Primary Attributes: *
• Secondary Attributes: *
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Embedded: -

73M1903C Data Sheet
DS_1903C_033
Figure 9: Frequency Response of TX Path for DC to 4 kHz in Band Signal at 8 kHz Sample Rate
6.4
Transmit Levels
The 16-bit transmit code word written by the DSP to the Digital Sigma-Delta Modulator (DSDM) (via TIF)
has a linear relationship with the analog output signal. So, decreasing a code word by a factor of 0.5 will
result in a 0.5 (-6dB) gain change in the analog output signal.
The following formula describes the relationship between the transmit code word and the output level at
the transmit pins (TXAP/TXAN):
Vout (V) = 2 * code/32,767 * DSDMgain * dacGAIN * VREF * TLPFgain * SMFLTgain * FreqFctr
Vout is the differential peak voltage at the TXAP and TXAN pins.
Code is the 16-bit, two’s compliment transmit code word written out by the DSP to the DSDM (via TIF).
The code word falls within a range of ± 32,767. For a sinusoidal waveform, the peak code word is used in
the formula to obtain the peak output voltage.
DSDMgain is the scaling factor used on the transmit code word to reduce the possibility of saturating the
modulator. This value is set to 0.640625(–3.555821dB) at dc in the 48 tap transmit interpolation filter
(TIF) that precedes DSDM.
dacGAIN is the gain of the DAC. The value dacGAIN is calculated based on capacitor values inside
DAC1 and dacGAIN=8/9=0.8889. The number 32,767 refers to the code word that generates an 82% 1’s
pulse density at the output of the DSDM. As can be seen from the formula, the D to A conversion is
dependent on the level of VREF. Also when DTMFBST bit is set, VREF is increased from 1.36 V to
1.586 V to allow higher transmit level or 16.6% increase in gain. This bit is intended for enhancing the
DTMF transmit level and should not be used in data mode.
TLPFgain is the gain of TLPF and nominally equals to 0.00 dB or 1.0.
SMFLTgain is the gain of SMFLT and nominally equal to 1.445 or 3.2 dB.
When TXBST0 bit is set, the gain is further increased by 1.65 dB (1.21) for the total of 4.85 dB. This is to
accommodate greater hybrid insertion loss encountered in some applications.
FreqFctr shows dependency of the entire transmit path on frequency. See Figure 8.
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Rev. 5.0