GC4114 Datasheet, PDF (14/45 Pages) Texas Instruments – QUAD TRANSMIT CHIP

GC4114 QUAD TRANSMIT CHIP
DATA SHEET REV 1.0
3.7 OVERALL GAIN
The overall gain of the chip is a function of the sum of the programmable filter coefficients (PFIR_SUM)
described in Section 3.4.1, the amount of interpolation in the CIC filters (N described in Section 3.4.3), the scale
circuit settings in the CIC filter (SCALE and BIG_SHIFT described in Section 3.4.3), and the sum tree scale factor
(SUM_SCALE described in Section 3.6). The overall gain is:
GAIN
=



--^G------
128



P----F---I--R----_---S---U-----M----
65536
^{

N³
2^–(SCALE
+
12
×
BIG_SHIFT
+
3
)
}
{
2 ^{–SUM_SCALE} }
where G can be different for each channel, but N, SCALE, BIG_SHIFT, SUM_SCALE and PFIR_SUM are common
to all channels.
The optimal gain setting is one which will keep the amplitude of the data within the channel as high as
possible without causing overflow. The recommended gain target is to keep the root-mean-squared amplitude of the
data close to one-fifth (0.2) full scale (a 14 dB crest factor). This level should be maintained throughout the channel
computations. This means that the products



--R----M-----S---
32768



--^G------
128



P----F---I--R----_---S---U-----M----
65536



and
 ^{3--R-2---M7---6--S-8--}



--^G------
128



P----F---I--R----_---S---U-----M----
65536
^{

N³
2^–(SCALE
+
12
×
BIG_SHIFT
+
3
)
}
should both be less than or equal to 0.2, where “RMS” is the root-mean-squared level of the input data. Other crest
factors can be used depending upon the application. For example, a crest factor of 12 dB is adequate if the final
number of bits going to a DAC is 12 bits. In most cases the input data will already have the correct crest factor for
the application, in
which
case the ratio



3--R-2---M7---6--S-8--



will be
equal
to the crest
factor
(e.g., 0.2) and the
gain settings
in the
channel should be set to unity.
The sum tree gain (SUM_SCALE) is used when the outputs from multiple channels are summed together.
The recommended gain when “M” uncorrelated channels are being added together is ^----1---- . For example, if all four
M
channels are active in a GC4114, then M is four and the gain should be 1/2, which can be achieved by setting
SUM_SCALE equal to 1. If four chips are cascaded to sum sixteen channels, then M is 16 and the gain should be
1/4, which implies SUM_SCALE should be 2.
The ^----1---- gain rule assumes that the channels can be treated as uncorrelated signals which will result in an
M
average amplitude gain of M . If the signals are correlated, however, the amplitude gain can be M and the sum
tree gain should be set to --1-- . Examples of correlated signals are pure tones or modem signals that have been
M
synchronized so that they might peak at the same time. These signals, however, require a much smaller crest factor,
such as 3 dB for pure tones and 6 dB for modem signals. In this case the crest factor of 14 dB will absorb much of
the difference in gain between M and M .
If overflow does occur, then the samples are saturated to plus or minus full scale. Overflow can be
monitored using the overflow status register, see Section 5.14 for details.
The values of N and BIG_SHIFT must also satisfy 2^{(12*BIG_SHIFT+18)} ≥ N³ (see Section 3.4.3 for details). If
N and BIG_SHIFT do not satisfy this relationship, then an overflow may occur which may not be detected.
If the auto flush mode is used, then the gain in the CIC must be less than or equal to unity. This means that
the values of N, SCALE and BIG_SHIFT must satisfy 2^{(SCALE+12*BIG_SHIFT+3)} ≥ N³ (see Section 3.4.1 for details).
Note that noise due to rounding errors is minimized by keeping the gain as close to unity as is possible. If
attenuation is necessary, for example when multiple channel outputs are to be added together, then the attenuation
should be added as close to the output of the chip as is possible. This means that the SUM_SCALE control should
be used to add attenuation before the SCALE and BIG_SHIFT controls are adjusted.
Texas Instruments Inc.
- 10 -
MAY 22, 2000
This document contains information which may be changed at any time without notice