GC5016 datasheet(32/88 Pages) TI

32 / 88 page

GC5016

SLWS142G − JANUARY 2003 − REVISED NOVEMBER 2005

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13.13 Overall Gain in Receive Mode

The overall gain in the receive mode is a function of zero padding (rinf_zpad), the CIC decimation (cic_dec), the cic

shift settings (cic_shift and cic_rshift), the sum of the programmable filter taps (PFIR_SUM), the filter output shift

(fir_shift) and the final gain in the agc circuit (G). The cmd5016 program, described later in the data sheet, sets the

cic_shift, cic_rshift, fir_shift, and G to their optimum levels for a targeted overall gain using the overall_gain keyword.

The calculated gain is done to limit the gain between stages, as well as provide an overall gain.

The overall gain is:

DDC_gain = zpad_gain x mix_gain x cic_gain x rshift_gain x fir_gain x agc_gain

Where the individual gains are:

zpad_gain = 1 / (rinf_zpad+1)

mix_gain = 1/2

cic_gain = cic_dec5 x 2(cic_shift−39)

rshift_gain = 2(cic_rshift−1)

fir_gain = PFIR_SUM x 2(fir_shift−21)

agc_gain = G / 4096

The restrictions on the gain settings are:

To prevent overflow in the CIC, cic_shift must be set such that:

zpad_gain x mix_gain x cic_gain

≤ 1

If rinf_zpad is greater than cic_dec, then cic_shift must be set such that:

(1/cic_dec) x mix_gain x cic_gain

≤ 1

For symmetric filters the maximum amplitude allowed into the fir is one-half, so cic_shift must be set such that:

zpad_gain x mix_gain x cic_gain x rshift_gain

≤ 1/2

The cic_rshift control is set to 1 (this control is only used to extend the allowable cic_dec range, and must be used

with care).

The fir_gain and agc_gain are used to adjust the overall gain to match the user’s desired gain (overall_gain). The

fir_shift control should be set such that:

zpad_gain x mix_gain x cic_gain x rshift_gain x fir_gain

≤ overall_gain

and the final agc_gain is set to give the desired overall_gain.

This equation gives unity gain for dc or complex data inputs. For real inputs, such as from an ADC, the DDC_gain

is typically set to 2 (6 dB). The gain of 2 compensates for the loss of 6 dB when tuning a signal to dc and filtering

out the negative image. Mathematically this is illustrated by using a an example input signal s(t) modulated up to a

frequency of ”

ω”. The input is defined as:

d(t) = s(t) x cos(

ωt) = s(t) x (ejωt + e−jωt) / 2

GC5016 Datasheet(PDF) 32 Page - Texas Instruments

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