Datasheet

LM97593

www.ti.com

SNWS019B –JULY 2007–REVISED APRIL 2013

This equation assumes that the DVGA gain control polarity is positive as is the case for the CLC5526. The gain

around the entire loop must be negative. Observe in Equation 10 that the control gain is dependent on operating

point G. If we instead compute the control gain with log conversion then:

(11)

which is no longer operating-point dependent. The log function is constructed by computing the CIC filter output

associated with each address (Equation 12) and converting these to dB. Full scale (dc signal) is

20log(511)=54dB.

The reference subtraction is constructed by subtracting the desired loop servo point (in dB) from the table values

computed in the previous paragraph. For example, if it is desired that the DVGA servo the ADC input level

(sinusoidal signal) to -6dBFS, the number to subtract from the data is:

(12)

The table data will then cross through zero at the address corresponding to this reference level. A deadband

wider than 6dB should then be constructed symmetrically about this point. This prevents the loop from hunting

due to the 6dB gain steps of the DVGA. Any deadband in excess of 6dB appears as hysteresis in the servo point

of the loop as illustrated in Figure 78. The deadband is constructed by loading zeros into those addresses on

either side of the one which corresponds to the reference level.

The last function of the RAM table is that of error amplification. All the operations preceding this one gave a table

slope S

RAM

= 1. This must now be adjusted in order to control the time constant of the loop given by:

(13)

The term G

in this equation is the loop gain:

(14)

The design equations are obtained by solving Equation 13 for G

and Equation 14 for S

RAM

. AGC_LOOP_GAIN

is a control register value that determines the number of bits to shift the output of the RAM down by. This allows

some of the loop gain to be moved out of the RAM so that the full output range of the table is utilized but not

exceeded. The valid range for AGC_LOOP_GAIN is from 0 to 3 which corresponds to a 0 to 3 bit shift to the left.

An example set of numbers to implement a loop having a reference of 6dB below full scale, a deadband of 8dB,

and a loop gain of 0.108 is:

-102 -102 -88 -80 -75 -70 -66

-63 -61 -56 -53 -50

-47 -42 -39 -36 -33 -29 -25

-22 -19 -15 -11 -0

0 0 0 0 0 13 17 20

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