Datasheet
LM97593
SNWS019B –JULY 2007–REVISED APRIL 2013
www.ti.com
Refer to Figure 7 for detailed timing information.
Figure 82. Timing diagram for AGC/DVGA interface, Channel A
The “FIXED TO FLOAT CONVERTER” takes the fixed point 9-bit output from the CIC filter and converts it to a
“floating point” number. This conversion is done so that the 32 values in the RAM can be uniformly assigned (dB
scale) to detected power levels (54 dB range). This provides a resolution of 1.7dB between detected power
levels. The truth table for this converter is given in Table 4. The upper three bits of the output represent the
exponent (e) and the lower 2 are the mantissa (m). The exponent is determined by the position of the leading ‘1’
out of the CIC filter. An output of ‘001XX’ corresponds to a leading ‘1’ in bit 2 (LSB is bit 0). The exponent
increases by one each time the leading ‘1’ advances in bit position. The mantissa bits are the two bits that follow
the leading ‘1’. If we define E as the decimal value of the exponent bits and M as the decimal value of the
mantissa bits, the output of the CIC filter, POUT, corresponding to a given “FIXED TO FLOAT CONVERTER”
output is:
(9)
The max() and min() operators account for row 1 of Table 4 which is a special case because M=P
OUT
. Equation 9
associates each address of the RAM with a CIC filter output.
Table 4. Fixed to Float Converter Truth Table
INPUT OUTPUT (eeemm)
0-3 000XX
4-7 001XX
8-15 010XX
16-31 011XX
32-63 100XX
64-127 101XX
128-255 110XX
256-511 111XX
As shown in Figure 81, the 32X8 RAM look-up table implements the functions of log converter, reference
subtraction, error amplifier, and deadband. The user must build each of these functions by constructing a set of
8-bit, 2’s complement numbers to be loaded into the RAM. Each of these functions and how to construct them
are discussed in the following paragraphs.
A log conversion is done in order to keep the loop gain independent of operating point. To see why this is
beneficial, the control gain of the DVGA computed without log conversion is:
where
• G is the decimal equivalent of GAIN
• G
o
accounts for the DVGA gain in excess of unity (10)
50 Submit Documentation Feedback Copyright © 2007–2013, Texas Instruments Incorporated
Product Folder Links: LM97593