AD6635BBSLASHPCB datasheet(29/60 Pages) AD

29 / 60 page

REV. 0

AD6635

–29–

of L/M that yields 0.25 will work (1/4, 2/8, or 4/16). However,

for the best dynamic range, the simplest ratio should be used.

For example, 1/4 gives better performance than 4/16.

Decimation and Interpolation Registers

rCIC2 decimation values are stored in register 0x90. This 12-bit

tion portion is stored in register 0x91. This 9-bit value holds the

interpolation less 1.

rCIC2 Scale

the circuit. The primary function is to store the scale value

computed in the sections above.

Bits 4–0 (rCIC2_LOUD[4:0]) of this register are used to contain

the scaling factor for the rCIC2 during conditions of strong

signals. These five bits represent the rCIC2 scalar calculated

above, plus any external signal scaling with an attenuator.

Bits 9–5 (rCIC2_QUIET[4:0]) of this register are used to con-

tain the scaling factor for the rCIC2 during conditions of weak

signals. In this register, no external attenuator would be consid-

ered and is not included. Only the value computed above for

rCIC2 compensation is stored in these bits.

Bit 10 of this register is used to indicate the value of the external

exponent. If this bit is set low, then external exponent represents

6 dB per step as in the AD6600. If this bit is set high, each

exponent represents a 12 dB step.

Bit 11 of this register is used to invert the external exponent

before internal calculation. This bit should be set high for gain-

ranging ADCs that use an increasing exponent to represent an

increasing signal level. This bit should be set low for gain-ranging

ADCs that use a decreasing exponent for representing an

increasing signal level.

In applications that do not require the features of the rCIC2, it

may be bypassed by setting the L/M ratio to 1/1. This effectively

bypasses all circuitry of the rCIC2 except the scaling which is

still in effect.

FIFTH-ORDER CIC FILTER

The third signal processing stage, CIC5, implements a sharper,

fixed-coefficient, decimating filter sharper than rCIC2. The

input rate to this filter is fSAMP2. The maximum input rate to this

filter is equal to the input rate into the AD6635, so bypassing

the rCIC2 filter is allowed.

The decimation ratio, MCIC5, may be programmed from 2 to

32 (all integer values). The frequency response of the filter is

given by the following equations. The gain and passband droop

of CIC5 should be calculated by these equations. Both parameters

may be compensated for in the RCF stage.

SCIC

MCIC

SCIC

CIC

SAMP

()

–

sin

–

=¥

ËÁ

¯˜

() =¥

ËÁ

¯˜

ËÁ

¯˜

5 5

The scale factor SCIC5 is a programmable, unsigned integer

between 0 and 20. It serves to control the attenuation of the

data into the CIC5 stage in 6 dB increments. For the best dynamic

range, SCIC5 should be set to the smallest value possible (low-

est attenuation) without creating an overflow condition. This

can be safely accomplished using the equation below, where

OLrCIC2 is the largest fraction of full scale possible at the input

to this filter stage. This value is output from the rCIC2 stage,

then pipelined into the CIC5.

ceil

CIC

5 5

=¥

()

= () ¥

log

–

The output rate of this stage is given by the equation below.

SAMP

CIC

CIC5 Rejection

Table IV illustrates the amount of bandwidth in percentage of the

clock rate (input rate) that can be protected with various decima-

tion rates and alias rejection specifications. The maximum input

rate into the CIC5 is 80 MHz when the rCIC2 decimates by 1. As

in the previous table, these are the 1/2 bandwidth characteristics

of the CIC5. Notice that the CIC5 stage can protect a much wider

band to any given rejection level compared to the rCIC2 stage.

This table helps to calculate an upper bound on decimation,

MCIC5, for given desired filter characteristics.

AD6635BB/PCB датащи(PDF) 29 Page - Analog Devices

AD6635BB/PCB датащи(HTML) 29 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

датащи скачать

ссылки URL