AD6635 datasheet(25/60 Pages) AD

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REV. 0

AD6635

–25–

IN13

IN2

IN1

IN0

EXP2

EXP1

EXP0

IEN

VDD

D11 (MSB)

D0 (LSB)

AD6645

AD6635

(ExpOff = 0, ExpInv = 0)

Figure 27. Typical Interconnection of the AD6645

Fixed-Point ADC and the AD6635

Scaling with Floating-Point or Gain-Ranging ADCs

An example of the exponent control feature combines the

AD6600 and the AD6635. The AD6600 is an 11-bit ADC with

3 bits of gain ranging. In effect, the 11-bit ADC provides the

mantissa, and the 3 bits of relative signal strength indicator

(RSSI) for the exponent. Only five of the eight available steps

are used by the AD6600. See the AD6600 data sheet for addi-

tional details.

For gain-ranging ADCs such as the AD6600,

scaled

input

Exp rCIC

– mod( –

)

=¥

72 32

ExpInv = 1, ExpWeight = 0

where IN is the value of IN[13:0], Exp is the value of EXP[2:0],

and rCIC2 is the rCIC scale register value (0x92 bits 9-5 and 4-0).

“mod” is the remainder function. For example, mod(1,32) = 1,

mod(2,32) = 2, and mod(34,32) = 2.

The RSSI output of the AD6600 grows numerically with increas-

ing signal strength of the analog input (RSSI = 5 for a large

signal, RSSI = 0 for a small signal). When the Exponent Invert

Bit (ExpInv) is set to zero, the AD6635 will consider the small-

est signal at the IN[13:0] to be the largest, and as the EXP word

increases, it shifts the data down internally (EXP = 5 will shift a

14-bit word to the right by 5 internal bits before passing the

data to the rCIC2). In this example, if ExpInv = 0, the AD6635

regards the RSSI[2:0] = 5 as smallest signal and RSSI[2:0] = 0

as the largest signal possible on the AD6600. Thus, we can use

the Exponent Invert Bit to make the AD6635 exponent agree

with the AD6600 RSSI. Setting ExpInv = 1 forces the AD6635

to shift the data up (left) for growing EXP instead of down.

The exponent invert bit should always be set high for use

with the AD6600.

The Exponent Offset is used to shift the data up. For example,

Table I shows that with no rCIC2 scaling, 12 dB of range is

lost when the ADC input is at the largest level. This is not

desired because it lowers the dynamic range and SNR of the

system by reducing the signal of interest relative to the quan-

tization noise floor.

Table I. AD6600 Transfer Function with

AD6635 ExpInv = 1, and no ExpOff

ADC

Signal

Input

AD6600

AD6635

Reduction

Level

RSSI[2:0]

Data

(dB)

Largest

101 (5)

/4 (>> 2)

–12

100 (4)

/8 (>>3)

–18

011 (3)

/16 (>> 4)

–24

010 (2)

/32 (>> 5)

–30

001 (1)

/64 (>> 6)

–36

Smallest

000 (0)

/128(>> 7)

–42

ExpInv = 1, rCIC2 Scale = 0)

To avoid this automatic attenuation of the full-scale ADC sig-

nal, the ExpOff is used to move the largest signal (RSSI = 5) up

to the point where there is no downshift. In other words, once

the Exponent Invert bit has been set, the Exponent Offset should

be adjusted so that mod(7-5 + ExpOff,32) = 0. This is the case

when Exponent Offset is set to 30 since mod(32,32) = 0. Table II

illustrates the use of ExpInv and ExpOff when used with the

AD6600 ADC.

Table II. AD6600 Transfer Function with

AD6620 ExpInv = 1, and ExpOff = 30

ADC

Signal

Input

AD6600

AD6635

Reduction

Level

RSSI[2:0]

Data

(dB)

Largest

101 (5)

/1 (>> 0)

100 (4)

/2 (>>1)

–6

011 (3)

/4 (>> 2)

–12

010 (2)

/8 (>> 3)

–18

001 (1)

/16 (>> 4)

–24

Smallest

000 (0)

/32(>> 5)

–30

ExpInv = 1, ExpOff = 30, Exp Weight = 0)

This flexibility in handling the exponent allows the AD6635 to

interface with other gain-ranging ADCs besides the AD6600.

The Exponent Offset can be adjusted to allow up to seven

RSSI(EXP) ranges to be used as opposed to the AD6600’s five.

It also allows the AD6635 to be tailored in a system that employs

the AD6600 but does not utilize all of its signal range. For

example, if only the first four RSSI ranges are expected to

occur, then the ExpOff could be adjusted to 29, which would

make RSSI = 4 correspond to the 0 dB point of the AD6635.

Note that the above scale factor set in the rCIC2 register is only

to account for the ExpOff required. This register should also

account for compensating rCIC2 filter gain. The value required

for this will be given in the CIC2 filter section. Hence the final

value set in the rCIC2 register will be the sum total of ExpOff

and rCIC2 scale required.

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