AD6634BCSLASHPCB datasheet(22/52 Pages) AD

22 / 52 page

REV. 0

–22–

AD6634

Table I. AD6600 Transfer Function with AD6634

ExpInv = 1, and No ExpOff

ADC Input

AD6600

AD6634

Signal

Level

RSSI[2:0]

Data

Reduction (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)

Table II. AD6600 Transfer Function with AD6620

ExpInv = 1, and ExpOff = 6

ADC Input

AD6600

AD6634

Signal

Level

RSSI[2:0]

Data

Reduction (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, ExpWeight = 0)

This flexibility in handling the exponent allows the AD6634 to

interface with gain-ranging ADCs other than 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 AD6634 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, the ExpOff

could be adjusted to 29, which would then make RSSI = 4

correspond to the 0 dB point of the AD6634.

IN 13

IN 2

IN 1

IN 0

IEN

D10 (MSB)

D0 (LSB)

AD6600

AD6634

EXP 1

RSSI 1

EXP 2

RSSI 2

EXP 0

RSSI 0

AB_OUT

Figure 29. Typical Interconnection of the AD6600

Gain-Ranging ADC and the AD6634

NUMERICALLY CONTROLLED OSCILLATOR

Frequency Translation

This processing stage comprises a digital tuner consisting of two

multipliers and a 32-bit complex NCO. Each channel of the

AD6634 has an independent NCO. The NCO serves as a quadra-

ture local oscillator capable of producing an NCO frequency

between –CLK/2 and +CLK/2 with a resolution of CLK/2

32 in

the complex mode. The worst-case spurious signal from the

NCO is better than –100 dBc for all output frequencies.

IN 13

IN 2

IN 1

IN 0

EXP 2

EXP 1

EXP 0

IEN

VDD

D11 (MSB)

D0 (LSB)

AD6640

AD6634

Figure 28. Typical Interconnection of the AD6640

Fixed Point ADC and the AD6634

Scaling with Floating-Point or Gain-Ranging ADCs

An example of the exponent control feature combines the AD6600

and the AD6634. The AD6600 is an 11-bit ADC with three bits

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

and the three 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 additional details.

For gain-ranging ADCs such as the AD6600,

SCALED

INPUT

EXPINV

EXPWEIGHT

MOD

EXP

rCIC

––

=×

()

72 32

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).

The RSSI output of the AD6600 numerically grows 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 AD6634 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 right by five internal bits before passing the data to

the rCIC2). In this example where ExpInv = 0, the AD6634

regards the largest signal possible on the AD6600 as the small-

est signal. Thus, we can use the Exponent Invert Bit to make

the AD6634 exponent agree with the AD6600 RSSI. By setting

ExpInv = 1, this forces the AD6634 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 undesirable

because it lowers the dynamic range and SNR of the system by reduc-

ing the signal of interest relative to the quantization noise floor.

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

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

the point where there is no down shift. 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.

AD6634BC/PCB датащи(PDF) 22 Page - Analog Devices

AD6634BC/PCB датащи(HTML) 22 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

датащи скачать

ссылки URL