AD9957SLASHPCBZ datasheet(46/62 Pages) AD

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Data Sheet

AD9957

Rev. E | Page 45 of 61

I/Q PATH LATENCY

The I/Q latency through the AD9957 is easiest to describe in

terms of system clock (SYSCLK) cycles and is a function of the

AD9957 configuration (that is, which mode and which optional

features are engaged). The I/Q latency is primarily affected by

the programmable CCI rate.

The values in Table 12 should be considered estimates because

observed latency may be data dependent. The latency was

calculated using the linear delay model for FIR filters. N = CCI

rate (programmable interpolation rate, 2 to 63, 1 if bypassed).

In BFI mode, the latency through the AD9957 may not be con-

stant for multiple transmissions. This is due to the relationship

between the phase of the clock that drives the first half-band

filter and the frame sync signal coming from the Blackfin,

which is unknown and denoted as x in Table 12. The design

successfully transfers data from the data assembler logic to the

signal process path by updating a parallel register at the proper

time. The data is transferred from the parallel register to the

signal processing chain and all timing has been verified regard-

less of the phase relationship between the updating of the parallel

Example

Quadrature modulation mode = 18-bit parallel data

Reference clock multiplier = bypassed

Input scale multiplier = off

Inverse CCI = off

CCI rate = 20

Inverse SINC = on

Output scale = off

Latency = (16 × 20) + (4 × 20) + (4 × 20) + (69 × 20) +

(4 × 20 + 8) + 22 + 8 + 2 + 8 = 1988 SYSCLKs

Table 12.

Stage

Quadrature Modulation Mode—Parallel

Quadrature Modulation Mode—BFI

Interpolation DAC Mode

Input Demuxplexer

16N

(16 + x)N

28N

where x = 0 to 15

Input Scale Multiplier

Active: 8N

Not available in BFI mode

Active: 8N

Bypassed: 4N

Inverse CCI Filter

Active: 8N

Bypassed: 4N

Half-Band Filters

69N

345N

69N

CCI Filter

Active: 4N + 8

Bypass: 2N + 4

Modulator

Inverse Sinc Filter

Active: 8

Bypass: 2

Output Scale Multiplier

Active: 12

Bypass: 2

DAC Interface

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