Datasheet

Data Sheet AD9957
Rev. C | Page 45 of 64
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
register and the signal processing clock.
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 Bypassed: 4N
Inverse CCI Filter
Active: 8
N
Active: 8
N
Active: 8
N
Bypassed: 4N Bypassed: 4N Bypassed: 4N
Half-Band Filters 69N 345N 69N
CCI Filter Active: 4N + 8 Active: 4N + 8 Active: 4N + 8
Bypass: 2N + 4 Bypass: 2N + 4 Bypass: 2N + 4
Modulator 22 22 0
Inverse Sinc Filter Active: 8 Active: 8 Active: 8
Bypass: 2 Bypass: 2 Bypass: 2
Output Scale Multiplier Active: 12 Active: 12 Active: 12
Bypass: 2 Bypass: 2 Bypass: 2
DAC Interface 8 8 8