AD6622SSLASHPCB datasheet(12/28 Pages) AD

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AD6622

–12–

REV. 0

1. Select the Impulse Response Length (NRCF) and the Inter-

polation Factor (LRCF). The Impulse Response Length

(NRCF) is limited in three ways: by the available calculation

time, by the data memory size (DMEM), and by the coefﬁ-

cient memory size (CMEM). The equation below shows

that NRCF is limited to the minimum of these three conditions.

Time

CMEM

Restriction

↓↓

RCF

≤×









min

128

(6)

↑

DMEM

Restriction

where:

L = LRCF

× L

CIC5

× L

CIC2

2. The interpolation rate (LRCF) may be any integer of NRCF

ranging from 1 to 128, while meeting the above equation.

Most ﬁlter designs can be optimized by choosing the small-

est LRCF that does not compromise the image rejection of

the subsequent CIC ﬁlter. The quality of an interpolating

ﬁlter is a strong function of the NRCF/LRCF ratio and a weaker

function of NRCF. The best ﬁlters are usually achieved by

maximizing NRCF/LRCF (no larger than 16) and then increasing

both NRCF and LRCF by the same ratio until the ﬁlter becomes

time or CMEM limited.

3. Once NRCF and LRCF are selected, Channel Register 0x0A

is programmed to NRCF – 1, and Channel Register 0x0C is

programmed to NRCF/LRCF – 1.

4. Determine the Impulse Response. The impulse response

relative to the RCF output rate can be calculated using ordi-

nary FIR design techniques. In most cases, it is desirable to

precompensate the inband frequency roll-off of the CIC ﬁl-

ter that follows. There are no symmetry requirements, so the

RCF can also be used for static phase equalization. The

impulse response must be quantized to 16-bit two’s comple-

ment numbers for the CMEM. The channel center gain and

worst-case peak can be calculated for each of the LRCF phases

(p) according to the equations below. A RCF coarse scale

factor (g) that ranges between 0 and 3 is provided to limit

the gain without excessive loss of resolution in the CMEM.

The coarse scale factor is located in Channel Register 0x0D.

ChannelCenterGain

h k

RCF

=×

∑

−

[]

–

(7)

5. The channel center gain is the response to a constant full-

scale input at every output phase. The summation is split

into phases because the interpolation of the data insures that

only NRCF/LRCF coefﬁcients can be active for any single output.

For LRCF = 1, there is only one phase and the channel center

gain is the simple sum of all the coefﬁcients, scaled by 2

–g. If

the channel center gain is not the same for every value of p,

some or all of the images of the channel center will be

imperfectly rejected by the RCF.

WorstCasePeak

h k

RCF

=×

∑

−

–

(8)

6. The worst-case peak is calculated similarly to the channel

center gain, except that the input sequence swings from full-

scale positive to full-scale negative to match the polarity of the

coefﬁcient by which it will be multiplied, so that each prod-

uct is positive. This results in a maximal that must be less

than one to guarantee no possibility of wrapping. Note that

when LRCF is greater than one, each phase may produce its

worst-case peak in response to a different input sequence.

7. Programming DMEM and CMEM. The DMEM must be

initialized to all zeros to avoid any unpredictable start-up

transients since a reset does not clear the memory. The

impulse response h[n] must be reordered by phase for the

CMEM as shown in the code below. Several ﬁlters with

impulse lengths that total less than 128 can be programmed

into the CMEM simultaneously and selected later using the

RCF offset pointer (ORCF) which is set by Channel Register

0x0B.

* Reorder Fir Coefﬁcients for AD6622 CMEM */

for (p=0; p<L_RCF; p++)

for (k=0; k<N_RCF/L_RCF; k++)

CMEM[O_RCF + p*N_RCF/L_RCF + k] = C[k*L_RCF +p];

* End of routine */

Table I. RCF Control Registers

Channel

Bit

Address

Width

Description

0x0A

7: Reserved (Must Be Written to 0)

6–0: NRCF–1

0x0B

7: Reserved (Must Be Written to 0)

6–0: ORCF

0x0C

7–6: Reserved

5–4: Reserved (Must Be Written to 0)

3–0: NRCF/LRCF–1

0x0D

7–6: RCF Coarse Scale:

00 = 0 dB

01 = –6 dB

10 = –12 dB

11 = –18 dB

5: Reserved (Must Be Written to 0)

4–0: Serial Clock Divider

0x0E

15–0: Reserved

0x0F

15–0: Reserved

0x10

15–0: Reserved (Must Be Written to 0)

0x11

15–0: Reserved (Must Be Written to 0)

0x20–0x3F

15–0: Data Memory (DMEM)

0x80–0xFF

15–0: Coefﬁcient Memory (CMEM)

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