AD6650SLASHPCB datasheet(21/44 Pages) AD

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AD6650

Rev. A | Page 21 of 44

Peak Detector

The peak detector always stores the input sample with the

largest magnitude. The absolute value of every input sample is

compared to what is currently in the peak detector’s holding

0; at this point, the dc offset estimate is updated and the peak

detector is set to the current input magnitude. The output of

each of the peak detectors is then encoded into a digital word

that represents the signal power in 6 dB steps relative to full

scale (FS).

DC Accumulator

The dc accumulator accumulates the 24-bit samples input from

the RCF filter until the control counter reaches 0. At this time,

the dc estimate in the holding register is updated, and the

accumulator is directly loaded with the new input sample to

begin work on the next estimate.

Control Counter

This counter controls the update of the dc correction block

based on the peak detector value and the input control registers.

The following three conditions are possible:

•

If the digital word from the peak detector indicates that the

desired signal is below the lower threshold, the counter

merely cycles through at the minimum period.

•

If the digital word from the peak detector indicates that the

desired signal is above the upper threshold, the control

counter is held at the minimum period value and does

not count down; therefore, no update is made. When the

signal returns below the upper threshold, this counter

resumes counting.

•

If the digital word from the peak detector indicates that the

desired signal is between the lower threshold and the upper

threshold, the fine dc correction circuit is in its normal mode

of operation. In this mode, the control counter starts with the

minimum period but is reloaded with 4× minimum period

every time the peak detector output words increment by

6 dB. This errs on the side of caution and ensures that the

dc correction integrates long enough to obtain a valid

estimate. If smaller integrations are preferred, the minimum

period can be decreased or the lower threshold can be raised.

The integration period is given by Equation 15 and Equation 16.

The factor of 2 in the exponent shows that as peak signal power

increases, the integration time is increased by a factor of 4. This

decreases the bandwidth of the estimation filter, thus providing

the additional processing gain in the dc estimation term.

When the desired signal power equals the upper threshold,

⎟

⎠

⎞

⎜

⎝

⎛

−

Threshold

Lower

Threshold

Upper

Ceil

Period

Min

(15)

When the desired signal power is less than the upper threshold,

⎟

⎠

⎞

⎜

⎝

⎛

−

Threshold

Lower

Power

Signal

Desired

Ceil

Period

Min

(16)

where Min_Period, Upper_Threshold, and Lower_Threshold are

To calculate the time required for the fine dc correction to

converge, use the following equation:

SYM

Converge

Fine

(17)

where:

TSYM is the output symbol rate of the AD6650.

Fine_DC_Converge is expressed in minutes, and for a GSM

application with 1× oversampling, it is 3.69 × 10−6.

USER-CONFIGURABLE BUILT-IN SELF-TEST (BIST)

The AD6650 includes a BIST to assess digital functionality. This

feature verifies the integrity of the main digital signal paths of

the AD6650. Each BIST register is independent, meaning that

each channel can be tested independently at the same time.

The BIST is a thorough test of the selected AD6650 digital

signal path. With this test mode, it is possible to use the internal

pseudorandom generator to produce known test data. A

signature register follows the fine dc correction block. This

signature. If the known good signature matches the register

value, the channel is fully operational.

If an error is detected, each internal block can be bypassed and

another test can be run to debug the fault. The I and Q paths are

tested independently. Use the following steps to perform this test:

Reset the AD6650.

Program the desired AD6650 channel parameters for the

desired application (these parameters include decimation

rates, scalars, and RCF coefficients). Also, ensure that the

start holdoff counter is set to a nonzero value.

Set Register 0xA, Bit 1, to 1 (PN_EN).

Set Register 0x21, Bit 8, to 0 (fine DCC to BIST).

Start the A and/or B channels with a microprocessor write

(Soft_SYNC) or a pulse on the SYNC pin (Pin_SYNC).

Wait at least 300 μs.

Read the four BIST registers and compare the values to a

known good device. This ensures that the AD6650 is

programmed correctly and that each channel is

functioning correctly.

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