AD9652BBCZ-310 datasheet(29/37 Pages) AD

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AD9652

Data Sheet

Rev. B | Page 28 of 36

FAST THRESHOLD DETECTION (FDA/FDB)

In receiver applications, it is desirable to have a mechanism to

reliably determine when the converter is about to be clipped. The

standard overflow indicator on the OR± pins provide delayed

information, which is synchronized with the output data. The

delayed indicator is of limited value in preventing clipping in this

case. Therefore, it is helpful to have a programmable threshold

below full scale that allows time to reduce external gain before the

clip occurs. In addition, because input signals can have significant

slew rates, latency of this function is of concern.

Using the SPI port, the user can provide a threshold above

which the fast detect (FD) output is active. Bit 0 of Register 0x45

enables the FD feature. Register 0x47 to Register 0x4C allow the

user to set the threshold levels and timing. As long as the signal

is below the selected threshold, the FD output remains low. In

this mode, the magnitude of the data is considered in the calcu-

lation of the condition, but the sign of the data (either positive

or negative) is not considered. The threshold detection responds

identically to positive and negative signals outside the desired

range (magnitude).

The fast detect indicators, FDA for Channel A and FDB for

Channel B, are asserted when the input magnitude exceeds the

value programmed in the fast detect upper threshold register,

The selected threshold register is compared with the signal

magnitude at the output of the ADC. The fast upper threshold

detection has a latency of seven clock cycles. The approximate

upper threshold is a 4-bit value defined by

Upper Threshold (% Full Scale) =

((Register 0x47 value)/8) × 100%

The FD indicators are not cleared until the signal drops below

the lower threshold for the programmed dwell time. The lower

threshold is programmed in the fast detect lower threshold

registers, Register 0x49 and Register 0x4A. The fast detect lower

threshold register is a 15-bit register that is compared with the

signal magnitude at the output of the ADC. This comparison is

subject to the ADC pipeline latency but is accurate in terms of

converter resolution. The lower threshold is defined by

Lower Threshold (% Full Scale) =

((Register 0x49/Register 0x4A value)/32767) × 100%

For example, to set an upper threshold of 50% full scale, write

0x04 to Register 0x47, and to set a lower threshold of 40% full

scale, write 0x3333 to Register 0x49 and Register 0x4A.

The dwell time can be programmed from 1 sample clock cycle

to 65,535 sample clock cycles by placing the desired value in the

fast detect dwell time registers, Register 0x4B and Register 0x4C

(see Figure 66).

UPPER THRESHOLD

LOWER THRESHOLD

FDA OR FDB

DWELL TIME

TIMER RESET BY

RISE ABOVE

LOWER

THRESHOLD

TIMER COMPLETES BEFORE

SIGNAL RISES ABOVE

LOWER THRESHOLD

DWELL TIME

Figure 66. Threshold Settings for FDA and FDB Signals

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