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AD9680

Data Sheet

Rev. C | Page 44 of 97

ADC OVERRANGE AND FAST DETECT

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

reliably determine when the converter is about to be clipped.

The standard overrange bit in the JESD204B outputs provides

information on the state of the analog input that is of limited

usefulness. Therefore, it is helpful to have a programmable

threshold below full scale that allows time to reduce the gain

before the clip actually occurs. In addition, because input

signals can have significant slew rates, the latency of this

function is of major concern. Highly pipelined converters can

have significant latency. The AD9680 contains fast detect

circuitry for individual channels to monitor the threshold and

assert the FD_A and FD_B pins.

ADC OVERRANGE

The ADC overrange indicator is asserted when an overrange is

detected on the input of the ADC. The overrange indicator can

be embedded within the JESD204B link as a control bit (when

CSB > 0). The latency of this overrange indicator matches the

sample latency.

The AD9680 also records any overrange condition in any of the

eight virtual converters. For more information on the virtual

converters, refer to Figure 146. The overrange status of each virtual

converter is registered as a sticky bit in Register 0x563. The

contents of Register 0x563 can be cleared using Register 0x562,

by toggling the bits corresponding to the virtual converter to set

and reset position.

FAST THRESHOLD DETECTION (FD_A AND FD_B)

The FD bit is immediately set whenever the absolute value of

the input signal exceeds the programmable upper threshold

level. The FD bit is only cleared when the absolute value of the

input signal drops below the lower threshold level for greater

than the programmable dwell time. This feature provides

hysteresis and prevents the FD bit from excessively toggling.

The operation of the upper threshold and lower threshold

registers, along with the dwell time registers, is shown in

Figure 141.

The FD indicator is asserted if the input magnitude exceeds the

value programmed in the fast detect upper threshold registers,

located at Register 0x247 and Register 0x248. 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 28 clock cycles (maximum). The approximate upper

threshold magnitude is defined by

Upper Threshold Magnitude (dBFS)

= 20 log (Threshold Magnitude/213)

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, located at Register 0x249 and Register 0x24A. The fast

detect lower threshold register is a 13-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

magnitude is defined by

Lower Threshold Magnitude (dBFS)

= 20 log (Threshold Magnitude/213)

For example, to set an upper threshold of −6 dBFS, write 0xFFF

to Register 0x247 and Register 0x248. To set a lower threshold

of −10 dBFS, write 0xA1D to Register 0x249 and Register 0x24A.

The dwell time can be programmed from 1 to 65,535 sample

clock cycles by placing the desired value in the fast detect dwell

time registers, located at Register 0x24B and Register 0x24C.

See the Memory Map section (Register 0x040, and Register 0x245

to Register 0x24C in Table 36) for more details.

UPPER THRESHOLD

LOWER THRESHOLD

FD_A OR FD_B

DWELL TIME

TIMER RESET BY

RISE ABOVE

LOWER

THRESHOLD

TIMER COMPLETES BEFORE

SIGNAL RISES ABOVE

LOWER THRESHOLD

DWELL TIME

Figure 141. Threshold Settings for FD_A and FD_B Signals

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