AD9640 datasheet(37/54 Pages) AD

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AD9640

Rev. B | Page 35 of 52

SIGNAL MONITOR

The signal monitor block provides additional information

about the signal being digitized by the ADC. The signal monitor

computes the rms input magnitude, the peak magnitude,

and/or the number of samples by which the magnitude exceeds

a particular threshold. Together, these functions can be used to

gain insight into the signal characteristics and to estimate the

peak/average ratio or even the shape of the complementary cumu-

lative distribution function (CCDF) curve of the input signal. This

information can be used to drive an AGC loop to optimize the

range of the ADC in the presence of real-world signals.

The signal monitor result values can be obtained from the part by

reading back internal registers at Address 0x116 to Address 0x11B,

using the SPI port or the signal monitor SPORT output. The output

contents of the SPI-accessible signal monitor registers are set via

the two signal monitor mode bits of the signal monitor control

signal monitor mode (Address 0x112). Separate SPI-accessible,

20-bit signal monitor result (SMR) registers are provided for each

ADC channel. Any combination of the signal monitor functions

can also be output to the user via the serial SPORT interface.

These outputs are enabled using the peak detector output enable

bit, the rms/ms magnitude output enable bit, and the threshold

crossing output enable bit in the signal monitor SPORT control

For each signal monitor measurement, a programmable signal

monitor period register (SMPR) controls the duration of the

measurement. This period of time is programmed as the number

of input clock cycles in a 24-bit signal monitor period register

located at Address 0x113, Address 0x114, and Address 0x115.

This register can be programmed with a period from 128 samples

to 16.78 (224) million samples.

Because the dc offset of the ADC can be significantly larger

than the signal of interest (affecting the results from the signal

monitor), a dc correction circuit is included as part of the signal

monitor block to null the dc offset before measuring the power.

PEAK DETECTOR MODE

The magnitude of the input port signal is monitored over a

programmable time period (determined by the SMPR) to give

the peak value detected. This function is enabled by programming

a Logic 1 in the signal monitor mode bits of the signal monitor

control register or by setting the peak detector output enable bit

in the signal monitor SPORT control register. The 24-bit SMPR

must be programmed before activating this mode.

After enabling this mode, the value in the SMPR is loaded into

a monitor period timer, and the countdown is started. The magni-

tude of the input signal is compared with the value in the internal

peak level holding register (not accessible to the user), and the

greater of the two is updated as the current peak level. The initial

value of the peak level holding register is set to the current ADC

input signal magnitude. This comparison continues until the

monitor period timer reaches a count of 1.

When the monitor period timer reaches a count of 1, the 13-bit

peak level value is transferred to the signal monitor holding register

(not accessible to the user), which can be read through the SPI port

or output through the SPORT serial interface. The monitor period

timer is reloaded with the value in the SMPR, and the countdown is

restarted. In addition, the magnitude of the first input sample is

updated in the peak level holding register, and the comparison and

update procedure, as explained previously, continues.

Figure 68 is a block diagram of the peak detector logic. The SMR

the peak detector logic.

SIGNAL MONITOR

HOLDING

MAGNITUDE

STORAGE

COMPARE

A>B

MEMORY

MAP/SPORT

FROM

MEMORY

MAP

FROM

INPUT

PORTS

LOAD

CLEAR

LOAD

IS COUNT = 1?

DOWN

COUNTER

SIGNAL MONITOR

PERIOD REGISTER

Figure 68. ADC Input Peak Detector Block Diagram

RMS/MS MAGNITUDE MODE

In this mode, the root-mean-square (rms) or mean-square (ms)

magnitude of the input port signal is integrated (by adding an

accumulator) over a programmable period of time (determined by

the SMPR) to give the rms or ms magnitude of the input signal.

This mode is set by programming Logic 0 in the signal monitor

mode bits of the signal monitor control register or by setting the

rms/ms magnitude output enable bit in the signal monitor SPORT

control register. The 24-bit SMPR, representing the period over

which integration is performed, must be programmed before acti-

vating this mode.

After enabling the rms/ms magnitude mode, the value in the SMPR

is loaded into a monitor period timer, and the countdown is started

immediately. Each input sample is converted to floating-point

format and squared. It is then converted to 11-bit, fixed-point

format and added to the contents of the 24-bit accumulator.

The integration continues until the monitor period timer reaches

a count of 1.

When the monitor period timer reaches a count of 1, the square

root of the value in the accumulator is taken and transferred,

after some formatting, to the signal monitor holding register, which

can be read through the SPI port or output through the SPORT

serial port. The monitor period timer is reloaded with the value

in the SMPR, and the countdown is restarted. In addition, the

first input sample signal power is updated in the accumulator,

and the accumulation continues with the subsequent input

samples.

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