AD9874ABST datasheet(32/41 Pages) AD

32 / 41 page

REV. A

AD9874

–31–

Table XII indicates which AGCA values are reasonable for

various decimation factors. The white cells indicate that the

(decimation factor/AGCA) combination works well; the light

gray cells indicate ringing and an increase in the AGC settling

time; and the dark gray cells indicate that the combination

results in instability or near instability in the AGC loop. Setting

AGCF = 1 improves the time-domain behavior at the expense

of increased spectral spreading.

Table XII. AGCA Limits if the DVGA is Enabled

AGCA

120

300

540

900

9 10 11 12 13 14 15

Lastly, consider the case of a strong out-of-band interferer (i.e.,

–18 dBm to –32 dBm for matched IF input) that is larger than

the target signal and large enough to be tracked by the control

loop based on the output of the DEC1. The ability of the con-

trol loop to track this interferer and set the VGA attenuation to

prevent clipping of the ADC is limited by the accuracy of the

digital signal estimation occurring at the output of DEC1. The

accuracy of the digital signal estimation is a function of the

frequency offset of the out-of-band interferer relative to the IF

frequency as shown in Figure 20. Interferers at increasingly

higher frequency offsets incur larger measurement errors, poten-

tially causing the control loop to inadvertently reduce the

amount of VGA attenuation that may result in clipping of the

ADC. Figure 21c shows the maximum measured interferer

signal level versus the normalized IF offset frequency (relative to

fCLK) tolerated by the AD9874 relative to its maximum target

input signal level (0 dBFS = –18 dBm). Note that the increase

in allowable interferer level occurring beyond 0.04

fCLK

results from the inherent signal attenuation provided by the

ADC’s signal transfer function.

NORMALIZED FREQUENCY OFFSET = (

IN – fIF)/fCLK

–6

0.02

–12

0.04

–3

–9

0.03

0.01

–15

0.05

Figure 21c. Maximum Interferer (or Blocker) Input

Level vs. Normalized IF Frequency Offset

Table XIII. SPI Registers Associated with AGC

Address

Bit

Default

(Hex)

Breakdown

Width

Value

Name

0x03

(7)

ATTEN

(6:0)

0x00

AGCG(14:8)

0x04

(7:0)

0x00

AGCG(7:0)

0x05

(7:4)

AGCA

(3:0)

0x00

AGCD

0x06

(7)

AGCV

(6:4)

AGCO

(3)

AGCF

(2:0)

AGCR

System Noise Figure (NF) vs. VGA (or AGC) Control

The AD9874’s system noise figure is a function of the ACG

attenuation and output signal bandwidth. Figure 22a plots the

nominal system NF as a function of the AGC attenuation for

both narrow-band (20 kHz) and wideband (150 kHz) modes

with fCLK = 18 MHz. Also shown on the plot is the SNR that

would be observed at the output for a –2 dBFS input. The

high dynamic range of the ADC within the AD9874 ensures

that the system NF increases gradually as the AGC attenuation

is increased. In narrow-band (BW = 20 kHz) mode, the system

noise figure increases by less than 3 dB over a 12 dB AGC

range, while in wideband (BW = 150 kHz) mode, the degra-

dation is about 5 dB. As a result, the highest instantaneous

dynamic range for the AD9874 occurs with 12 dB of AGC

attenuation, since the AD9874 can accommodate an addi-

tional 12 dB peak signal level with only a moderate increase

in its noise floor.

As Figure 22a shows, the AD9874 can achieve an SNR in

excess of 100 dB in narrow-band applications. To realize the

full performance of the AD9874 in such applications, it is recom-

mended that the I/Q data be represented with 24 bits. If 16-bit

data is used, the effective system NF will increase because of the

quantization noise present in the 16-bit data after truncation.

912

VGA ATTENUATION – dB

SNR = 90.1dBFS

BW = 50kHz

BW = 150kHz

SNR = 82.9dBFS

BW = 10kHz

SNR = 95.1dBFS

SNR = 103.2dB

Figure 22a. Nominal System Noise Figure and

Peak SNR vs. AGCG Setting (fIF = 73.35 MHz, fCLK =

18 MSPS, and 24-bit I/Q data)

AD9874ABST датащи(PDF) 32 Page - Analog Devices

AD9874ABST датащи(HTML) 32 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

датащи скачать

ссылки URL