AD9864 datasheet(38/48 Pages) AD

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Data Sheet

AD9864

Rev. A | Page 37 of 47

Finally, consider the case of a strong out-of-band interferer (that

is, −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

control 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 61. Interferers at increasingly

higher frequency offsets incur larger measurement errors,

potentially causing the control loop to inadvertently reduce the

amount of VGA attenuation that may result in clipping of the

ADC. Figure 65 shows the maximum measured interferer signal

level versus the normalized IF offset frequency (relative to fCLK)

tolerated by the AD9864 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 signal

transfer function of the ADC.

NORMALIZED FREQUENCY OFFSET = (

fIN–fIF)/fCLK

–6

0.02

–12

0.04

–3

–9

0.03

0.01

–15

0.05

Figure 65. Maximum Interferer (or Blocker) Input Level vs. Normalized IF

Frequency Offset

Table 22. SPI Registers Associated with AGC

Address (Hex)

Bit(s)

Width

Default Value

Name

0x03

ATTEN

[6:0]

0x00

AGCG [14:8]

0x04

[7:0]

0x00

AGCG [7:0]

0x05

[7:4]

AGCA

[3:0]

0x00

AGCD

0x06

AGCV

[6:4]

0x00

AGCO

AGCF

[2:0]

0x00

AGCR

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

The system noise figure of the AD9864 is a function of the ACG

attenuation and output signal bandwidth. Figure 66 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 is

observed at the output for a −2 dBFS input. The high dynamic

range of the ADC within the AD9864 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 degradation is

approximately 5 dB. Therefore, the highest instantaneous

dynamic range for the AD9864 occurs with 12 dB of AGC

attenuation, since the AD9864 can accommodate an additional

12 dB peak signal level with only a moderate increase in its

noise floor.

As Figure 66 shows, the AD9864 can achieve an SNR in excess

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

performance of the AD9864 in such applications, it is

recommended that the I/Q data be represented with 24 bits. If

16-bit data is used, the effective system NF increases because of

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

VGA ATTENUATION (dB)

SNR = 90.1dBFS

BW = 50kHz

BW = 150kHz

SNR = 82.9dBFS

BW = 10kHz

SNR = 95.1dBFS

SNR = 103.2dB

Figure 66. Nominal System Noise Figure and Peak SNR vs. AGCG Setting

(fIF = 73.35 MHz, fCLK = 18 MSPS, and 24-bit I/Q Data)

Figure 67 plots the nominal system NF with 16-bit output data

as a function of AGC in both narrow-band and wideband

mode. In wideband mode, the NF curve is virtually unchanged

relative to the 24-bit output data because the output SNR before

truncation is always less than the 96 dB SNR that 16-bit data

can support. However, in narrow-band mode, where the output

SNR approaches or exceeds the SNR that can be supported with

16-bit data, the degradation in system NF is more severe.

Furthermore, if the signal processing within the DSP adds noise

at the level of an LSB, the system noise figure can be degraded

even more than Figure 67 shows. For example, this might occur

in a fixed 16-bit DSP whose code is not optimized to process

the AD9864 16-bit data with minimal quantization effects. To

limit the quantization effects within the AD9864, the 24-bit data

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