AD9864 datasheet(34/44 Pages) AD

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AD9864

NORMALIZED FREQUENCY OFFSET = (

fIN–fIF)/fCLK

–6

0.02

–12

0.04

–3

–9

0.03

0.01

–15

0.05

option, the use of 24-bit data is preferable to using the DVGA.

Table 17 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 th

time-domain behavior at the expense of increased spectral spreading.

Table 17. AGCA Limits if the DVGA Is Enabled

AGCA

10 11 12 13 14 15

120

300

540

900

Figure 63. Maximum Interferer (or Blocker) Input Level

vs. Normalized IF Frequency Offset

Table 18. SPI Registers Associated with AGC

Address

(Hex)

Bit

Breakdown

Width

Default

Value

Name

0x03

(7)

(6:0)

0x00

ATTEN

AGCG (14:8)

0x04

(7:0)

0x00

AGCG (7:0)

0x05

(7:4)

(3:0)

0x00

AGCA

AGCO

0x06

(7)

(6:4)

(3)

(2:0)

AGCV

AGCO

AGCF

AGCR

Finally, 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 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 60. 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 clip-

ping of the ADC. Figure 63 shows the maximum measured

interferer signal level versus the normalized IF offset frequency

(relative to fCLK) tolerated by the AD9864 relative to its maxi-

mum 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.

SYSTEM NOISE FIGURE (NF) VERSUS VGA (OR

AGC) CONTROL

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

attenuation and output signal bandwidth. Figure 64 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 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 degrada-

tion is about 5 dB. As a result, 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 64 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 recom-

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

Rev. 0 | Page 34 of 44

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