AD9874EB datasheet(31/40 Pages) AD

31 / 40 page

REV. 0

AD9874

–31–

Figure 22b 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.

SNR = 98.8dBFS

BW = 50kHz

BW = 150kHz

SNR = 83dBFS

SNR = 94.1dBFS

BW = 10kHz

SNR = 89.9dBFS

VGA ATTENUATION – dB

Figure 22b. Nominal System Noise Figure and Peak SNR

vs. AGCG Setting (fIF = 73.35 MHz, fCLK = 18 MSPS, and

16-bit I/Q data)

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 22b

shows. For example, this could occur in a fixed 16-bit DSP whose

code is not optimized to process the AD9874’s 16-bit data with

minimal quantization effects. To limit the quantization effects

within the AD9874, the 24-bit data undergoes noise shaping just

prior to 16-bit truncation, thus reducing the in-band quantization

noise by 5 dB (with 2

oversampling). This explains why 98.8 dBFS

SNR performance is still achievable with 16-bit data in a 10 kHz BW.

APPLICATION CONSIDERATIONS

Frequency Planning

The LO frequency (and/or ADC clock frequency) must be chosen

carefully to prevent known internally generated spurs from mixing

down along with the desired signal, thus degrading the SNR

performance. The major sources of spurs in the AD9874 are the

ADC clock and digital circuitry operating at 1/3 of fCLK. Thus,

the clock frequency (fCLK) is the most important variable in

determining which LO (and therefore IF) frequencies are viable.

–60

–70

–80

–90

–50

250

300

200

150

100

LO FREQUENCY – MHz

Figure 23a. Total In-Band Noise + Spur Power with No Signal Applied as a Function of the LO Frequency

(fCLK = 18 MHz and Output Signal Bandwidth of 150 kHz)

–60

–70

–80

–90

–50

250

300

LO FREQUENCY – MHz

200

150

100

Figure 23b. Same as Figure 23a Excluding LO Frequencies Known to Produce Large In-Band Spurs

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