AD6676EBZ datasheet(25/90 Pages) AD

25 / 90 page

Data Sheet

AD6676

Rev. A | Page 25 of 90

DAC

ADC

LOOP FILTER

H(s)

–

DAC

H(z)

–

ADC

U(z)

V(z) =

H(z)

1 + H(z)

STF

E(z)

1 + H(z)

NTF

Figure 69. Simplified Model of a Σ-Δ ADC Showing Origins of STF and NTF

In the case of the AD6676, the loop filter consists of three cascaded

resonators to implement a sixth-order band-pass response, thus

allowing the oversampling ratio of the AD6676 to be kept to

moderate levels (≥10) such that useable bandwidths of up to

160 MHz can be realized. The loop filter utilizes a feedback

architecture so that the STF has minimal out-of-band gain

peaking while the NTF suppresses the in-band quantization

noise. Figure 70 shows an example of the STF and the shaped

noise of the Σ-Δ ADC when it is configured for BW = 80 MHz,

FIF = 300 MHz, and FADC = 3.2 GHz. Note that the NSD near FIF

is much lower than the NSD elsewhere and that the STF is quite

broadband.

–120

–100

–80

–60

–40

–20

200

400

600

800

1000

1200

1400

1600

FREQUENCY (MHz)

STF

NSD = –161.5dBFS/Hz

NTF

SHAPED NOISE

NBW = 146.5kHz

Figure 70. STF and NTF Shaped Noise of the Σ-Δ ADC

(FIF = 300 MHz, BW = 80 MHz, FADC = 3.2 GHz, LEXT = 19 nH)

Figure 71 focuses on the IF pass band region to compare the

measured vs. ideal shaped noise with the theoretical NSD curve

accounting only for the ideal ADC quantization effect. The

resonator zero locations are highlighted on the theoretical trace

and are recognizable in the measured response. Note that the

region with the lowest NSD performance or the deepest notch is

always centered about the FIF setting. This is because the gain of

RESON1 peaks at FIF and the noise from stages which follow

RESON1 is input referred by dividing by the gain of RESON1.

–60

–130

–120

–130

–140

–150

–160

–170

–180

–120

–110

–100

–90

–80

–70

100

150

200

250

300

350

400

450

500

FREQUENCY (MHz)

RESON2

RESON1

RESON3

THEORETICAL

NSD FROM

ADC QUANTIZATION

OBSERVED

NSD

NBW = 146.5kHz

Figure 71. Measured vs. Ideal NTF

(FIF = 300 MHz, BW = 80 MHz, FADC = 3.2 GHz, LEXT = 19 nH)

Unlike conventional ADCs, the NSD of a Σ-Δ ADC is not flat due

to its frequency dependent loop filter, H(s), which shapes the

quantization noise as well as various other noise sources. Because

the Σ-Δ ADC is highly programmable, its NSD can be optimized

for the user specified application parameter settings. In general, the

NSD performance varies based on the application parameter

settings in the following ways:

•

Operating with a high oversampling ratio (OSR > 20) results

in the lowest and flattest NSD performance. This is because

the resonant frequencies (or zero locations) associated with

RESON1, RESON2, and RESON3 are close together when the

oversampling ratio is high thereby reducing the quantization

noise to the point where thermal noise from the first stage

IDAC1 dominates.

•

Operating at reduced oversampling ratio (oversampling

ratio < 20) causes the quantization noise contribution to

become more significant, causing humps to appear in the

NSD. Bumpiness in the NSD occurs because the resonant

frequencies associated RESON2 and RESON3 are further

offset from RESON1 to accommodate the increase in BW;

therefore, resulting in less overall loop gain to suppress this

increasingly dominant noise source. The effect of different

oversampling ratios on the NSD is shown in Figure 72.

•

Operating at a lower FIF while keeping the same oversampling

ratio results in degraded NSD performance at the pass band

edges, as shown in Figure 73.

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