AD6676EBZ datasheet(34/90 Pages) AD

34 / 90 page

AD6676

Data Sheet

Rev. A | Page 34 of 90

–10

–12

–14

–16

–18

–20

–22

–24

–26

–28

–30

100

200

300

400

500

600

800

1000

700

900

FREQUENCY (MHz)

0dB

2dB

4dB

6dB

8dB

10dB

12dB

14dB

Figure 94. Differential S11 vs. Frequency for Different Attenuator Settings

The accuracy of the attenuator is an important consideration in

applications implementing AGC or system calibration. The

attenuator remains monotonic over its full operating range.

Figure 95, which shows a typical devices attenuation error vs.

attenuation state at −40°C, +25°C, and +85°C, demonstrates the

near instrumentation level accuracy of the AD6676 attenuator.

0.10

–0.10

–0.08

–0.06

–0.04

–0.02

0.02

0.04

0.06

0.08

ATTENUATOR SETTING (dB)

–40°C

+25°C

+85°C

Figure 95. Typical Attenuation Step Size Error vs. Setting over Temperature

The linearity performance of the attenuator is another

consideration when determining the largest input drive levels

before its nonlinearity may dominate over that of the Σ-Δ ADC.

The effective PIN_0dFS level of the AD6676 is increased decibel-

per-decibel by the attenuator setting. At large attenuator settings,

the peak-to-peak voltage swing seen at the VIN+ and VIN− pins

increases as well as the current that is steered into the attenuator

shunt resistance. At a certain level, the IMD contribution from

the attenuator begins to dominate over the Σ-Δ ADC contribution.

Figure 96 plots the worst third-order IMD spurious vs. attenuator

setting for IDAC1FS of 4 mA and 2 mA with the power of the

dual tones increased to maintain a constant −8 dBFS level

measured by the Σ-Δ ADC. The effective PIN_0dBFS is also

plotted to show the maximum continuous wave signal level

into the device that results in a 0 dBFS level.

Note the following conditions and observations:

•

The AD6676 is configured as follows: IF = 180 MHz, BW =

80 MHz, and fCLK = 3.2 GHz. Tones are situated at 177.5 MHz

and 182.5 MHz.

•

The PIN_0dBFS level is reduced by 6 dB when IDAC1FS is

reduced to 2 mA.

•

The IMD performance remains below −80 dBc until an

attenuator setting of 9 dB.

•

Further increases in the two-tone power lead to a

corresponding steady decline in the IMD performance due

to the nonlinearity of the attenuator.

•

Although not shown, the NSD performance centered about

the IF improves a few dB with increased attenuation.

–10

–90

–80

–70

–60

–50

–40

–30

–20

–10

–7

–4

–1

ATTENUATOR SETTING (dB)

PIN_0dBFS_IDAC1FS = 2mA

PIN_0dBFS_IDAC = 4mA

IMD_IDAC1FS = 4mA

IMD_ IDAC1FS = 2mA

Figure 96. IMD Component Degradation as Two-Tone Centered at an IF of

180 MHz Is Increased 1 dB for Every 1 dB Increase in Attenuator Setting, Such

That Two-Tone Level Remains at −8 dBFS

The effects of switching transients are another important

consideration for AGC implementations that digitally calibrate

gain changes in the signal path of the receiver that can otherwise

degrade the demodulation of the desired signals.

Figure 97 and Figure 98 show the IQ envelope response when

the attenuator state is switched between 0 dB and 6 dB via an

external control signal using the AGC2 input pin at a rate of

3.3 MHz. Note that the settling response is dominated by the

response of the digital filter (decimate by 12) and shows no

signs of glitch.

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