AFE5808A datasheet(52/66 Pages) TI1

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AFE5808A

SLOS729B – OCTOBER 2011 – REVISED APRIL 2012

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As discussed in the theory of operation, the attenuator architecture uses seven attenuator segments that are

equally spaced in order to approximate the linear-in-dB gain-control slope. This approximation results in a

monotonic slope; the gain ripple is typically less than ±0.5dB.

The control voltage input (VCNTLM/P pins) represents a high-impedance input. The VCNTLM/P pins of multiple

AFE5808A devices can be connected in parallel with no significant loading effects. When the voltage level

(VCNTLP-VCNTLM) is above 1.5V or below 0V, the attenuator continues to operate at its maximum attenuation level

or minimum attenuation level respectively. It is recommended to limit the voltage from -0.3V to 2V.

When the AFE5808A operates in CW mode, the attenuator stage remains connected to the LNA outputs.

Therefore, it is recommended to power down the VCA using the PDN_VCA register bit. In this case, VCNTLP-

VCNTLM voltage does not matter.

The AFE5808A gain-control input has a –3dB bandwidth of approximately 800KHz. This wide bandwidth,

although useful in many applications (e.g. fast VCNTL response), can also allow high-frequency noise to modulate

the gain control input and finally affect the Doppler performance. In practice, this modulation can easily be

avoided by additional external filtering (RVCNTL and CVCNTL) at VCNTLM/P pins as Figure 81 shows. However, the

external filter's cutoff frequency cannot be kept too low as this results in low gain response time. Without external

filtering, the gain control response time is typically less than 1

μs to settle within 10% of the final signal level of

1VPP (–6dBFS) output as indicated in Figure 51 and Figure 52.

Typical VCNTLM/P signals are generated by an 8bit to 12bit 10MSPS digital to analog converter (DAC) and a

differential operation amplifier. TI’s DACs, such as TLV5626 and DAC7821/11 (10MSPS/12bit), could be used to

generate TGC control waveforms. Differential amplifiers with output common mode voltage control (e.g.

THS4130 and OPA1632) can connect the DAC to the VCNTLM/P pins. The buffer amplifier can also be configured

as an active filter to suppress low frequency noise. More information can be found in the literatures SLOS318F

and SBAA150. The VCNTL vs Gain curves can be found in Figure 2. The below table also shows the absolute

gain vs. VCNTL, which may help program DAC correspondingly.

In PW Doppler and color Doppler modes, VCNTL noise should be minimized to achieve the best close-in phase

noise and SNR. Digital VCNTL feature is implemented to address this need in the AFE5808A. In the digital VCNTL

mode, no external VCNTL is needed.

Table 12. VCNTLP–VCNTLM vs Gain Under Different LNA and PGA Gain Settings (Low Noise Mode)

Gain (dB)

VCNTLP–VCNTLM

LNA = 12 dB

LNA = 18 dB

LNA = 24 dB

LNA = 12 dB

LNA = 18 dB

LNA = 24 dB

(V)

PGA = 24 dB

PGA = 30 dB

36.45

42.45

48.45

42.25

48.25

54.25

0.1

33.91

39.91

45.91

39.71

45.71

51.71

0.2

30.78

36.78

42.78

36.58

42.58

48.58

0.3

27.39

33.39

39.39

33.19

39.19

45.19

0.4

23.74

29.74

35.74

29.54

35.54

41.54

0.5

20.69

26.69

32.69

26.49

32.49

38.49

0.6

17.11

23.11

29.11

22.91

28.91

34.91

0.7

13.54

19.54

25.54

19.34

25.34

31.34

0.8

10.27

16.27

22.27

16.07

22.07

28.07

0.9

6.48

12.48

18.48

12.28

18.28

24.28

1.0

3.16

9.16

15.16

8.96

14.96

20.96

1.1

–0.35

5.65

11.65

5.45

11.45

17.45

1.2

–2.48

3.52

9.52

3.32

9.32

15.32

1.3

–3.58

2.42

8.42

2.22

8.22

14.22

1.4

–4.01

1.99

7.99

1.79

7.79

13.79

1.5

–4

1.8

7.8

13.8

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