AFE5803ZCF datasheet(43/52 Pages) TI

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AFE5803

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SLOS763A

– JANUARY 2012 – REVISED JANUARY 2012

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

±0.5 dB.

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

AFE5803 devices can be connected in parallel with no significant loading effects. When the voltage level (VCNTLP

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

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

The AFE5803 gain-control input has a

–3 dB 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 be avoided by additional

external filtering (RVCNTL and CVCNTL) at VCNTLM/P pins as Figure 69 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 (–6 dBFS)

output as indicated in Figure 48 and Figure 49.

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 (10 MSPS/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 documents SLOS318

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 AFE5803. In the digital VCNTL

mode, no external VCNTL is needed.

Table 10. 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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