AFE5808A datasheet(51/66 Pages) TI1

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W0004-01

(a) Single-Ended Input at V

CNTLP

(b) Differential Inputs at V

and V

CNTLP

CNTLM

TGC Gain

= 0V

CNTLM

CNTLP

1.5V

XdB

X+40dB

TGC Gain

CNTLM

CNTLP

XdB

X+40dB

1.5V

0.75V

AFE5808A

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SLOS729B – OCTOBER 2011 – REVISED APRIL 2012

LNA Gain Switch Response

The LNA gain is programmable through SPI. The gain switching time depends on the SPI speed as well as the

LNA gain response time. During the switching, glitches might occur and they can appear as artifacts in images.

LNA gain switching in a single imaging line may not be preferred, although digital signal processing might be

used here for glitch suppression.

VOLTAGE-CONTROLLED-ATTENUATOR

The attenuator in the AFE5808A is controlled by a pair of differential control inputs, the VCNTLM/P pins. The

differential control voltage spans from 0 V to 1.5 V. This control voltage varies the attenuation of the attenuator

based on its linear-in-dB characteristic. Its maximum attenuation (minimum channel gain) appears at VCNTLP -

VCNTLM= 1.5 V, and minimum attenuation (maximum channel gain) occurs at VCNTLP-VCNTLM = 0. The typical gain

range is 40 dB and remains constant, independent of the PGA setting.

When only single-ended VCNTL signal is available, this 1.5Vpp signal can be applied on the VCNTLP pin with the

VCNTLM pin connected to ground. As shown in Figure 86, TGC gain curve is inversely proportional to the VCNTLP-

VCNTLM.

Figure 86. VCNTLP and VCNTLM Configurations

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