AFE5808A datasheet(37/66 Pages) TI1

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LNAx

INPx

INMx

ACTx

INPUT

AFE

ACT

BYPSS

CLAMP

DC Offset

Correction

AFE5808A

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SLOS729B – OCTOBER 2011 – REVISED APRIL 2012

The AFE5808A can be terminated passively or actively. Active termination is preferred in ultrasound application

for reducing reflection from mismatches and achieving better axial resolution without degrading noise figure too

much. Active termination values can be preset to 50, 100, 200, 400

Ω; other values also can be programmed by

users through register 52[4:0]. A feedback capacitor is required between ACTx and the signal source as

Figure 65 shows. On the active termination path, a clamping circuit is also used to create a low impedance path

when overload signal is seen by the AFE5808A. The clamp circuit limits large input signals at the LNA inputs and

improves the overload recovery performance of the AFE5808A. The clamp level can be set to 350mVpp,

600mVpp, 1.15Vpp automatically depending on the LNA gain settings when register 52[10:9]=0. Other clamp

voltages, such as 1.15Vpp, 0.6Vpp, and 1.5Vpp, are also achievable by setting register 52[10:9]. This clamping

circuit is also designed to obtain good pulse inversion performance and reduce the impact from asymmetric

inputs.

Figure 65. AFE5808A LNA with DC Offset Correction Circuit

VOLTAGE-CONTROLLED ATTENUATOR

The voltage-controlled attenuator is designed to have a linear-in-dB attenuation characteristic; that is, the

average gain loss in dB (refer to Figure 2) is constant for each equal increment of the control voltage (VCNTL) as

shown in Figure 66. A differential control structure is used to reduce common mode noise. A simplified attenuator

structure is shown in the following Figure 66 and Figure 67.

The attenuator is essentially a variable voltage divider that consists of the series input resistor (RS) and seven

shunt FETs placed in parallel and controlled by sequentially activated clipping amplifiers (A1 through A7). VCNTL

is the effective difference between VCNTLP and VCNTLM. Each clipping amplifier can be understood as a

specialized voltage comparator with a soft transfer characteristic and well-controlled output limit voltage.

Reference voltages V1 through V7 are equally spaced over the 0V to 1.5Vcontrol voltage range. As the control

voltage increases through the input range of each clipping amplifier, the amplifier output rises from a voltage

where the FET is nearly OFF to VHIGH where the FET is completely ON. As each FET approaches its ON state

and the control voltage continues to rise, the next clipping amplifier/FET combination takes over for the next

portion of the piecewise-linear attenuation characteristic. Thus, low control voltages have most of the FETs

turned OFF, producing minimum signal attenuation. Similarly, high control voltages turn the FETs ON, leading to

maximum signal attenuation. Therefore, each FET acts to decrease the shunt resistance of the voltage divider

formed by Rs and the parallel FET network.

Additionally, a digitally controlled TGC mode is implemented to achieve better phase-noise performance in the

AFE5808A. The attenuator can be controlled digitally instead of the analog control voltage VCNTL. This mode can

be set by the register bit 59[7]. The variable voltage divider is implemented as a fixed series resistance and FET

as the shunt resistance. Each FET can be turned ON by connecting the switches SW1-7. Turning on each of the

switches can give approximately 6dB of attenuation. This can be controlled by the register bits 59[6:4]. This

digital control feature can eliminate the noise from the VCNTL circuit and ensure the better SNR and phase noise

for TGC path.

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