AFE5808ZCF datasheet(38/68 Pages) TI1

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Control

Input

VCNTL

Attenuator

Output

C - C Clipping Amplifiers

A1 - A7 Attenuator Stages

Attenuator

Input

VHIGH

Attenuator

Output

Attenuator

Input

SW1

SW2

SW3

SW4

SW5

SW6

SW7

AFE5808

SLOS688C – SEPTEMBER 2010 – REVISED APRIL 2012

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

AFE5808. 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.

Figure 65. Simplified Voltage Controlled Attenuator (Analog Structure)

Figure 66. Simplified Voltage Controlled Attenuator (Digital Structure)

The voltage controlled attenuator’s noise follows a monotonic relationship to the attenuation coefficient. AAt

higher attenuation, the input-referred noise is higher and vice-versa. The attenuator’s noise is then amplified by

the PGA and becomes the noise floor at ADC input. In the attenuator’s high attenuation operating range, i.e.

VCNTL is high, the attenuator’s input noise may exceed the LNA’s output noise; the attenuator then becomes the

dominant noise source for the following PGA stage and ADC. Therefore the attenuator’s noise should be

minimized compared to the LNA output noise. The AFE5808’s attenuator is designed for achieving very low

noise even at high attenuation (low channel gain) and realizing better SNR in near field. The input referred noise

for different attenuations is listed in the below table:

Table 10. Voltage-Controlled-Attenuator noise vs Attenuation

Attenuation (dB)

Attenuator Input Referred noise (nV/rtHz)

–40

10.5

–36

–30

–24

8.5

–18

–12

–6

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