AFE5808A datasheet(38/66 Pages) TI1

38 / 66 page

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

AFE5808A

SLOS729B – OCTOBER 2011 – REVISED APRIL 2012

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Figure 66. Simplified Voltage Controlled Attenuator (Analog Structure)

Figure 67. Simplified Voltage Controlled Attenuator (Digital Structure)

The voltage controlled attenuator’s noise follows a monotonic relationship to the attenuation coefficient. At 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 AFE5808A’s attenuator is designed for achieving 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

PROGRAMMABLE GAIN AMPLIFIER (PGA)

After the voltage controlled attenuator, a programmable gain amplifier can be configured as 24dB or 30dB with a

constant input referred noise of 1.75 nV/rtHz. The PGA structure consists of a differential voltage-to-current

converter with programmable gain, current clamp( bias control) circuits, a transimpedance amplifier with a

programmable low-pass filter, and a DC offset correction circuit. Its simplified block diagram is shown below:

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