Datasheet
R
S
Q
1
V
B
Q
2
Q
3
Q
4
Q
5
Q
6
Q
7
VHIGH
Attenuator
Output
Attenuator
Input
SW1 SW2 SW3
SW4
SW5 SW6 SW7
R
S
Control
Input
Q
1
V
B
Q
2
Q
3
C
1
V1
Q
4
Q
5
Q
6
Q
7
C
2
V2
C
3
V3
C
4
V4
C
5
V5
C
6
V6
C
7
V7
A1
VCNTL
A1 A1 A1 A1 A1 A1
Attenuator
Output
C - C Clipping Amplifiers
1 8
A1 - A7 Attenuator Stages
Attenuator
Input
AFE5808A
SLOS729B –OCTOBER 2011–REVISED APRIL 2012
www.ti.com
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 10
–30 9
–24 8.5
–18 6
–12 4
–6 3
0 2
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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