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
AFE5808
SLOS688C –SEPTEMBER 2010–REVISED APRIL 2012 www.ti.com
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 10
–30 9
–24 8.5
–18 6
–12 4
–6 3
0 2
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