AFE5804ZCF datasheet(53/63 Pages) TI1

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LNA

PGA

Attenuator

VCNTL

AFE5804

www.ti.com

SBOS442C

– JUNE 2008 – REVISED OCTOBER 2011

VCA

—GAIN CONTROL

When the AFE5804 operates in CW mode, the

attenuator stage remains connected to the LNA

The attenuator (VCA) for each of the eight channels

outputs. Therefore, it is recommended to set the

of the AFE5804 is controlled by a single-ended

VCNTL voltage to +1.2V in order to minimize the

control signal input, the VCNTL pin. The control voltage

internal

the

LNA

outputs.

Small

range spans from 0V to 1.2V, referenced to ground.

improvements in reduced power dissipation and

This control voltage varies the attenuation of the VCA

improved

distortion

performance

may

also

based on its linear-in-dB characteristic with its

realized.

maximum attenuation (minimum gain) at VCNTL = 0V,

and minimum attenuation (maximum gain) at VCNTL =

1.2V. Table 19 shows the nominal gains for each of

the four PGA gain settings. The total gain range is

typically 46dB and remains constant independent of

the PGA selected; the Max Gain column reflects the

absolute gain of the full signal path comprised of the

fixed LNA gain of 20dB and the programmable PGA

gain.

Table 19. Nominal Gain Control Ranges for Each

of the Four PGA Gain Settings

MIN GAIN

MAX GAIN AT

PGA GAIN

AT VCNTL = 0V

VCNTL = 1.2V

20dB

–5.5dB

40.5dB

25dB

–1.0dB

45.0dB

27dB

1.0dB

47.0dB

30dB

3.0dB

49.0dB

As previously discussed, the VCA architecture uses

eight attenuator segments that are equally spaced in

Figure 103. External Filtering of the VCNTL Input

order to approximate the linear-in-dB gain-control

slope. This approximation results in a monotonic

slope; gain ripple is typically less than

±0.5dB.

CW DOPPLER PROCESSING

The

AFE5804

gain-control

input

has

–3dB

The AFE5804 integrates many of the elements

bandwidth

approximately

1.5MHz.

This

wide

necessary to allow for the implementation of a CW

bandwidth, although useful in many applications, can

doppler processing circuit, such as a V/I converter for

allow high-frequency noise to modulate the gain

each channel and a cross-point switch matrix with an

control input. In practice, this modulation can easily

8-input into 10-output (8

×10) configuration.

be avoided by additional external filtering (RF and CF)

of the control input, as Figure 103 shows. Stepping

In order to switch the AFE5804 from the default TGC

the control voltage from 0V to 1.2V, the gain control

mode operation into CW mode, bit D5 of the control

response time is typically less than 500ns to settle

within 10% of the final signal level of a 1VPP

enables access to all other registers that determine

(

–6dBFS) output.

the switch matrix configuration (see the Input Register

Bit Map tables). In order to process CW signals, the

The control voltage input (VCNTL pin) represents a

LNA internally feeds into a differential V/I amplifier

high-impedance input. Multiple AFE5804 devices can

stage. The transconductance of the V/I amplifier is

be connected in parallel with no significant loading

typically 13.5mA/V with a 100mVPP input signal. For

effects using the VCNTL pin of each device. Note that

proper operation, the CW outputs must be connected

when the VCNTL pin is left unconnected, it floats up to

to an external bias voltage of +2.5V. Each CW output

a potential of about +3.7V. For any voltage level

is designed to sink a small dc current of 0.9mA, and

above 1.2V and up to 5.0V, the VCA continues to

can deliver a signal current up to 2.9mAPP.

operate at its minimum attenuation level; however, it

is recommended to limit the voltage to approximately

1.5V or less.

AFE5804ZCF Datasheet(PDF) 53 Page - Texas Instruments

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