AFE5805 datasheet(39/53 Pages) TI2

39 / 53 page

Clamp

Gain

Control

Bits

From

Attenuator

Clamp

Control

Bit

Low-Pass

Filter

PGA

VCM

(+1.65V)

ToADC

Inputs

AFE5805

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SBOS421D – MARCH 2008 – REVISED MARCH 2010

PROGRAMMABLE POST-GAIN AMPLIFIER

(PGA)

PROGRAMMABLE CLAMPING

Following the VCA is a programmable post-gain

amplifier

(PGA).

Figure

shows

simplified

To further optimize the overload recovery behavior of

schematic of the PGA, including the clamping stage.

a complete TGC channel, the AFE5805 integrates a

The gain of this PGA can be configured to four

programmable

clamping

stage,

shown

different gain settings: 20dB, 25dB, 27dB, and 30dB,

Figure

49.

This

clamping

stage

precedes

the

programmable through the serial port; see Table 10.

low-pass filter in order to prevent the filter circuit from

being driven into overload, the result of which would

The

PGA

structure

consists

differential,

extended

recovery

time.

Programmable

programmable-gain

voltage-to-current

converter

through the serial interface, the clamping level can be

stage followed by transimpedance amplifiers to buffer

either set to clamp the signal level to approximately

each side of the differential output. Low input noise is

1.7VPP differential, or be disabled. Disabling the

also a requirement for the PGA design as a result of

clamp function increases the current consumption on

the large amount of signal attenuation that can be

the 3.3V analog supply (AVDD2) by about 3mA for

applied in the preceding VCA stage. At minimum

the full device. Note that with the clamp function

VCA attenuation (used for small input signals), the

enabled, the third-harmonic distortion increases.

LNA noise dominates; at maximum VCA attenuation

(large input signals), the attenuator and PGA noise

LOW-PASS FILTER

dominate.

The AFE5805 integrates an anti-aliasing filter in the

form of a programmable low-pass filter (LPF) for each

channel. The LPF is designed as a differential, active,

second-order

filter

that

approximates

Bessel

characteristic, with typically 12dB per octave roll-off.

Figure 49 shows the simplified schematic of half the

differential

active

low-pass

filter.

Programmable

through the serial interface, the –3dB frequency

corner can be set to either 10MHz or 15MHz. The

filter bandwidth is set for all channels simultaneously.

Figure 48. Post-Gain Amplifier

(Simplified Schematic)

Figure 49. Clamping Stage and Low-Pass Filter (Simplified Schematic)

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