AFE5808ZCF datasheet(48/68 Pages) TI1

48 / 68 page

S0498-01

CLAMP

AFE

LNAx

DCOffset

Correction

ACTx

ACT

BYPASS

INPx

INMx

Input

AFE5808

SLOS688C – SEPTEMBER 2010 – REVISED APRIL 2012

www.ti.com

A typical application circuit diagram is listed above. The configuration for each block is discussed below.

LNA CONFIGURATION

LNA Input Coupling and Decoupling

The LNA closed-loop architecture is internally compensated for maximum stability without the need of external

compensation components. The LNA inputs are biased at 2.4V and AC coupling is required. A typical input

configuration is shown in Figure 82. CIN is the input AC coupling capacitor. CACT is a part of the active termination

feedback path. Even if the active termination is not used, the CACT is required for the clamp functionality.

Recommended values for CACT ≥ 1µF and CIN are ≥ 0.1µF. A pair of clamping diodes is commonly placed

between the T/R switch and the LNA input. Schottky diodes with suitable forward drop voltage (e.g. the

BAT754/54 series, the BAS40 series, the MMBD7000 series, or similar) can be considered depending on the

transducer echo amplitude.

Figure 82. LNA Input Configurations

This architecture minimizes any loading of the signal source that may otherwise lead to a frequency-dependent

voltage divider. The closed-loop design yields very low offsets and offset drift. CBYPASS (≥0.015µF) is used to set

the high-pass filter cut-off frequency and decouple the complimentary input. Its cut-off frequency is inversely

proportional to the CBYPASS value, The HPF cut-off frequency can be adjusted through the register 59[3:2] a

Table 11 lists. Low frequency signals at T/R switch output, such as signals with slow ringing, can be filtered out.

In addition, the HPF can minimize system noise from DC-DC converters, pulse repetition frequency (PRF)

trigger, and frame clock. Most ultrasound systems’ signal processing unit includes digital high-pass filters or

band-pass filters (BPFs) in FPGAs or ASICs. Further noise suppression can be achieved in these blocks. In

addition, a digital HPF is available in the AFE5808 ADC. If low frequency signal detection is desired in some

applications, the LNA HPF can be disabled.

Table 11. LNA HPF Settings (CBYPASS = 15 nF)

Reg59[3:2] (0x3B[3:2])

Frequency

100 KHz

50 KHz

200 KHz

150 KHz

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