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Digital Offset

Default=No

ADC

Output 12/14b

Final

Digital

Output

12/14b

Digital

Gain

Default=0

Channel

Average

Default=No

Digital HPF

Default = No

( )

( ) (

) (

)

y n =

x n

x n 1

y n 1

- +

AFE5808A

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SLOS729B – OCTOBER 2011 – REVISED APRIL 2012

ADC Register/Digital Processing Description

The ADC in the AFE5808A has extensive digital processing functionalities which can be used to enhance

ultrasound system performance. The digital processing blocks are arranged as in Figure 63.

Figure 63. ADC Digital Block Diagram

AVERAGING_ENABLE: Address: 2[11]

When set to 1, two samples, corresponding to two consecutive channels, are averaged (channel 1 with 2, 3 with

4, 5 with 6, and 7 with 8). If both channels receive the same input, the net effect is an improvement in SNR. The

averaging is performed as:

•

Channel 1 + channel 2 comes out on channel 3

•

Channel 3 + channel 4 comes out on channel 4

•

Channel 5 + channel 6 comes out on channel 5

•

Channel 7 + channel 8 comes out on channel 6

ADC_OUTPUT_FORMAT: Address: 4[3]

The ADC output, by default, is in 2’s-complement mode. Programming the ADC_OUTPUT_FORMAT bit to 1

inverts the MSB, and the output becomes straight-offset binary mode.

DIGITAL_GAIN_ENABLE: Address: 3[12]

Setting this bit to 1 applies to each channel i the corresponding gain given by DIGTAL_GAIN_CHi <15:11>. The

gain is given as 0dB + 0.2dB × DIGTAL_GAIN_CHi<15:11>. For instance, if DIGTAL_GAIN_CH5<15:11> = 3,

channel 5 is increased by 0.6dB gain. DIGTAL_GAIN_CHi <15:11> = 31 produces the same effect as

DIGTAL_GAIN_CHi <15:11> = 30, setting the gain of channel i to 6dB.

DIGITAL_HPF_ENABLE

•

CH1-4: Address 21[0]

•

CH5-8: Address 33[0]

DIGITAL_HPF_FILTER_K_CHX

•

CH1-4: Address 21[4:1]

•

CH5-8: Address 3[4:1]

This group of registers controls the characteristics of a digital high-pass transfer function applied to the output

data, following Equation 1.

(1)

These digital HPF registers (one for the first four channels and one for the second group of four channels)

describe the setting of K. The digital high pass filter can be used to suppress low frequency noise which

commonly exists in ultrasound echo signals. The digital filter can significantly benefit near field recovery time due

to T/R switch low frequency response. Table 3 shows the cut-off frequency vs K, also see Figure 58.

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