AFE5808A datasheet(53/66 Pages) TI1

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INT /EXT

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AFE5808A

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SLOS729B – OCTOBER 2011 – REVISED APRIL 2012

CW OPERATION

CW Summing Amplifier

In order to simplify CW system design, a summing amplifier is implemented in the AFE5808A to sum and convert

8-channel mixer current outputs to a differential voltage output. Low noise and low power are achieved in the

summing amplifier while maintaining the full dynamic range required in CW operation.

This summing amplifier has 5 internal gain adjustment resistors which can provide 32 different gain settings

(register 54[4:0], Figure 85 and Table 7). System designers can easily adjust the CW path gain depending on

signal strength and transducer sensitivity. For any other gain values, an external resistor option is supported. The

gain of the summation amplifier is determined by the ratio between the 500

Ω resistors after LNA and the internal

or external resistor network REXT/INT. Thus the matching between these resistors plays a more important role than

absolute resistor values. Better than 1% matching is achieved on chip. Due to process variation, the absolute

resistor tolerance could be higher. If external resistors are used, the gain error between I/Q channels or among

multiple AFEs may increase. It is recommended to use internal resistors to set the gain in order to achieve better

gain matching (across channels and multiple AFEs). With the external capacitor CEXT, this summing amplifier has

1st order LPF response to remove high frequency components from the mixers, such as 2f0±fd. Its cut-off

frequency is determined by:

(8)

Note that when different gain is configured through register 54[4:0], the LPF response varies as well.

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AFE5808A Datasheet(PDF) 53 Page - Texas Instruments