EVAL-AD73311LEB View Datasheet(PDF) - Analog Devices

Part Name

Description

MFG CO.

EVAL-AD73311LEB

Low Cost, Low Power CMOS General Purpose Analog Front End

Analog Devices 

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AD73311L

The dc biasing of the analog input signal is accomplished with

an on-chip voltage reference. If the input signal is not biased at

the internal reference level (via REFOUT), it must be ac-coupled

with external coupling capacitors. CIN should be 0.1 µF or larger.

The dc biasing of the input can then be accomplished using

resistors to REFOUT as in Figures 27 through 29.

If the ADC is being connected in single-ended mode, the

AD73311L should be programmed for single-ended mode using

the SEEN and INV bits of CRF, and the inputs connected as

shown in Figure 28. When operated in single-ended input mode,

the AD73311L can multiplex one of the two inputs to the ADC

input, as shown in Figures 28 and 29.

VINP

VINN

0/38dB

PGA

VREF

0.1␮F 100⍀

0.047

10k⍀

␮F

VINP

VINN

0/38dB

PGA

VREF

OPTIONAL

BUFFER

VOUTP

VOUTN

REFOUT

REFCAP

0.1␮F

+6/15dB

PGA

CONTINUOUS

TIME

LOW-PASS

FILTER

REFERENCE

AD73311L

Figure 26. Analog Input (DC-Coupled) Using External

Amplifiers

The AD73311L’s ADC inputs are biased about the internal

reference level (REFCAP level), therefore it may be neces-

sary to bias external signals to this level using the buffered

REFOUT level as the reference. This is applicable in either dc-

or ac-coupled configurations. In the case of dc coupling, the signal

(biased to REFOUT) may be applied directly to the inputs

(using amplifier bypass), as shown in Figure 25, or it may be

conditioned in an external op amp where it can also be biased

to the reference level using the buffered REFOUT signal as

shown in Figure 26.

In the case of ac-coupling, a capacitor is used to couple the

signal to the input of the ADC. The ADC input must be biased

to the internal reference (REFCAP) level which is done by

connecting the input to the REFOUT pin through a 10 kΩ

resistor as shown in Figure 27.

0.1␮F 100⍀

0.1␮F 10k⍀

100⍀

10k⍀

VINP

0.047␮F

VINN

0.047␮F

0/38dB

PGA

VREF

VOUTP

VOUTN

REFOUT

REFCAP

0.1␮F

+6/–15dB

PGA

CONTINUOUS

TIME

LOW-PASS

FILTER

REFERENCE

AD73311L

Figure 27. Analog Input (AC-Coupled) Differential

VOUTP

VOUTN

REFOUT

REFCAP

0.1␮F

+6/–15dB

PGA

CONTINUOUS

TIME

LOW-PASS

FILTER

REFERENCE

AD73311L

Figure 28. Analog Input (AC-Coupled) Single-Ended

VINP

0.1␮F 100⍀

0.047

10k⍀

␮F

VINN

0/38dB

PGA

VREF

VOUTP

VOUTN

REFOUT

REFCAP

0.1␮F

+6/15dB

PGA

CONTINUOUS

TIME

LOW-PASS

FILTER

REFERENCE

AD73311L

Figure 29. Analog Input (AC-Coupled) Single-Ended

(Alternate Input)

Interfacing to an Electret Microphone

Figure 30 details an interface for an electret microphone which

may be used in some voice applications. Electret microphones

typically feature a FET amplifier whose output is accessed on

the same lead that supplies power to the microphone; therefore,

this output signal must be capacitively coupled to remove the

power supply (dc) component. In this circuit the AD73311L

input channel is being used in single-ended mode where the

internal inverting amplifier provides suitable gain to scale the

input signal relative to the ADC’s full-scale input range. The

buffered internal reference level at REFOUT is used via an

external buffer to provide power to the electret microphone.

This provides a quiet, stable supply for the microphone. If this

is not a concern, the microphone can be powered from the

system power supply.

REV. A

–23–

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