AFE5804 datasheet(55/60 Pages) TI

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REFERENCE CIRCUIT

VREFT=1.5V+

VCM

1.5V

(3)

VREFB=1.5V -

VCM

1.5V

(4)

REFT

REFB

ISET

0.1 F

2.2 F

56.2kW

2.2 F

0.1 F

AFE5804

www.ti.com ................................................................................................................................................ SBOS442A – JUNE 2008 – REVISED SEPTEMBER 2008

The device also supports the use of external

reference voltages. There are two methods to force

The digital beam-forming algorithm in an ultrasound

the references externally. The first method involves

system relies on gain matching across all receiver

pulling INT/EXT low and forcing externally REFT and

channels. A typical system would have about 12 octal

REFB to 2.5V and 0.5V nominally, respectively. In

AFEs on the board. In such a case, it is critical to

this mode, the internal reference buffer goes to a

ensure that the gain is matched, essentially requiring

3-state output. The external reference driving circuit

the reference voltages seen by all the AFEs to be the

should be designed to provide the required switching

same. Matching references within the eight channels

current for the eight ADCs inside the AFE5804. It

of a chip is done by using a single internal reference

should be noted that in this mode, VCM and ISET

voltage buffer. Trimming the reference voltages on

continue to be generated from the internal bandgap

each

chip

during

production

ensures

that

the

voltage, as in the internal reference mode. It is

reference voltages are well-matched across different

therefore important to ensure that the common-mode

chips.

voltage of the externally-forced reference voltages

matches to within 50mV of VCM.

All bias currents required for the internal operation of

the device are set using an external resistor to

The second method of forcing the reference voltages

ground at the ISET pin. Using a 56k

Ω resistor on

externally can be accessed by pulling INT/EXT low,

ISET generates an internal reference current of 20

µA.

and programming the serial interface to drive the

This current is mirrored internally to generate the bias

external reference mode through the VCM pin

current for the internal blocks. Using a larger external

(register bit called EXT_REF_VCM). In this mode,

resistor at ISET reduces the reference bias current

VCM becomes configured as an input pin that can be

and thereby scales down the device operating power.

driven from external circuitry. The internal reference

However, it is recommended that the external resistor

buffers driving REFT and REFB are active in this

be within 10% of the specified value of 56k

Ω so that

mode. Forcing 1.5V on the VCM pin in the mode

the internal bias margins for the various blocks are

results in REFT and REFB coming to 2.5V and 0.5V,

proper.

respectively. In general, the voltages on REFT and

REFB in this mode are given by Equation 3 and

Buffering the internal bandgap voltage also generates

Equation 4:

the common-mode voltage VCM, which is set to the

midlevel of REFT and REFB. It is meant as a

reference voltage to derive the input common-mode if

the input is directly coupled. It can also be used to

derive the reference common-mode voltage in the

external reference mode. Figure 107 shows the

suggested decoupling for the reference pins.

The state of the reference voltage internal buffers

during various combinations of the PD, INT/EXT, and

EXT_REF_VCM register bits is described in Table 19.

Figure 107. Suggested Decoupling on the Reference Pins

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