AFE5805_1 Datasheet, PDF (46/51 Pages) Texas Instruments – FULLY-INTEGRATED, 8-CHANNEL ANALOG FRONT-END FOR ULTRASOUND 0.85nV/√Hz, 12-Bit, 50MSPS, 122mW/Channel

AFE5805
SBOS421C – MARCH 2008 – REVISED OCTOBER 2008................................................................................................................................................. www.ti.com
REFERENCE CIRCUIT
The digital beam-forming algorithm in an ultrasound
system relies on gain matching across all receiver
channels. A typical system would have about 12 octal
AFEs on the board. In such a case, it is critical to
ensure that the gain is matched, essentially requiring
the reference voltages seen by all the AFEs to be the
same. Matching references within the eight channels
of a chip is done by using a single internal reference
voltage buffer. Trimming the reference voltages on
each chip during production ensures that the
reference voltages are well-matched across different
chips.
All bias currents required for the internal operation of
the device are set using an external resistor to
ground at the ISET pin. Using a 56kΩ resistor on
ISET generates an internal reference current of 20µA.
This current is mirrored internally to generate the bias
current for the internal blocks. Using a larger external
resistor at ISET reduces the reference bias current
and thereby scales down the device operating power.
However, it is recommended that the external resistor
be within 10% of the specified value of 56kΩ so that
the internal bias margins for the various blocks are
proper.
Buffering the internal bandgap voltage also generates
the common-mode voltage V_CM, 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 58 shows the
suggested decoupling for the reference pins.
The device also supports the use of external
reference voltages. There are two methods to force
the references externally. The first method involves
pulling INT/EXT low and forcing externally REFT and
REFB to 2.5V and 0.5V nominally, respectively. In
this mode, the internal reference buffer goes to a
3-state output. The external reference driving circuit
should be designed to provide the required switching
current for the eight ADCs inside the AFE5805. It
should be noted that in this mode, CM and ISET
continue to be generated from the internal bandgap
voltage, as in the internal reference mode. It is
therefore important to ensure that the common-mode
voltage of the externally-forced reference voltages
matches to within 50mV of V_CM.
The second method of forcing the reference voltages
externally can be accessed by pulling INT/EXT low,
and programming the serial interface to drive the
external reference mode through the CM pin (register
bit called EXT_REF_VCM). In this mode, CM
becomes configured as an input pin that can be
driven from external circuitry. The internal reference
buffers driving REFT and REFB are active in this
mode. Forcing 1.5V on the CM pin in the mode
results in REFT and REFB coming to 2.5V and 0.5V,
respectively. In general, the voltages on REFT and
REFB in this mode are given by Equation 3 and
Equation 4:
VREFT = 1.5V + ^VCM
1.5V
(3)
VREFB = 1.5V - ^VCM
1.5V
(4)
The state of the reference voltage internal buffers
during various combinations of the ADS_PD,
INT/EXT, and EXT_REF_VCM register bits is
described in Table 18.
AFE5805
REFT
ISET
REFB
56.2kW
+
+
0.1mF 2.2mF
2.2mF
0.1mF
Figure 58. Suggested Decoupling on the Reference Pins
46
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