THS1040 Datasheet, PDF (16/32 Pages) Texas Instruments – 3-V, 10-BIT , 40-MSPS CMOS ANALOG TO DIGITAL CONVERTER

THS1040
SLAS290C − OCTOBER 2001 − REVISED OCTOBER 2004
PRINCIPLES OF OPERATION
ADC reference generation
The THS1040 ADC references REFT and REFB can be driven from external (off-chip) sources or from the
internal (on-chip) reference buffer A2. The voltage at the MODE pin determines the ADC references source.
Connecting MODE to AGND enables external ADC references mode. In this mode the internal buffer A2 is
powered down and the user must provide the REFT and REFB voltages by connecting external sources directly
to these pins. This mode is useful where several THS1040 devices must share common references for best
matching of their ADC input ranges, or when the application requires better accuracy and temperature stability
than the on-chip reference source can provide.
Connecting MODE to AV_DD or AV_DD/2 enables internal ADC references mode. In this mode the buffer A2 is
powered up and drives the REFT and REFB pins. External reference sources should not be connected in this
mode. Using internal ADC references mode when possible helps to reduce the component count and hence
the system cost.
When MODE is connected to AV_DD, a buffered AV_DD/2 voltage is available at the BIASREF pin. This voltage
can be used as a dc bias level for any ac-coupling networks connecting the input signal sources to the AIN+
and AIN− pins.
MODE PIN
AGND
AVDD/2
AVDD
REFERENCE SELECTION
External
Internal
Internal
BIASREF PIN FUNCTION
High impedance
High impedance
AVDD/2 for AIN± bias
external reference mode (MODE = AGND)
AIN+
AIN−
X1 ^Sample
and
X−1 _Hold
ADC
Core
VREF
Internal
Reference
Buffer
REFT
REFB
Figure 21. ADC Reference Generation, MODE = AGND
Connecting pin MODE to AGND powers down the internal references buffer A2 and disconnects its outputs from
the REFT and REFB pins. The user must connect REFT and REFB to external sources to provide the ADC
reference voltages required to match the THS1040 input range to their application requirements. The
common-mode reference voltage must be AV_DD/2 for correct THS1040 operation:
(REFT
)
2
REFB)
+
AVDD
2
(6)
16
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