THS1041_15 Datasheet, PDF (17/42 Pages) Texas Instruments – 10-Bit, 40-MSPS ANALOG-TO-DIGITAL CONVERTER WITH PGA AND CLAMP

THS1041
SLAS289C − OCTOBER 2001 − REVISED OCTOBER 2004
PRINCIPLES OF OPERATION
ADC reference generation
The THS1041 ADC references REFT and REFB can be driven from external (off-chip) sources or from the
internal A2 reference buffer. 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 THS1041 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 also available at the CLAMPOUT 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
CLAMPOUT PIN FUNCTION
Clamp
Clamp
AVDD/2 for AIN± bias
external reference mode (MODE = AGND)
AIN+
AIN−
X1 ^Sample
and
X−1 _Hold
PGA
ADC
Core
VREF
Internal
Reference
Buffer
REFT
REFB
Figure 26. 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 THS1041 input range to their application requirements. The
common-mode reference voltage must be AV_DD/2 for correct THS1041 operation:
(REFT
)
2
REFB)
+
AVDD
2
(7)
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