THS1040

SLAS290C − OCTOBER 2001 − REVISED OCTOBER 2004

16

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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.

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.

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

REFERENCE SELECTION

BIASREF PIN FUNCTION

AGND

External

High impedance

AVDD/2

Internal

High impedance

AVDD

Internal

AVDD/2 for AIN± bias

external reference mode (MODE = AGND)

Sample

and

Hold

X1

X−1

ADC

Core

AIN+

AIN−

Internal

Reference

Buffer

VREF

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

(REFT

) REFB)

2

+

AV

DD

2

(6)