THS1041

SLAS289C − OCTOBER 2001 − REVISED OCTOBER 2004

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PRINCIPLES OF OPERATION

sample-and-hold

Differential input signal sources can be connected directly to the AIN+ and AIN− pins using either dc- or

ac-coupling.

For single-ended sources, the signal can be dc- or ac-coupled to one of AIN+ or AIN−, and a suitable reference

voltage (usually the midscale voltage, see operating configuration examples) must be applied to the other pin.

Note that connecting the signal to AIN− results in it being inverted during sampling.

The sample and hold differential output voltage VP = VP+ − VP− is given by

VP = (AIN+) − (AIN−)

A clamp is available for dc restoration of ac-coupled single-ended inputs (see clamp operation).

programmable gain amplifier

VP is amplified by the PGA and fed to the ADC as a voltage VQ = VQ+ − VQ− where

VQ = Gain

× VP = Gain × [(AIN+) − (AIN−)]

analog-to-digital converter

VQ is digitized by the ADC, using the voltages at pins REFT and REFB to set the ADC zero-scale (code 0) and

full-scale (code 1023) input voltages.

VQ (ZS) = − (REFT − REFB)

VQ (FS) = (REFT − REFB)

Any inputs at AIN+ and AIN− that give VQ voltages less than VQ(ZS) or greater than VQ(FS) lie outside the

ADC’s conversion range and attempts to convert such voltages are signalled by driving pin OVR high when the

conversion result is output. VQ voltages less than VQ(ZS) digitize to give ADC output code 0, and VQ voltages

greater than VQ(FS) give ADC output code 1023.

complete system and system input range

Combining the above equations to find the input voltages [(AIN+) − (AIN−)] that correspond to the limits of the

ADC’s valid input range gives:

(REFB

* REFT)

Gain

v [(AIN)) * (AIN*)] v

(REFT

* REFB)

Gain

For both single-ended and differential inputs, the ADC can thus handle signals with a peak-to-peak input range

[(AIN+) − (AIN−)] of:

pk−pk input range

+ 2

(REFT

* REFB)

Gain

[(AIN+) − (AIN−)]

The next sections describe the options available to the user for setting the REFT and REFB voltages to obtain

the desired input range and performance in their THS1041 applications.

(1)

(2)

(3)

(4)

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