THS1031

3-V TO 5.5-V, 10-BIT, 30 MSPS

CMOS ANALOG-TO-DIGITAL CONVERTER

SLAS242E – NOVEMBER 1999 – REVISED MARCH 2002

24

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

operating configuration examples (continued)

Figure 27 shows an example of top/bottom mode operation on an input span of 800 mV with mid-scale value

1.5 V. Pin REFTS is set to 2.5 V and pin REFBS to 0.5 V, making their average value equal to the mid-scale

full-scale range of the ADC core for best resolution. The PGA gain then has to be set to 2.5, to amplify the

AIN

REFTS

REFBS

REFTF

REFBF

MODE

AVDD

1.9 V

1.1 V

0.1

µF

10

µF

0.1

µF

0.1

µF

1.5 V

DC SOURCE = 2.5 V

DC SOURCE = 0.5 V

REFSENSE

Figure 27. Top/Bottom Mode, PGA Gain 2.5

clamp operation

10-Bit

DAC

_

+

CLAMPIN

CLAMP

AIN

RIN

CIN

V(Clamp)

Control Register (Bit CLINT)

S/H

VIN

SW1

Figure 28. Schematic of Clamp Circuitry

The THS1031 provides a clamp function for restoring a dc reference level to the signal at AIN which has been

lost through ac-coupling from the signal source to this pin.

Figure 29 shows an example of using the clamp to restore the black level of a composite video input ac coupled

to AIN. While the clamp pin is held high, the clamp amplifier forces the voltage at AIN to equal the clamp

reference voltage, setting the dc voltage at AIN for the video black level.

After power up, the clamp reference voltage is the voltage on the CLAMPIN pin. This reference can instead be

taken from the internal CLAMP DAC by suitably programming the THS1031 clamp and control registers.

Clamp acquisition and clamp droop design calculations are discussed later.