THS1031CPWR datasheet(32/41 Pages) TI

32 / 41 page

THS1031

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

CMOS ANALOG-TO-DIGITAL CONVERTER

SLAS242E – NOVEMBER 1999 – REVISED MARCH 2002

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

PRINCIPLES OF OPERATION

driving REFTF and REFBF (full external reference mode)

AVDD

AGND

AVDD

AGND

680

Ω

To REFBS

(For Kelvin Connection)

To REFTS

(For Kelvin Connection)

REFTF

REFBF

Figure 38. Equivalent Circuit of REFTF and REFBF Inputs

Note the need for off-chip decoupling.

clamp operation

The clamp voltage output level may be established by an analog voltage on the CLAMPIN pin or by

programming the on-chip clamp DAC.

clamp acquisition time

Figure 39 shows the basic operation of the clamp circuit with the analog input AIN coupled via an RC circuit.

10-Bit

DAC

CLAMPIN

CLAMP

AIN

RIN

CIN

VCLAMP

Control Register

S/H

VIN

SW1

Figure 39. Schematic of Clamp Circuitry

After powerup, the clamp circuit requires SW1 to be closed to charge the coupling capacitor, CIN, to the voltage

required to set the dc clamp level at AIN. The charging time required to set the correct clamp voltage is called

the clamp acquisition time, tACQ:

ACQ +

Vc is the difference between the dc bias voltage level of the input signal, VIN, and the target clamp output voltage,

V(clamp). Ve is the difference between the ideal Vc and the actual Vc obtained during the acquisition time. The

maximum tolerable error depends on the application requirements.

(19)

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