THS1041 Datasheet, PDF (24/37 Pages) Texas Instruments – 10-Bit, 40-MSPS ANALOG-TO-DIGITAL CONVERTER WITH PGA AND CLAMP

THS1041
SLAS289B – OCTOBER 2001 – REVISED FEBRUARY 2002
PRINCIPLES OF OPERATION
operating configuration examples (continued)
Figure 37 shows a configuration using external ADC references for digitizing a differential input with span 0.8 V.
To maximize the signal swing at the ADC core, the PGA gain is set to 2.5 to give a 2-V_p-p output from the PGA.
MODE is tied to ground to disable the internal reference buffer. The external ADC reference sources must set
REFT 1 V higher than REFB to set the ADC input span to 2 V_p-p, and the voltages provided by the external
sources must be centered near AV_DD/2 for best ADC operation. REFSENSE is shown tied to AV_DD to disable
the internal bandgap refence (A1), though other components in the system may use the VREF output if desired.
External ADC references are best suited to applications which require the tighter reference voltage tolerance
and temperature coefficient than the internal bandgap reference (A1) can provide, or where the references are
to be shared among several THS1041 ADCs for best matching of their ADC channels.
1.7 V
1.5 V
1.3 V
1.7 V
1.5 V
1.3 V
20 Ω
20 Ω
AIN+
20 pF
AIN–
20 pF
MODE
2V
10 µF
1V
REFSENSE
REFT
10 µF
0.1 µF
REFB
10 µF
AVDD
Figure 37. Operating Configuration: 0.8-V Differential Input and External ADC References
clamp operation
CLAMPIN
10-Bit
DAC
CLAMP
CLAMPOUT
Control Register (Bit CLINT)
+
_
V(Clamp)
CIN _RIN
SW1
VIN
S/H
AIN+
Figure 38. Schematic of Clamp Circuitry
The THS1041 provides a clamp function for restoring a dc reference level to the signal at AIN+ or AIN– which
has been lost through ac-coupling from the signal source to this pin.
Figure 38 and Figure 39 show 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 THS1041 clamp and control registers.
Clamp acquisition and clamp droop design calculations are discussed later.
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