THS1031IPW Datasheet, PDF (14/41 Pages) Texas Instruments – 3-V TO 5.5-V, 10-BIT, 30 MSPS CMOS ANALOG-TO-DIGITAL CONVERTER

THS1031
3-V TO 5.5-V, 10-BIT, 30 MSPS
CMOS ANALOG-TO-DIGITAL CONVERTER
SLAS242E – NOVEMBER 1999 – REVISED MARCH 2002
PRINCIPLES OF OPERATION
sample-and-hold
The analog input signal A_IN is applied to the AIN pin, either dc-coupled, ac-coupled, or ac-coupled with dc
restoration using the THS1031 clamp circuit.
The differential sample and hold processes A_IN with respect to the voltages applied to the REFTS and REFBS
pins, to give a differential output VP+ – VP– = VP given by:
VP = A_IN – VM
(1)
Where:
+ ⁾ VM
(REFTS REFBS)
2
(2)
For single-ended input signals, VM is a constant voltage; usually the AIN mid-scale input voltage. However if
MODE = AV_DD/2 then REFTS and REFBS can be connected together to operate with AIN as a complementary
pair of differential inputs (see Figures 15 and 16).
programmable gain amplifier
VP is amplified by the PGA and fed into the ADC as a differential voltage VQ⁺ – VQ^– = VQ
VQ = Gain × VP = Gain × [A_IN – VM]
The default PGA gain at power up is 1, but can be programmed from 0.5 to 4.0 via the control register.
analog-to-digital converter
In all operating configurations, VQ is digitized against ADC reference voltages REFTF and REFBF, full-scale
values of VQ being given by
)+ ^{) *} VQFS
(REFTF REFBF)
2
(3)
*+ ^{* *} VQFS
(REFTF REFBF)
2
VQ voltages outside the range VQFS– to VQFS+ lie outside the conversion range of the ADC. Attempts to
convert out-of-range inputs are signalled to the application by driving the OVR output pin high. VQ voltages less
than VQFS – give ADC output code 0. VQ voltages greater than VQFS+ give output code 1023.
complete system
Combining equations 1 to 3, the analog full-scale input voltages at AIN which give VQFS+ and VQFS– at the
PGA output are:
+ )+ ) ^* AIN FS
VM
(REFTF REFBF)
(2 Gain)
(4)
and
+ *+ * ^* AIN FS
VM
(REFTF REFBF)
(2 Gain)
(5)
The analog input span (voltage range) that lies within the ADC conversion range is:
+ ) * * + * ń Input span [(FS ) (FS )] (REFTF REFBF) Gain
(6)
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