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TDC1044A Datasheet(PDF) 9 Page - Fairchild Semiconductor |
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TDC1044A Datasheet(HTML) 9 Page - Fairchild Semiconductor |
9 / 16 page PRODUCT SPECIFICATION TDC1044A 9 Applications Discussion Calibration To calibrate the TDC1044A, adjust VRT and VRB to set the 1st and 15th thresholds to the desired voltages. Assuming a 0V to -1V desired range, continuously strobe the converter with -0.0033V (1/2 LSB from 0.000V) on the analog input, and adjust VRT for output toggling between codes 0000 and 0001. Then apply -0.976V (1/2 LSB from -1.000V) and adjust VRB for toggling between codes 1110 and 1111. Instead of adjusting VRT, RT can be connected to analog ground and the 0V end of the range calibrated with an ampli- fier offset control. RB is a convenient point for gain adjust- ment that is not in the analog signal path. Typical Interface Circuit The TDC1044A does not require a special input buffer amplifier to drive the analog input because of its low input capacitance. A terminated low-impedance transmission line (<100 Ohms) connected to the VIN terminal of the device is sufficient if the input voltage levels match those of the A/D converter. However, many driver circuits lack sufficient offset control, drive current, or gain stability. The typical interface circuit in Figure 4 shows a simple amplifier and voltage reference cir- cuit that may be used with the device. U2 is a wide-band operational amplifier with a gain factor of -1. As the video input increases from zero to one volt, VIN of the TDC1044A decreases from zero to -1 volt. With true binary selected (NMINV = 1 and NLINV = 1), output codes increase from 0000 to 1111. A small value resistor, R12, serves to isolate the small input capacitance of the A/D converter from the amplifier output and insure frequency stability. Pulse and frequency response of the amplifier are optimized by variable capacitor C12. The reference voltage for the TDC1044A is generated by ampli- fier U3. System gain is adjusted by varying R9, which con- trols the reference voltage level to the A/D converter. Input voltage range and input impedance for the circuit are determined by resistors R1 and R2. Formulas for calculating values for these input resistors are: and where VR is the input voltage range of the circuit, ZIN is the input impedance of the circuit, and the constant 1000 comes from the value of R3. As shown, the circuit is set up for 1Vp-p 75 Ohm video input. R1 1 2VR Z IN ------------ èø æö 1 1000 ------------ – ----------------------------------- = R2 Z IN 1000 R1 1000 R1 + ------------------------- èø æö – = |
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