TDC1044AR4C FAIRCHILD [Fairchild Semiconductor], TDC1044AR4C Datasheet - Page 9

• Manufacturer Part Number: TDC1044AR4C
• Description: Monolithic Video A/D Converter 4-Bit, 25 Msps
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

PRODUCT SPECIFICATION
Applications Discussion
Calibration
To calibrate the TDC1044A, adjust V
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 V
0001. Then apply -0.976V (1/2 LSB from -1.000V) and
adjust V
Instead of adjusting V
ground and the 0V end of the range calibrated with an ampli-
fier offset control. R
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 V
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
RB
for toggling between codes 1110 and 1111.
RT
for output toggling between codes 0000 and
B
RT
is a convenient point for gain adjust-
, R
T
can be connected to analog
IN
terminal of the device is
RT
and V
RB
to set the
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 V
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
R2
=
=
---------------------------------- -
æ
è
Z
R
2VR
----------- -
Z
IN
is the input voltage range of the circuit, Z
IN
–
ö
ø
æ
è
1
------------------------ -
1000
–
1000 R1
----------- -
1000
1
+
R1
ö
ø
TDC1044A
IN
is the
9