®
®
ADCDS-1410
14-Bit, 10 Megapixels/Second CCD Image Converter
Offset Adjustment
For Example:
Manual offset adjustment for the ADCDS-1410 can be accomplished
using the adjustment circuit shown in Figure 3. A software controlled
D/A converter can be substituted for the 20KΩ potentiometer. The offset
adjustment feature allows the user to adjust the Offset/Dark Current level
of the ADCDS-1410 until the output bits are 00 0000 0000 0000 and the
LSB flickers between 0 and 1. Offset adjust should be performed before
gain adjust to avoid interaction. The ADCDS-1410's offset adjustment is
dependent on the value of the external series resistor used in the offset
adjust circuit (Figure 3). The Offset Adjustment graph (Figure ꢀ) illustrates
the typical relationship between the external series resistor value and its
offset adjustment capability utilizing 5V supplies.
External 50KΩ resistor:
1. 10mV of noise or voltage variation at the potentiometer
will produce 0.25LSB's of output variation.
2. 100mV of noise or voltage variation at the potentiometer
will produce 2.5LSB's of output variation.
The Offset Adjustment Sensitivity graph (Figure 7) illustrates the offset
adjustment sensitivity over a wide range of external resistor and noise
values. If a large offset voltage is required, it is recommended that a very
low noise external reference be used in the offset adjust circuit in place
of power supplies. The ADCDS-1410's +2.4V reference output could be
configured to provide the reference voltage for this type of application.
Fine Gain Adjustment
OFFSET ADJUSTMENT SENSITIVITY
Fine gain adjustment is provided to compensate for the tolerance of the
external coarse gain resistor (Rext) and/or the unavailability of exact
coarse gain resistor (Rext) values. Note, the fine gain adjustment will not
change the expected input amplifier's full scale VOUT (2.8Vp-p.) Instead,
the gain of the ADCDS-1410's internal A/D is adjusted allowing the actual
input amplifier's full scale VOUT to produce an output code of all ones (11
1111 1111 1111).
It should be noted that with increasing amounts of offset adjustment
(smaller values of external series resistors), the ADCDS-1410 becomes
more susceptible to power supply noise or voltage variations seen at the
wiper of the offset potentiometer.
ADCDS-1410
DAC
Fine gain adjustment for the ADCDS-1410 is accomplished using the
adjustment circuit shown below (Figure 5). A software controlled D/A
converter can be substituted for the 20KΩ potentiometer. The fine gain
adjust circuit ensures that the video input signal (saturated signal) will be
properly scaled to obtain the desired Full Scale digital output of 11 1111
1111 1111, with the LSB flickering between 0 and 1. Fine gain adjust
should be performed following the offset adjust to avoid interaction. The
fine gain adjust provides 25ꢀ codes of adjust when 5V supplies are
used for the Fine Gain Adjust Circuit.
or
Fine
Gain
Adjust
1
+5V
100
Ω
20K
–5V
Figure 5. Fine Gain Adjustment Circuit
Offset Adjustment Sensitivity
External Series Resistor vs. Output Variation (LSB's)
Offset Adjustment vs. External Series Resistor
10000
1000
100
100
Peak-Peak
variation at
potentiometer
10
1
100mV
10mV
1mV
0.1
0.01
10
0
5K 10K 15K 20K 25K 30K 35K 40K 45K 50K 55K 60K
0
5k 10k 15k 20k 25k 30k 35k 40k 45k 50k 55k 60k
External Series Resistor (Ohm's)
External Series Resistor Value (Ohms)
Figure 6. Offset Adjustment vs. External Series Resistor
Figure 7. Offset Adjustment Sensitivity
DATEL
•
11 Cabot Boulevard, Mansfield, MA 02048-1151 USA
•
Tel: (508) 339-3000
•
www.datel.com
•
e-mail: help@datel.com
31 Mar 2011 MDA_ADCDS-1410.B04 Page 5 of 14
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