As lead times for obtaining the Analog Devices AD1674J analog-to-digital converter (ADC) have grown very long, we have investigated several alternatives. If you wish to substitute the Sipex SI1674J for the originally recommended Analog Devices ADC, there appears to be only one minor modification necessary.
The AD1674J part uses an operational amplifier input with a summing junction. In the original Cookbook camera circuit, the bipolar offset pin was purposely left open since it has no affect on the operation of the ADC and it allows the builder to add an offset adjustment if desired. The SI1674J has a switched capacitor ratio DAC as the input. The input to the DAC is connected to a resistor divider network. If the bipolar offset pin 12 is not connected to ground, the gain of this device is 1.25 times that of the AD1674J part.
Because the Sipex part has a higher gain, it will not pass the ADC test in the CARD test software program. The test program will read 2.50V with only 2.0V on the ADC input. You have several options for using the Sipex ADC, but take only one of the options.
The Sipex ADC is sensitive to substrate injection since the part is a positive-supply-only device. Sipex operates properly with up to -3V below ground on the input, and voltages to -16.5V will not damage the part. If the preamplifier output voltage goes below -3V, however, when P1 is adjusted through its full range, the ADC output will suddenly jump. Measure pin 13 on the U5 ADC to check the preamp voltage, and adjust P1 until pin 13 is around zero volts.
The preamp output voltage in the 211 camera normally runs 0 to 11V. It will only go below ground if the reset level is adjusted below ground. If the image is very bright and goes past full scale, some -2V transients are present.
The preamp output voltage in the 245 camera also runs between 0 and 11V unless the reset level is adjusted below ground. Adjusting the reset to about 100 PV in order to get a bit more dynamic range is within the -3V limit.
When performing Card test 5. Card Functional Test #2, Converter; Test-1 step [10], it is okay for the match to read in error by +-0.05V.
Both the 211 camera and the 245 camera have been tested with the Sipex part. Noise comparisons on the 245 showed short-exposure dark images had ADC spans of about 4 to 5 for both chips. The peak noise was about 6 with either chip and the RMS noise was 1.7 ADC units for the AD1674J and 1.5 ADC units for the SI1674J.
I am currently using a Sipex ADC that I installed in my 245 camera and I have had no problems during field testing on a telescope. Other than the gain fix, there is no reason not to substitute the Sipex SI1674J for the Analog Devices AD1674J ADC at this time.
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