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| Why Digital Electronics for Flow Cytometry? |
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Last Updated
6th of January, 2009
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The major flow cytometer manufacturers have converted from analog to digital electronics during the past 15 years. There are various blends of analog and digital electronics, but since main advantage of digital electronics is accurate logarithmic conversions, all the blends incorporate this feature. Most blends implement the full compensation matrix with digital electronics. Also, triggering on any parameter is easier with digital electronics.
One disadvantage of digital electronics is possible loss of resolution at the low end of the log scale. To implement a 4 decade log scale that spans 1mV to 10V, the lowest bin width is 9 micro-volts, so there would need to be approximately a million digital bins to provide low end resolution. This is 2 to the 20th power, so a 20 bit ADC would be required for full resolution at the low end of a 4 decade scale.
One manufacturer uses a 14 bit ADC, so full resolution cannot be provided at the low end with a peak sense (single value) circuit. However, since their ADCs run at 10MHz, approximately 20 samples can be collected per pulse, giving an effective 18 bits for an area measurement. More recently 18 bit ADCs have become available, so it isn’t necessary to sample a pulse several times to obtain 18 bit resolution.
Another disadvantage of digital area is so called aliasing, where the digital area has a slight variation depending on the synchronization between the pulse and ADC clock. This effect is reduced with faster sampling ADCs. With a 10MHz ADC, this effect adds perhaps 1-2% to the area CVs, and perhaps 3-4% to the pulse height CVs. Another disadvantage of digital width with a 10MHz sampling rate is the fairly large uncertainty of when the pulse begins and ends, or one sample per micron at 10u/us. This means the uncertainty in the pulse width measurement is a micron.
Since digital pulse processed parameters are not as good as analog generated pulse processed signals, some manufacturers have chosen to retain analog pulse area and width parameters.
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| Visitor Comments |
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Comment #1 (Posted by Wolesale Dealer
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Yeah you are right their is no doubt that digital electronics are now playing very important role in the field of technology but the analog is still better than digital in some cases
http://www.dailytrader.com
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Comment #2 (Posted by arjun pratap singh
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i m interested to know about DE but i have no notes so plz give me a basic ntes for DE
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