ADCs digitize at different rates. What conversion rate is required if a chromato

Question

ADCs digitize at different rates. What conversion rate is required if a chromatographic peak is to be sampled and digitized 25 times between the first positive deflection from the baseline until the peak returns to the baseline? The total baseline-to-baseline time is (a) 20 s and (b)1 s. According to the Nyquist sampling criterion (see Section 5C-2), a signal must be digitized at a rate at least twice that of the highest frequency in the signal to avoid a sampling error. If a particular 12-bit ADC has a conversion time of 5 s, what is the highest frequency that can be accurately digitized while satisfying the Nyquist criterion?

Explanation / Answer

given, the signal has to be digitized 25 times from the first p[ositive deflection untill the peak returns to baseline

a. total baseline to baseline time = t = 20 s

now, for digitisation 25 times in this interval

dt = t/25

so conversion rate = 1/dt = 25/t = 25/20 = 1.25 Hz

b. for t = 1 s

dt = t/25

rate = 25/1 = 25 Hz

a 12 bit ADC has conversion time of 5 micro s = t

so, conversion freq = 1/t

hence this freq must be atlewast three times the signal freq f

so, 1/t = f

highest frequency that can be recorded by ADC accurately = 1/3t = 1/3*5*10^-6 = 66,666.66 Hz

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