FID Answer

  1. At 10 Hz, dual slope, Σ-Δ, and voltage-to-frequency converter ADCs are all viable. One might also use a digital signal processor and a faster ADC such as a successive approximations converter, doing multiple averages to emulate the slower devices.
  2. 1 pA = 10-12 A. 1 A = 6.18 × 1018 e- s-1, so in 0.1 s, we will see on average 618,000 electrons. N = S1/2, or 786. That's an RMS noise of 0.1 pA/786 = 0.13 fA. Detection limit is thus 0.4 fA. Maximum current is given as 1 µA, giving a dynamic range of 1 × 10-6 A/ 0.4 × 10-15 A = 2.5 × 109. This is an impossibly large dynamic range. The highest resolution commonly available is 24 bits, and that's 224 = 4 × 106 dynamic range. To get the additional dynamic range of a factor of almost 1000 will require an adjustable gain amplifier.
  3. Given the wide dynamic range, slow conversion rate, and need to signal average, Σ-Δ is probably the best choice.