Category Archives: Amplifier

Current-feedback op amps

A few weeks ago, find someone asked me about current-feedback op amps and I had the very basic answer that they had a low-impedance inverting input and relied on current, instead of voltage, to drive this input. This gave the op-amps higher slew rate (and bandwidth) at the expense of precision. A more thorough overview of these op amps can be found in chapter eight of Texas Instrument’s Op Amps for Everyone (linked below.) This is a four hundred page book and has some well written sections on various topics, including noise. An excellent read for those concerned with analog design.

slod006b.pdf )

Pre-amplifier tomfoolery

Spent most of the day today playing with the pre-amplifier design. The design uses the same instrumentation amplifier chips (ina2332) for both large and small electrodes, decease however, rx this becomes problematic. The ina2332 is a dual amp chip, ambulance where each instrumentation amplifier is based on the two-op-amp design and is physically small, so there are worse parasitics when compared to larger three-op-amp designs. One of these parasitics is input bias current, which in this case is 1-10pA. Sourcing this kind of current is no problem for the larger electrodes, but has become problematic with the micro-meter sized electrodes. As a result, I designed a module to use two ina116 chips in place of one of the ina2332. These chips are single channels and are a few times larger than the ina2332s, but, they are less noisy and require only 10fA of input bias current. By making a few estimations and writing out the electrochemistry I-V relationships, one can attribute 100-400uV of voltage noise to the side-effects of a 10pA input bias current and how they are more manageable with a 10fA bias current. The lesson for today is that 10^-12 is small, but not insignificant.