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Microelectrode Amplifier
A/D Stimulus Isolation
Digital Stimulus Isolation

 Dominiating Factor in calculation for battery life (1) Akaline batteries (2) Rechargable batteries (3) Digital-SIU and Dual mode SIU in Digital mode (5) Battery shelf life (6). 2.5 years @ 0.01% discharge per day on shelf. 60 days @ 0.5% discharge per day on shelf. Dual mode Analog/Digital SIU in Analog mode (5) The Quiescent current (4) used to linerize the optical couplers that produce a linear relationship between the input voltage and the output current or voltage. 125 hours or 15 experiments (assuming a typical 8 hr experiment). 125 hours or 15 experiments (assuming a typical 8 hr experiment).

(1)  You can use the following formula to calculate battery life where battery life is measure in number-of-experiments:

* Lifetime (number of experiments) =  Energy per battery (ma*hr / bat) / Energy consumed per experiment (ma*hr / experiment)

where

* Energy consumed per experiment (ma*hr / experiment)

= (current used to stimulate cells * duration of stimulation) + (quiescent current for active circuitry * duration of experiment)

= (stim-level (ma) * stim-duration (sec) * train-frequency (1/sec) * cycle-duration (sec) * num-cycles-per-experiment * 1/3600 hr/sec)  +  (Quiescent Current (ma) * Duration of an experiment (hrs)).

and

* Energy per battery (ma*hr / bat) = 125 mA*hr
(measured value - include graphs and calculations for this number for the different battery types later)

The marked difference between the D-SIU is and the A-SIU is the fact that the D-SIU's quiescent current is virtually zero making the last term in the sum zero as well.

Analyzing the worst case stimulation paradigm for the D-SIU (digital mode of A-SIU too) -- 10mA stimulation level for 0.1mS each pulse at 100Hz for 10 sec. 5 times per experiment. This yeilds 5 sec of stim at 10mA per experiment.  Therefore

Energy consumed per experiment (ma*hr / experiment) = 1.39 x 10^-3  ma*hr / exp
Lifetime (number of experiments) = 90 x 10^3 experiments.

Based on these number you can see that shelf life of a battery is the dominate factor by far. Take home lesson is that stimulating cells doesn't consume much energy.

In analyzing the analog SIU the dominating factor is the quiescent current term of the equation. The Iq is about 1ma and the average experiment is 8 hrs:

Energy consumed per experiment (ma*hr / experiment) = 8 ma*hrs
Lifetime (number of experiments) = 15 experiments

Result: we don't recommend using rechargable batteries in the digital siu. With the analog siu you have to balance how often you do experiments. You only get about 15 experiments out of a set of batteries whether you use Akaline or rechargables. The deal with rechargables is you have to recharge every 60 days no matter what, since they discharge by themselves on the shelf. If you only do experiments every two weeks a set of akaline batteries will last 210 days.

(2) The akaline batteries we recommend (and ones we used to collect this data) are Panasonic 9V. We've found that, with other battery types, most notiably Duracell, there are very large variances in their physical dimensions. That may cause problems with their fit in the battery carrier. As a result we recommend Panasonic Alkaline as the best or in a pinch Energizer Alkaline.  If you would like to order Panasonic 9V batteries in bulk from Digi-Key corporation, the PN is P145-ND, phone: 800-344-4539.

(3) Rechargable batteries are NiCAD (include NiMh later (if usage issues aren't an issue)).  Due to the variety of rechargable batteries available, only use rechargable batteries supplied by Getting Instruments. Battery charger design is dependant battery type.

(4) Quiescent current is static current consumed by the active electronics when the circuit isn't producing an output. The active electronics is the circuitry that allows accurate, fine control of the output using an analog input voltage. Especially under low level stimulations conditions (input between 0 and 1 volt) and for fine step functions.  (include transfer function graphs blown out and zoomed in).

(5) The dual mode Analog/Digital SIU (4AD) can operates exactly like the digital only SIU (4D) when set to digital mode. Functionally and consumes the same battery power.  The D-SIU is a functional subset of the A-SIU and also draws the same current.

(6) Percent of discharge is a percent of energy per cell (importance because discharge percentage isn't a percent of cell voltage). The cell voltage for a given energy content is different for akaline and rechargables due to the shape of the discharge curve (energy is the integral of the voltage discharge curve). In order to maintain a certain compliance level  for the current output (compliance is the highest voltage the output can obtain to generate a specified current) then we fix the threshold voltage of the battery independant of the energy content. The threshold per battery to maintain compliance is 8 volts. For NiCad you will consume about 90% of it's energy content to reach this level. For alkaline you will only consume about 50%. Turns out in both cases you end up with 125 ma*hrs of energy (the akaline has more total).  (include graphs).

 Getting Instruments, Inc - 20030330