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Dual Channel Differential Electrometer

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FD223a Datasheet
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FD223a Instruction Manual
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The FD223a is a dual differential, high impedance amplifier/electrometer designed specifically for electrochemical measurements using ion specific (K+, Na+, Cl-, etc.) or pH glass microelectrodes.

The instrument is very stable, drift free, and features a built in provision for measuring and adjusting input leakage current. DC levels may be independently adjusted for each probe channel.

The ability to locate the sensing probes directly at the measurement site overcomes the noise introduced by the long cables usually needed to bring the measured potential to the instrument. Signal-driven guards at the probe input maintains the specified high resistance and reduces the stray capacitance of the probes.

Careful design, coupled with quality component selection, particularly in the headstage, results in an excellent amplifier with low noise and wide bandwidth. The FD223a will faithfully reproduce the measured signal.

To reduce the noise and stray capacity even farther the probe housing includes a signal driven guard. A portion of this inner driven shell is exposed at the probe tip allowing a spring shield to be extended over the electrode holder and microelectrode.


Input Impedance > 10^15 ohm, shunted by 0.5pF 
Input Capacitance 1pF, nominal
Leakage Current 75fA max
Gain  1.000± 0.1%
Output Resistance 50 ohm
Input Swing Voltage ±10V
Rise Time (10 to 90%) 5us, small signal 
Noise (0.1 Hz to 10 kHz) < 100uV p-p, input shorted
Baseline Stability ±0.1mV/day
Position Controls Range ±600mV 
Physical Dimensions Case:8.8 x 21.0 x 17.5 cm (H x W x D) Probe:12.7 x 65 mm (D x L), 1.8 m cable
Power 90-265VAC, 50/60Hz, 10VA 
Probe Handle 6.5 x 65 mm (D x L) 
Shipping Weight 2.5 kg
Operating Conditions

Equipment is intended to be operated in a controlled laboratory environment.
Temperature: 0-40°C; altitude:sea level to 2000m; relative humidity: 0-95%.



Mousavi, S. A. R., Nguyen, C. T., Farmer, E. E., & Kellenberger, S. (2014). Measuring surface potential changes on leaves. Nature Protocols, 9(8), 1997–2004.

Yoshida, T., Nin, F., Murakami, S., Ogata, G., Uetsuka, S., Choi, S., … Hibino, H. (2016). The unique ion permeability profile of cochlear fibrocytes and its contribution to establishing their positive resting membrane potential. Pflügers Archiv - European Journal of Physiology, 468(9), 1609–1619.

Oshima, H., Ikeda, R., Nomura, K., Yamazaki, M., Hidaka, H., Katori, Y., … Kobayashi, T. (2014). Change in Endocochlear Potential During Experimental Insertion of a Simulated Cochlear Implant Electrode in the Guinea Pig. Otology & Neurotology, 35(2), 234–240.

Campbell, J. B., Andersen, M. K., Overgaard, J., & Harrison, J. F. (2018). Paralytic hypo-energetic state facilitates anoxia tolerance despite ionic imbalance in adult Drosophila melanogaster. The Journal of Experimental Biology, jeb.177147.

Karunasinghe, R. N., Grey, A. C., Telang, R., Vlajkovic, S. M., & Lipski, J. (2017). Differential spread of anoxic depolarization contributes to the pattern of neuronal injury after oxygen and glucose deprivation (OGD) in the Substantia Nigra in rat brain slices. Neuroscience, 340, 359–372.

Rubio, L., García, D., García-Sánchez, M. J., Niell, F. X., Felle, H. H., & Fernández, J. A. (2017). Direct uptake of HCO 3 − in the marine angiosperm Posidonia oceanica (L.) Delile driven by a plasma membrane H + economy. Plant, Cell & Environment, 40(11), 2820–2830.

Lippert, F., Al Dehailan, L., Castiblanco, G. A., Tagelsir, A. A., Buckley, C., & Eckert, G. J. (2018). Enhancing predicted fluoride varnish efficacy and post-treatment compliance by means of calcium-containing gummy bears. Journal of Dentistry.

Staun Larsen, L., Baelum, V., Tenuta, L. M. A., Richards, A., & Nyvad, B. (2018). Fluoride in saliva and dental biofilm after 1500 and 5000 ppm fluoride exposure. Clinical Oral Investigations, 22(3), 1123–1129.

Heiny, J. A., Cannon, S. C., & Di Franco, M. (2018). A Four Microelectrode Method to Study Intracellular Ion Concentration and Transport in Skeletal Muscle Fibers. Biophysical Journal, 114(3), 627a.




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