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Chapter 3 -- Instrument Circuitry--Reference 3000 Schematic/Block Diagrams
Notes for Figure 3-1
The 4x booster following the Control Amp can operate with two combinations of compliance current
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and compliance voltage. One is ± 1.5 amps at ± 30 volts, the other is ± 3 amps at ± 15 volts.
Only Potentiostat Mode circuitry is shown in this figure. In this mode the voltage difference between
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the Reference and Working Sense leads (called Esig) is feedback into the control amplifier.
In Galvanostat Mode, the feedback is from Isig.
In the ZRA and stack modes, the feedback is from a differential amplifier measuring the difference
between the Counter Sense and Working Sense leads of the cell cable. The counter sense circuitry is
not shown. It is conceptually similar to the voltage sensing circuit that generates Esig, except that it can
measure voltage differences as large as ± 36 volts.
The Bias DAC and PFIR (Positive Feedback IR compensation) DAC are set using a computer bus that is
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not shown.
Switches are either reed relays or MOS switches as appropriate. All switches are under computer
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control (obviously, since the Reference 3000 does not have a knob and dial front panel).
The variable current measurement resistor, Rm, is one of eleven fixed value resistors selected using
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relays. The resistor's values vary by decades: 50 mΩ, 500 mΩ, 5Ω, 50 Ω ...500 MΩ. The lower
value resistors require software gain corrections. Correction values are measured at Gamry's test
facility and stored in an EEPROM on the Reference 3000 potentiostat board. Software calibration of
the instrument by a customer does not change these Rm gain corrections.
Other components shown as being variable (IEStab capacitor and CASpeed capacitor) are actually
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several fixed value components switched into the circuit, not continuously variable .
The monitor BNC connectors for Isig and Esig are lightly filtered using an RLC circuit.
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The ADC channel for Esig is actually switchable between Esig (the reference voltage minus the working
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sense voltage) and Zsig (the counter sense voltage minus the working sense voltage). The Zsig
connection allows the Reference 3000 to measure the voltage of battery or fuel cell stacks.
The programmable attenuator on Esig prior to the ADC channel scales the Esig voltage to make it
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compatible with the A/D channel's ± 3 volt input range. The 0.25 gain setting allows the Reference
3000 to measure potential signals slightly in excess of 10 volts (on a 12 volt full scale range). Isig is
gained to be 3 volts full-scale so it does not require a similar attenuation function.
All the resistors summing voltages into the Control Amplifier input do not have values shown on the
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diagram – their values depend on scaling factors too complex to discuss in this chapter.
Calibration components are not shown.
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Gamry's software can disconnect the signal generator from the Potentiostat. Once disconnected it can
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be used for other experimental control tasks.
Overload protection and overload detection are not shown. Good engineering practice demands that
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any possible misconnection of the cell leads will not damage the instrument. This practice has been
followed in the Reference 3000 design.
The overload protection can handle overloads of up to 30 amps for very short times. Fuses in the
Working and Counter Sense leads always open up before overload conditions can damage the
instrument. Misconnection of a battery, fuel cell, or super-capacitor stack can open the fuse, but will
not cause hardware failures.
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