This project, a femto ammeter from
1985, (A Femto Amp is pretty miniscule = 10e-15A), shows how an innovative thermoelectric cooling/heating element (TEC) is used to stabilize the measurement accuracy of the circuit.
The major problem with these type of circuits is that the incredibly small currents to be measured are usually swamped by external noise.
So the design has to be extremely clever so that even the internal currents in the system do not wipe out the measurement results!
Another problem is that traditional designs would use huge time constants (of several hours) to average out the noise. This design avoids all that in a very clever way.
A femtoammeter is disclosed which resists noise induced reversed bias of the ammeter amplifier and avoids long time constant recovery with unusually high measurement accuracy. The amplifier ammeter has a feedback loop which is provided with paired opposingly faced log diodes in parallel across the feedback circuit.
Transients of negative or positive bias are accommodated without biasing the amplifier to a reverse polarity and having unacceptably long time constant recovery. Conventional temperature compensation, not possible with the parallel reversed diode configuration, is supplied by heat sinking the ammeter components, providing a log diode temperature sensor for the heat sinked components and controlling the heat sink operating temperature with a thermoelectric cooler.
The temperature sensing log diodes output to a computer lookup table. The same temperature sensing log diode outputs to a bridge circuit operating the thermoelectric cooler. The thermoelectric cooler is polarized to heat or cool the heat sink to thermally adjust the ammeter to an optimum operating temperature.
Automated bootstrap calibration is disclosed with reed switching for prevention of stray currents. Accuracy includes heretofore unattainable results.
1. Field of the Design
This design relates to ammeters and particularly to a femtoammeter for measuring currents in the range of 10-15 amperes.
Femtoammeters measure incredibly small currents. Unfortunately, the small currents measured are oftentimes smaller than the noise transients which can be anticipated within the ammeter. Conventional ammeter circuitry can easily give erroneous readings.
Femtoammeters find application in sensitive instrumentations in nuclear power plants such as main steamline radiation monitors, out of core wide range radiation monitors, out of core intermediate range monitors, and area and process radiation monitor functions.
Where ammeters are placed in these locations, they simply cannot be off line for inordinate periods of time. Ammeter reliability must be high; the instruments must not be inoperative due to transients. Moreover, the instrument must not cause false readings, as false readings cause nuclear power plants to go off line and lose operating revenues.
2. Summary Of The Prior Art
A femtoammeter is disclosed which resists noise induced reversed bias of the ammeter amplifier and avoids long time constant recovery with unusually high measurement accuracy. The amplifier ammeter has a feedback loop which is provided with paired opposingly faced log diodes in parallel across the feedback circuit.
Transients of negative or positive bias are accommodated without biasing the amplifier to a reverse polarity and having unacceptably long time constant recovery. Conventional temperature compensation, not possible with the parallel reversed diode configuration, is supplied by heat sinking the ammeter components, providing a log diode temperature sensor for the heat sinked components and controlling the heat sink operation temperature with a thermoelectric cooler.
The temperature sensing log diodes output to a computer lookup table. The same temperature sensing log diode outputs to a bridge circuit operating the thermoelectric cooler. The thermoelectric cooler is polarized to heat or cool the heat sink to thermally adjust the ammeter to an optimum operating temperature.
Automated bootstrap calibration is disclosed with reed switching for prevention of stray currents. Accuracy includes heretofore unattainable results.
OBJECTS AND ADVANTAGES OF THE DESIGN
An object of this design is to disclose a femtoammeter circuit in which reverse biasing of the amplifier by transient noise is provided with an acceptable recovery time constant. According to this aspect of the design, an amplifier is provided with an input to a first leg with a reference voltage to a second leg.
A feedback loop is provided from the amplifier output to the input leg and provided with paired opposing log diodes connected in parallel across the loop. Voltage transients biasing the amplifier negatively or positively are provided with rapid recovery paths through the log diodes.
An advantage of this aspect of the design is that the resultant ammeter may be reliably used with sensitive safety instrumentation. Long down times due to recovery from transients are avoided.
Unfortunately, the disclosed parallel and opposed log diodes have a secondary effect. They render completely inoperative temperature compensation circuits such as temperature compensating log diode 19 of FIG. 1. This being the case, it is a further object of this design to provide a temperature control for a femtoammeter.
According to this aspect of the design, the first and most sensitive amplification stage of the femtoammeter is enclosed within a heat sink. Two log diodes connected in series are provided with a constant current input. These log diodes output as a direct effect of the first order, a voltage which is directly proportional to temperature.
The output voltage is compared at a bridge circuit. The bridge circuit output goes to a thermoelectric cooler biasing the cooler to heat the sink where the sink temperature is too low, and biasing the cooler to cool the sink where the sink temperature is too high.
An advantage of this aspect of the design is that operating temperature of the ammeter amplifier is accurately controlled.
A further advantage of the log diode temperature sensing is that a direct output of heat sink temperature is provided. This output of heat sink temperature can adjust the ammeter output calibration on a real time basis. Such calibration can occur through three dimensional lookup tables selected on the basis of current and operating temperature.
An advantage of this aspect of the design is that not only is the ammeter read in real time for its present operating temperature, but long term thermal movement to and towards an optimum operating temperature is enabled.
A further aspect of this design is to disclose a bootstrap calibration system computer operated for periodically calibrating the ammeter. According to this aspect of the design, a reference resistance placed in parallel with gross resistive values calibrates the ammeter at sequential operating levels approximately two magnitudes apart over a range of six magnitudes. Magnetic switching of the bootstrapping resistors avoids stray current in the ammeter and assures regular automated calibration.
An advantage of the disclosed bootstrap calibration is that the three dimensonal lookup table is recreated at each calibration. Consequently, the instrument is calibrated substantially concurrently with each use.
The entire ammeter operates with a level of accuracy to within .+-.1% at 10-13 amps. Reliable instrument operation results.
Other objects, features and advantages of this design will become more apparent after referring to the following specification and attached drawings in which:
Referring to FIG. 2 the design is schematically described.
Referring to FIG. 2, input 24 passes through a summing point 28 to the leg of an amplifier 30. Amplifier 30 has an output through an A-to-D converter 14. D-to-A converter 16 provides summing point 18 with a temperature compensated input.
Amplifier 30 is provided with a feedback loop 31 which feedback loop includes opposed log diodes 34, 36 to allow amplifier 30 to operate in either polarity while permitting acceptable recovery times from inevitable voltage transients. A 1011 ohm resistor 38 and a small capacitor 32 (10 picofarads) form the remainder feedback loop for amplifier 30 for damping amplifier vibration.
Amplifier 30 is contained within a heat sink 42. Heat sink 42 has its temperature measure by constant current log diodes schematically shown at 45 outputting a voltage which is a function of temperature. This voltage is used for comparison at a bridge circuit 47 for powering of a thermoelectric cooler 48.
Comparison at bridge circuirt 47 biases circuit output. Where heat sink 42 temperature is too high, current to thermoelectric cooler 48 is operated to cool the heat sink. Where heat sink 42 temperature is too low, current is reversed to the thermoelectric cooler 48 to raise the temperature of the heat sink.
At the same time, the output of constant current temperature sensing diodes 45 is converted to a digital value and used to address lookup tables. The digital converter 14 causes the computer control to address an appropriate lookup table adjusted in real time to the desired current level. There results ammeter output which is substantially continuous and calibrated to reliable levels.
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