Patent GB-1256981-A (GB)
IPC: G01R15/00
Filed: 1969-03-14
Abstract: 1,256,981. Current or voltage measurement. COMMISSARIAT A L'ENERGIE ATOMIQUE. March 14, 1969 [March 15, 1968; Feb. 27, 1969], No. 13428/69. Heading G1U. In an instrument for measuring a function, e.g. magnitude or time integral, of a signal representing a direct current or voltage, the signal is applied to the input of an amplification network incorporating two amplifier circuits 2, 3 in series, there being a multi-element impedance feedback network, e.g. 22, 23, connected between the input and output of the first circuit and a logarithmic feedback network, e.g. 9, connected between the input and output of the second circuit. An arrangement 15 for reading the magnitude of the output signal of each amplifier circuit comprises linear and logarithmic scale metering units functioning simultaneously connected to the outputs of the first and second circuits respectively. The various elements of the multi-element network are switched automatically according to the value of the output of the amplification network. A reference input for the logarithmic scale metering unit is supplied with a voltage determined in accordance with the switching of the multi-element network. One embodiment of the instrument is suitable for measuring very small current or voltage over several orders of magnitude on linear and logarithmic scales simultaneously, range changing in the linear scale being effected automatically when a particular input magnitude is reached. In another embodiment, the integral of charges applied to the input is determined. In the first embodiment the input signal I is applied to an input impedance matching device 1, whose output at S is applied to the amplifier 2. This amplifier has linear feedback through one or both of resistors 22, 23, controlled by switch 8, to provide two ranges, and provides outputs to the linear input of a reading system 15 and to the amplifier. The amplifier 3 has logarithmic feedback via diode 9 and provides an output, via a matching circuit 10, to the logarithmic input of the system 15 and to the threshold detection amplifier 16 which has a threshold level established by variable negative feedback resistor 17. A voltage tapped from a potentiometer 11 in series with the resistor pair 24, 25 across a source 14 is applied via the matching device 10 to reference inputs for the logarithmic part of the reading system 15 and for the threshold detector/amplifier 16. This voltage changes when the ganged switches 8, 13 are changed in such a way that continuity of reading on the logarithmic scale is maintained when the sensitivity of the linear amplifier 2 is changed. The range changing switch 8 for the linear scale is operated by a relay 18 whose energization is under the control of the threshold detector 16. The relay is energized from a source 21 via a delay 20 such that following initial switch on, the higher range scale is first adapted to protect the system. In the second embodiment Fig. 3 (not shown) linear and logarithmic outputs representing the integral of the input are provided by a system similar to that of Fig. 2, except that feedback capacitances are provided across the amplifier 2, the capacitances being switched automatically to determine the appropriate range. The system of Fig. 3 (not shown) may be used to provide a digital indication of the integrated output by connecting a counter to the output of the threshold detector circuit 16 and using the operation of the realy 18 to effect a temporary short circuiting of the integrating capacitors so that these are discharged at each count.