| 摘要 |
Optically generated signals are processed to form a two-wire 4-20 mA signal by generating (U7) a control signal having pulses at a selected frequency to drive a light emitter (10) which generates light pulses, transmitting the light pulses to a light detector (20) over a transmission line (15) having variable attenuation to form a sensor signal, and amplifying the sensor signal in an operational amplifier (U1). The variations in attenuation follow a process variable to be measured. To save power the amplifier (U1) has a low-current mode into which it is switched whenever no pulse is present in the sensor signal. The amplifier (U1) is switched into a high-current mode only when a pulse is present in the sensor signal. Switching is controlled by the control signal for the light emitter (10). Peaks in the signal from the amplifier (U1) are sampled and held (C1,CR1,S1) and then subjected to low pass filtering (R15,C2) to remove the selected frequency component and leave a cyclic filtered signal. The amplifier (U1) is driven towards ground by a feedback clamping signal which changes slowly with respect to the cyclic filtered signal and is generated by feedback means (U4,U3d). The filtered signal triggers a multivibrator (U10) to form a pulse signal having pulses of fixed length and amplitude of each cycle of the filtered signal. The pulse signal is then averaged (R22,R42,R34,C20,C14,C15) with respect to its voltage, subjected to zero adjustment (U11b,R24) and span adjustment (U11a,R28) and then converted (40) to a two-line 4-20 mA current signal. |
