Meter-Scale Magnifier

20 029 The resolution of moving-coil meters is general- ly no better than 1%, because the scale normally is given no more than 100 marker lines for the given dimensions: more lines might detract from the legi- bility. Most digital meters have a resolution of 0.05% or better. The resolution of moving-coil instruments may be improved in two ways: physi- cally enlarging the scale, which is possible only in the factory, or electronically enlarging the scale, which is what this article is all about. The circuit divides the scale into five sections, each of which is then extended over the full scale. This therefore gives a five-fold improvement in resolution. The input signal (200 mV = full-scale deflection - f.s.d.) is amplified by IC1 to a value of 2.5 V. The amplified signal is fed to four comparators that divide the input signal into segments of 40 mV. Which segment is indicated on the meter-scale is shown by the LEDs. The LEDS are shunted by a 10 kSI resistor, so that the open-co...