HEWLETT . PACI { ARD JOUMHL o MARCH 1969

Some common analog-to-digital conversion techniques, and ben-etits derived trom dual slope as used in the new digital instrument. Drcrrar voLTMETERS DTFFER BASTcALLy from the commonly used analog voltmeters in one respect-their display or readout is a numeric display rather than a meter indication. Analog voltmeters seldom have accuracies greater than about 0.25%, and to achieve even these accuracies, a great deal of care is necessary in reading the meter. In the digital voltmeter, the analog signal is 'converted' into digital form. Since the required resolution in reading is obtained by the numeric readout, the accuracy of the signal being measured depends upon the accuracy of the conversion technique. The digital form of the signal may be easily read by the user, or the conversion mav be done manv times each second and the resulting binary signal transmitted to external processing or recording devices, such as computers or printers. By comparison to the meter instruments of 0.25% reading accuracies, there are digital voltmeters with accuracies on the order of O.OO2% of readins. Conversion Methods Four basic analog to digital conversion techniques are being used.* These are: (1) single ramp, (2) successive approximation or potentiometric technique, (3) voltage *ln addition to the iour logic techniques, there are several A-to-D Converters that use a combination of these to improve performance, For example the HP Model 3460A uses a potentiometric/integrating combination and the HP Model 2402A uses a voltage-to-frequency/interpolation combination, Fag. 1. With the ramp technique (a), dc voltage is measured by counting the number ol cycles allowed through the gate during time T(b). Fig. 2. (left) Potentiomettic digital voltmeter uses a comparatot amptilier to drive a logical programmer. The prcgtammer adlusts a precision power supply and resistive divider to null out the unknown voltage at the comparator input. The tinat setting ot the divider is converted to a readout of input voltage. Fig.3.(tight) ln an integrating (tigitat voltmeter, the time interval tather than trequency is lixed. The unknown input vottage is convefted to a frequency ptoportional to that voltage. to frequency conversion, and (4) dual-slope technique. to-analog converter represents the unknown voltage. Each technique has advantages, but any voltmeter design (The D-to-A converter usually consists of resistive net-using a single technique may differ significantly due to works that divide the dc reference voltage into discrete certain basic compromises that must be made. quantities.) Reading speed is high since the reading rate is limited …